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Vertical clearance ??
Ok, now that I have had time to compare NOAA charts with the reproductions
in the Maptech portfolio, I see that vertical clearance is from MHW (mean high water), as stated in the title block of each NOAA chart. It seems Maptech 'washes out' the title block as they copy and crop the charts for their book pages. But, that still leaves me with the question: How do you calculate or adjust the stated Vertical Clearance when all you have is a chart and a tide table. In my example; The charts show a vertical clearance of Hood Canal Bridge, east span, as 55 ft and the tide table shows a height of 8 ft at the time I want to pass under it. My mast height is 54' 7", including the VHF antenna... Do I need the NOAA tide tables or can this be calculated from the usual, locally reproduced, convience tables?? I realize, after the fact, that there would have been sufficient clearance yesterday, but there may well come a day when I arrive at the bridge at something slightly higher than MHW. Steve s/v Good Intentions |
Vertical clearance ??
Steve wrote:
Were is vertical clearance measured from (what datum)? Should be Mean High Water. I can't seem to find this information in any of my reference books. Could have sworn it was in Chapman's, but my copy is on my bedside table and my wife is still asleep. Chart 1 (http://www.carolinaglobalmaps.com/pr...cal/chart1.htm) says: the shoreline is "usually the mean high water line" (p 4) and heights of landmarks and structures are "referred to the shoreline plane of reference" (p8). regards, dave M/V Auspicious "a sailor fallen from grace" |
Vertical clearance ??
Steve wrote:
But, that still leaves me with the question: How do you calculate or adjust the stated Vertical Clearance when all you have is a chart and a tide table. In my example; The charts show a vertical clearance of Hood Canal Bridge, east span, as 55 ft and the tide table shows a height of 8 ft at the time I want to pass under it. My mast height is 54' 7", including the VHF antenna... You have to know MHW. If it isn't on your tide table, I'd ask the bridge tender. If he doesn't know, I'd ask him to open up. |
Vertical clearance ??
Steve,
Does this bridge not have a water height scale? I thought all bridges on navigable water had them. Ron |
Vertical clearance ??
The answers to all your questions are somewhere in:
http://pollux.nss.nima.mil/NAV_PUBS/APN/Chapt-09.pdf Yes, you do have to be careful. Astronomical predictions often show tides a foot or two high than MHW in some locations. Also, a storm or even a persistent wind can do the same. And both can occur at the same time. And, although most bridges are actually a few inches higher than listed, some are actually lower, or have gear hanging down. "Steve" wrote in message ... Ok, now that I have had time to compare NOAA charts with the reproductions in the Maptech portfolio, I see that vertical clearance is from MHW (mean high water), as stated in the title block of each NOAA chart. It seems Maptech 'washes out' the title block as they copy and crop the charts for their book pages. But, that still leaves me with the question: How do you calculate or adjust the stated Vertical Clearance when all you have is a chart and a tide table. In my example; The charts show a vertical clearance of Hood Canal Bridge, east span, as 55 ft and the tide table shows a height of 8 ft at the time I want to pass under it. My mast height is 54' 7", including the VHF antenna... Do I need the NOAA tide tables or can this be calculated from the usual, locally reproduced, convience tables?? I realize, after the fact, that there would have been sufficient clearance yesterday, but there may well come a day when I arrive at the bridge at something slightly higher than MHW. Steve s/v Good Intentions |
Vertical clearance ??
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Vertical clearance ??
Vertical clearance is usually measured above high water. The only
significant exception is small scale charts where the datum may be Mean Sea Level. Such charts rarely show bridge clearances, so the difference isn't an issue. The exact datum varies from country to country. On US domestic charts, it's always Mean High Water (the average of all high tides). On foreign charts, including US charts made from foreign data, it's usually a higher datum, Mean Higher High Water (the average of the higher of the two high tides each day), Mean High Water Springs (the average of all spring tides), Highest Astronomical Tide (the highest the water ever gets without help from a storm) or maybe something else. May I suggest that you buy a copy of Bowditch (The American Practical Navigator -- NIMA Pub 9) which will answer this question and every other question you ever had about navigation? Bowditch just had its 200th anniversary of being continuously in print, through about 75 editions. It's the standard source of basic information on all kinds of navigation. There's no need to buy it new -- any edition from the last forty years will do fine. I might add, that as a serious navigator, I own two copies of Bowditch (1962 and 1977) and don't own Chapman. Jim Woodward www.mvfintry.com "Steve" wrote in message ... Were is vertical clearance measured from (what datum)? I can't seem to find this information in any of my reference books. What I find in Chapman's, in a section on chart "Plane of Reference", states "Different planes are on different charts of various boating areas. For charts along the Atlantic coast the National Ocean Survey uses mean low water as the datum for soundings. On the Pacific coast it is the mean lower low water that is used for the reference plane....." No mention of vertical clearance. Here in the Pacific NW where tide difference are signicant the chart datum is not metioned in my Maptech Reg. 15 portfolio (or I just can't find it). Example: Today the vertical clearance under two different bridges was just too close to take a chance on and it was a +8 ft tide. I ended up playing it safe and had the draw span opened on the Hood Canal floating bridge and went out of my way to avoid going through the the Port Townsend Canal with an overhead bridge span. In each case the "stated" Vertical Clearance was enough for my 54 ft 7 inch requirement, but I had no idea what tidal state their datum was based on. (Hood Canal Bridge, east span was 55ft vert. clearance while P.T. Canal bridge was 58ft.) Sure hope the 10-20 min traffic delay for some 200 cars and trucks wasn't due to my cautions and lack of knowledge on this matter.. Please enlighten me. Steve s/v Good Intentions |
Vertical clearance ??
The link that I posted before was to the tide chapter in Bowditch. The entire contents
are online at: http://pollux.nss.nima.mil/pubs/pubs...s.html?rid=187 There are two versions of the latest Bowditch available in hard copy - the one from the government includes a CD which has a wrapper program to do some of the navigation for you. It also has a calculator and a few other features. I don't know if the "commercial" version - roughly the same price, also has that. Old editions of Bowditch are fun - I have several, including a first and one from 1870 - I like to see what the practices and terminology was back then. "Jim Woodward" wrote in message om... Vertical clearance is usually measured above high water. The only significant exception is small scale charts where the datum may be Mean Sea Level. Such charts rarely show bridge clearances, so the difference isn't an issue. The exact datum varies from country to country. On US domestic charts, it's always Mean High Water (the average of all high tides). On foreign charts, including US charts made from foreign data, it's usually a higher datum, Mean Higher High Water (the average of the higher of the two high tides each day), Mean High Water Springs (the average of all spring tides), Highest Astronomical Tide (the highest the water ever gets without help from a storm) or maybe something else. May I suggest that you buy a copy of Bowditch (The American Practical Navigator -- NIMA Pub 9) which will answer this question and every other question you ever had about navigation? Bowditch just had its 200th anniversary of being continuously in print, through about 75 editions. It's the standard source of basic information on all kinds of navigation. There's no need to buy it new -- any edition from the last forty years will do fine. I might add, that as a serious navigator, I own two copies of Bowditch (1962 and 1977) and don't own Chapman. Jim Woodward www.mvfintry.com "Steve" wrote in message ... Were is vertical clearance measured from (what datum)? I can't seem to find this information in any of my reference books. What I find in Chapman's, in a section on chart "Plane of Reference", states "Different planes are on different charts of various boating areas. For charts along the Atlantic coast the National Ocean Survey uses mean low water as the datum for soundings. On the Pacific coast it is the mean lower low water that is used for the reference plane....." No mention of vertical clearance. Here in the Pacific NW where tide difference are signicant the chart datum is not metioned in my Maptech Reg. 15 portfolio (or I just can't find it). Example: Today the vertical clearance under two different bridges was just too close to take a chance on and it was a +8 ft tide. I ended up playing it safe and had the draw span opened on the Hood Canal floating bridge and went out of my way to avoid going through the the Port Townsend Canal with an overhead bridge span. In each case the "stated" Vertical Clearance was enough for my 54 ft 7 inch requirement, but I had no idea what tidal state their datum was based on. (Hood Canal Bridge, east span was 55ft vert. clearance while P.T. Canal bridge was 58ft.) Sure hope the 10-20 min traffic delay for some 200 cars and trucks wasn't due to my cautions and lack of knowledge on this matter.. Please enlighten me. Steve s/v Good Intentions |
Vertical clearance ??
I did ask the bridge operator, but the answer, sounded more like a guess
(seat of the pants) since he is about a mile from the fixed span I was interested in passing under. There is a clearance scale on the fixed span but according to my cruising guide, it has some error (on the conservative side), several feet less than the vert. clearance on the charts. Even if I could trust these scale markers, I wouldn't hate to get in close enough to read them only to find there wasn't enough clearance, with several knot of current running.. Bad situation for a sail boat, trying to come about with a deep draft. Many of these scales that I have seen in New England, were badly faded or covered with marine growth/bird droppings, etc. I'm going to search out a Gov. tide table and see what the charted datum is for both of these bridges. Thanks again for the good response on this question. A good exercise. 40+ years of boating but not to proud to ask the dumb questions. Steve s/v Good Intentions |
Vertical clearance ??
You need either a "Tide Table" or tide program, so you can figure out
the clearance based on the stage of the tide at the time you will pass under. The tables or a program will give you the mean tide ranges (or in the case of a tide program it will frequently tell you MHW) from which you can calculate YOUR height above or below MHW for the particular time or stage of the tide, in question. If the bridge clearance is 55', and you find that MHW for a particular bridge ( or nearest datum point)is 5' and you will be passing under the bridge at a time when the tide is at +2', you (technically EG) should have 58' of clearance ..... or if you are in some extreme tides and the tide at the particular time will be +8', you will only have 52' of clearance. Always approach these numbers with caution and check any local "tide gauge" on a bridge pier when available. Steve wrote: Ok, now that I have had time to compare NOAA charts with the reproductions in the Maptech portfolio, I see that vertical clearance is from MHW (mean high water), as stated in the title block of each NOAA chart. It seems Maptech 'washes out' the title block as they copy and crop the charts for their book pages. But, that still leaves me with the question: How do you calculate or adjust the stated Vertical Clearance when all you have is a chart and a tide table. In my example; The charts show a vertical clearance of Hood Canal Bridge, east span, as 55 ft and the tide table shows a height of 8 ft at the time I want to pass under it. My mast height is 54' 7", including the VHF antenna... Do I need the NOAA tide tables or can this be calculated from the usual, locally reproduced, convience tables?? I realize, after the fact, that there would have been sufficient clearance yesterday, but there may well come a day when I arrive at the bridge at something slightly higher than MHW. Steve s/v Good Intentions |
Vertical clearance ??
I think it's mean MHHW (Mean Higher High Water) on the Wesy coast, not MHW.
Paul L "Steve" wrote in message ... Ok, now that I have had time to compare NOAA charts with the reproductions in the Maptech portfolio, I see that vertical clearance is from MHW (mean high water), as stated in the title block of each NOAA chart. It seems Maptech 'washes out' the title block as they copy and crop the charts for their book pages. But, that still leaves me with the question: How do you calculate or adjust the stated Vertical Clearance when all you have is a chart and a tide table. In my example; The charts show a vertical clearance of Hood Canal Bridge, east span, as 55 ft and the tide table shows a height of 8 ft at the time I want to pass under it. My mast height is 54' 7", including the VHF antenna... Do I need the NOAA tide tables or can this be calculated from the usual, locally reproduced, convience tables?? I realize, after the fact, that there would have been sufficient clearance yesterday, but there may well come a day when I arrive at the bridge at something slightly higher than MHW. Steve s/v Good Intentions |
Vertical clearance ??
I think it's mean MHHW (Mean Higher High Water) on the Wesy coast, not MHW.
Paul L "Steve" wrote in message ... Ok, now that I have had time to compare NOAA charts with the reproductions in the Maptech portfolio, I see that vertical clearance is from MHW (mean high water), as stated in the title block of each NOAA chart. It seems Maptech 'washes out' the title block as they copy and crop the charts for their book pages. But, that still leaves me with the question: How do you calculate or adjust the stated Vertical Clearance when all you have is a chart and a tide table. In my example; The charts show a vertical clearance of Hood Canal Bridge, east span, as 55 ft and the tide table shows a height of 8 ft at the time I want to pass under it. My mast height is 54' 7", including the VHF antenna... Do I need the NOAA tide tables or can this be calculated from the usual, locally reproduced, convience tables?? I realize, after the fact, that there would have been sufficient clearance yesterday, but there may well come a day when I arrive at the bridge at something slightly higher than MHW. Steve s/v Good Intentions |
Vertical clearance ??
I think it's mean MHHW (Mean Higher High Water) on the Wesy coast, not MHW.
Paul L "Steve" wrote in message ... Ok, now that I have had time to compare NOAA charts with the reproductions in the Maptech portfolio, I see that vertical clearance is from MHW (mean high water), as stated in the title block of each NOAA chart. It seems Maptech 'washes out' the title block as they copy and crop the charts for their book pages. But, that still leaves me with the question: How do you calculate or adjust the stated Vertical Clearance when all you have is a chart and a tide table. In my example; The charts show a vertical clearance of Hood Canal Bridge, east span, as 55 ft and the tide table shows a height of 8 ft at the time I want to pass under it. My mast height is 54' 7", including the VHF antenna... Do I need the NOAA tide tables or can this be calculated from the usual, locally reproduced, convience tables?? I realize, after the fact, that there would have been sufficient clearance yesterday, but there may well come a day when I arrive at the bridge at something slightly higher than MHW. Steve s/v Good Intentions |
Vertical clearance ??
Coast Pilot 7, says MHW, unless otherwise stated.
You need to check your chart or Particular Coast Pilot, etc., if you have any doubts ..... For instance, I believe the Mississippi river, above a point will use "river stage". If in doubt, G use the one that gives you the least clearance. otn Paul L wrote: I think it's mean MHHW (Mean Higher High Water) on the Wesy coast, not MHW. Paul L "Steve" wrote in message ... Ok, now that I have had time to compare NOAA charts with the reproductions in the Maptech portfolio, I see that vertical clearance is from MHW (mean high water), as stated in the title block of each NOAA chart. It seems Maptech 'washes out' the title block as they copy and crop the charts for their book pages. But, that still leaves me with the question: How do you calculate or adjust the stated Vertical Clearance when all you have is a chart and a tide table. In my example; The charts show a vertical clearance of Hood Canal Bridge, east span, as 55 ft and the tide table shows a height of 8 ft at the time I want to pass under it. My mast height is 54' 7", including the VHF antenna... Do I need the NOAA tide tables or can this be calculated from the usual, locally reproduced, convience tables?? I realize, after the fact, that there would have been sufficient clearance yesterday, but there may well come a day when I arrive at the bridge at something slightly higher than MHW. Steve s/v Good Intentions |
Vertical clearance ??
Hi Steve,
Bridge heights are measured from Mean High Water - although I can't remember where I saw the reference... Bill "Steve" wrote in message ... Were is vertical clearance measured from (what datum)? I can't seem to find this information in any of my reference books. What I find in Chapman's, in a section on chart "Plane of Reference", states "Different planes are on different charts of various boating areas. For charts along the Atlantic coast the National Ocean Survey uses mean low water as the datum for soundings. On the Pacific coast it is the mean lower low water that is used for the reference plane....." No mention of vertical clearance. Here in the Pacific NW where tide difference are signicant the chart datum is not metioned in my Maptech Reg. 15 portfolio (or I just can't find it). Example: Today the vertical clearance under two different bridges was just too close to take a chance on and it was a +8 ft tide. I ended up playing it safe and had the draw span opened on the Hood Canal floating bridge and went out of my way to avoid going through the the Port Townsend Canal with an overhead bridge span. In each case the "stated" Vertical Clearance was enough for my 54 ft 7 inch requirement, but I had no idea what tidal state their datum was based on. (Hood Canal Bridge, east span was 55ft vert. clearance while P.T. Canal bridge was 58ft.) Sure hope the 10-20 min traffic delay for some 200 cars and trucks wasn't due to my cautions and lack of knowledge on this matter.. Please enlighten me. Steve s/v Good Intentions |
Vertical clearance ??
Steve wrote:
Were is vertical clearance measured from (what datum)? The vertical clearance is measured from 'Mean High Water' in places with a single diurnal tide. In the Pacific Northwest, it is measured from 'Mean Higher High Water' (MHHW). The tide tables provide 'Mean Tide' for each subordinate station, and 'Mean Range'. You can figure out what MHHW is for a place (within a foot or so) by taking 'Mean Tide level' and adding one-half the Mean Tidal Range. When I do that, I come within a foot of the MHHW given in my 'Tides and Currents Pro' program, and the error results in a number less (more safe) than the listed one. The vertical clearance of the east span of Hood Canal Bridge is 55 ft. Yesterday the highest tide was 8.8 feet (at 1500). The Mean Tide for Port Gamble is listed as: 6.10 ft. The Mean Range is listed as: 6.70. My computation results in a figure of 9.45 feet for the MHHW. Therefore, at the high tide (daylight hours) there was 55.6 feet under the east span high rise. Of course, prudence leaves 2' of wiggle room to make up for unplanned things like wind effect and such, or some dot.com yahoo with more boat than brains zipping through at 55. But if your boat height over water is 53' or less, well, just don't go on rec.auto.vacationing and mention it. ;=) This information comes from Chapman, but also from U.S. Power Squadron courses in Piloting and Advanced Piloting. Not a bad investment in time and a few bucks. |
Vertical clearance ??
Chuck Bollinger wrote: Steve wrote: Were is vertical clearance measured from (what datum)? The vertical clearance is measured from 'Mean High Water' in places with a single diurnal tide. In the Pacific Northwest, it is measured from 'Mean Higher High Water' (MHHW). Interesting ..... Can you show me where this information comes from? In looking at CP 7, it list all heights as above MHW, unless otherwise stated, and the only major change to this I can find is for the Columbia River, which uses MLLW below Harrington Point, and "Columbia River Datum", between there and Bonneville Dam. The tide tables provide 'Mean Tide' for each subordinate station, and 'Mean Range'. You can figure out what MHHW is for a place (within a foot or so) by taking 'Mean Tide level' and adding one-half the Mean Tidal Range. When I do that, I come within a foot of the MHHW given in my 'Tides and Currents Pro' program, and the error results in a number less (more safe) than the listed one. On the right track, but be careful that the meaning of "mean Tide" and "mean range", given in the program and tables you are using, mean G what you want. Many define "Mean tide" as "the level half way between mean high water and mean low water" and "mean range" as "the difference in height between MHW and MLW" .... see the problem? My particular tide program, gives me MHHW and "Mean Tide". In this case, I would take the "mean Tide" X 2 and apply it to Zero tide (MLLW) and use this as MHW .... It should, normally, give a built in safety factor. At any rate, as I said before, be careful. There are many variables which can come into play, and you should NEVER push the envelope too closely. BTW, I think the program you are using gives MHHW otn |
Vertical clearance ??
Steve wrote:
Were is vertical clearance measured from (what datum)? In the European canal system it is measured from PHEN, which is a French acronym for "the highest navigable waters"(les plus hautes eaux navigables). When the water gets higher than that they cancel all navigation. So a posted vertical height is always the worst case. Cheers, Andy |
Vertical clearance ??
Yea, you are right. I always 'assumed' it was MHHW, but the Pilot clearly
say MHW. It seems like MHHW would make more sense out here. (sorry for the multiple posts earlier - Outrage acting up). Paul "otnmbrd" wrote in message .net... Coast Pilot 7, says MHW, unless otherwise stated. You need to check your chart or Particular Coast Pilot, etc., if you have any doubts ..... For instance, I believe the Mississippi river, above a point will use "river stage". If in doubt, G use the one that gives you the least clearance. otn Paul L wrote: I think it's mean MHHW (Mean Higher High Water) on the Wesy coast, not MHW. Paul L "Steve" wrote in message ... Ok, now that I have had time to compare NOAA charts with the reproductions in the Maptech portfolio, I see that vertical clearance is from MHW (mean high water), as stated in the title block of each NOAA chart. It seems Maptech 'washes out' the title block as they copy and crop the charts for their book pages. But, that still leaves me with the question: How do you calculate or adjust the stated Vertical Clearance when all you have is a chart and a tide table. In my example; The charts show a vertical clearance of Hood Canal Bridge, east span, as 55 ft and the tide table shows a height of 8 ft at the time I want to pass under it. My mast height is 54' 7", including the VHF antenna... Do I need the NOAA tide tables or can this be calculated from the usual, locally reproduced, convience tables?? I realize, after the fact, that there would have been sufficient clearance yesterday, but there may well come a day when I arrive at the bridge at something slightly higher than MHW. Steve s/v Good Intentions |
Vertical clearance ??
otnmbrd wrote:
Chuck Bollinger wrote: Steve wrote: Were is vertical clearance measured from (what datum)? The vertical clearance is measured from 'Mean High Water' in places with a single diurnal tide. In the Pacific Northwest, it is measured from 'Mean Higher High Water' (MHHW). Interesting ..... Can you show me where this information comes from? In looking at CP 7, it list all heights as above MHW, unless otherwise stated, and the only major change to this I can find is for the Columbia River, which uses MLLW below Harrington Point, and "Columbia River Datum", between there and Bonneville Dam. We're just going out and tomorrow going to Port Ludlow. It will be Monday evening before I'll be on the internet again. Working on it. The tide tables provide 'Mean Tide' for each subordinate station, and 'Mean Range'. You can figure out what MHHW is for a place (within a foot or so) by taking 'Mean Tide level' and adding one-half the Mean Tidal Range. When I do that, I come within a foot of the MHHW given in my 'Tides and Currents Pro' program, and the error results in a number less (more safe) than the listed one. On the right track, but be careful that the meaning of "mean Tide" and "mean range", given in the program and tables you are using, mean G what you want. Many define "Mean tide" as "the level half way between mean high water and mean low water" and "mean range" as "the difference in height between MHW and MLW" .... see the problem? Frankly, no. One is a tide level and the other is a range. But something bothers me about Mean Tidal Level being half way between MHW and MLW, especially where there are two diurnal highs and lows. Can't put my finger on it, but that seems like one of those shortcuts that can introduce errors. Kind of like those situations where computing from the results of a computation introduces error. Another thing to research. My particular tide program, gives me MHHW and "Mean Tide". In this case, I would take the "mean Tide" X 2 and apply it to Zero tide (MLLW) and use this as MHW .... It should, normally, give a built in safety factor. At any rate, as I said before, be careful. There are many variables which can come into play, and you should NEVER push the envelope too closely. Hmm. More later on that. Literally have to go. BTW, I think the program you are using gives MHHW Yes. otn |
Vertical clearance ??
x-no-archive:yes
Chuck Bollinger wrote: otnmbrd wrote: Chuck Bollinger wrote: Steve wrote: Were is vertical clearance measured from (what datum)? The vertical clearance is measured from 'Mean High Water' in places with a single diurnal tide. In the Pacific Northwest, it is measured from 'Mean Higher High Water' (MHHW). That's what I remember being taught, and then at some subsequent time being told that all the charts were going to go to MHHW (or maybe it was MHW that all of them were going to go to). I don't remember why, nor have I been able to find a reference on the internet. Interesting ..... Can you show me where this information comes from? In looking at CP 7, it list all heights as above MHW, unless otherwise stated, and the only major change to this I can find is for the Columbia River, which uses MLLW below Harrington Point, and "Columbia River Datum", between there and Bonneville Dam. We're just going out and tomorrow going to Port Ludlow. It will be Monday evening before I'll be on the internet again. Working on it. The tide tables provide 'Mean Tide' for each subordinate station, and 'Mean Range'. You can figure out what MHHW is for a place (within a foot or so) by taking 'Mean Tide level' and adding one-half the Mean Tidal Range. When I do that, I come within a foot of the MHHW given in my 'Tides and Currents Pro' program, and the error results in a number less (more safe) than the listed one. On the right track, but be careful that the meaning of "mean Tide" and "mean range", given in the program and tables you are using, mean G what you want. Many define "Mean tide" as "the level half way between mean high water and mean low water" and "mean range" as "the difference in height between MHW and MLW" .... see the problem? Frankly, no. One is a tide level and the other is a range. But something bothers me about Mean Tidal Level being half way between MHW and MLW, especially where there are two diurnal highs and lows. Can't put my finger on it, but that seems like one of those shortcuts that can introduce errors. Kind of like those situations where computing from the results of a computation introduces error. Another thing to research. My particular tide program, gives me MHHW and "Mean Tide". In this case, I would take the "mean Tide" X 2 and apply it to Zero tide (MLLW) and use this as MHW .... It should, normally, give a built in safety factor. At any rate, as I said before, be careful. There are many variables which can come into play, and you should NEVER push the envelope too closely. Hmm. More later on that. Literally have to go. BTW, I think the program you are using gives MHHW Yes. otn grandma Rosalie |
Vertical clearance ??
Chuck Bollinger wrote: But something bothers me about Mean Tidal Level being half way between MHW and MLW, especially where there are two diurnal highs and lows. Can't put my finger on it, but that seems like one of those shortcuts that can introduce errors. Kind of like those situations where computing from the results of a computation introduces error. Another thing to research. This is part of my problem with this. If we need to find the height of MHW and our tide datum is based on MLLW, I'm not sure how we can directly convert with any certainty from the info given. Also: Diurnal - Single high and low Semi Diurnal - two high and low Mixed - Variations/inequalities in highs and lows .... what we have on the West Coast, with variations in local This is one area I've always been weak on, so BG hopefully this old dog can learn some new tricks. otn |
Vertical clearance ??
On Fri, 08 Aug 2003 18:07:42 GMT, otnmbrd
wrote: Chuck Bollinger wrote: But something bothers me about Mean Tidal Level being half way between MHW and MLW, especially where there are two diurnal highs and lows. Can't put my finger on it, but that seems like one of those shortcuts that can introduce errors. Kind of like those situations where computing from the results of a computation introduces error. Another thing to research. This is part of my problem with this. If we need to find the height of MHW and our tide datum is based on MLLW, I'm not sure how we can directly convert with any certainty from the info given. Also: Diurnal - Single high and low Semi Diurnal - two high and low Mixed - Variations/inequalities in highs and lows .... what we have on the West Coast, with variations in local This is one area I've always been weak on, so BG hopefully this old dog can learn some new tricks. There is no need to convert anything. They are different measurements. On US charts use MWH to deal with clearances and heights. The clearance is normally the minimum clearance available under a bridge, overhead lines, etc.. Use your tide tables to determine if you have additional clearance. Mean High Water (MHW): A tidal datum. The average of all the high water heights observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mhw.html) Use MLLW to deal with depths on US charts. MLLW will normally be the shallowest that the water will be. Use your tide tables to determine how much water you have under you on that day at that time. Also this information will let know how much additional depth you have over underwater rocks that are a danger to navigation, how much water is over rocks awash and whether or not drying rocks are visible. Mean Lower Low Water (MLLW): A tidal datum. The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mllw.html) Always read the title block to establish datum for clearances and depths, and ensure that you use the appropriate tide tables. Canadian datum is based on Lowest Normal Tide, clearances are based on Higher High Water, Large Tides. For US charts use US tide tables, use Canadian tide tables for Canadian charts. BTW - the space between MWH and WLLW on US charts is the green stuff (foreshore). A couple of years I attempted to create an online lesson for reading tide tables (http://www.acs.ucalgary.ca/~jodale/e...t/content.html). Jack __________________________________________________ Jack Dale Swiftsure Sailing Academy Director/ISPA and CYA Instructor http://www.swiftsuresailing.com Phone: 1 (800) 470-SAIL (toll free) __________________________________________________ |
Vertical clearance ??
Most of this is correct. However, I would take issue with the statement
Use MLLW to deal with depths on US charts. MLLW will normally be the shallowest that the water will be. Since this is the average of the lowest tide for each day, roughly half the days will have a lower tide. In some locations this might not be significant, but in Boston, for instance, there are several days every month that are more than a foot below MLLW. There are several days a year that are two feet or more below MLLW. "Jack Dale" wrote in message ... On Fri, 08 Aug 2003 18:07:42 GMT, otnmbrd wrote: Chuck Bollinger wrote: But something bothers me about Mean Tidal Level being half way between MHW and MLW, especially where there are two diurnal highs and lows. Can't put my finger on it, but that seems like one of those shortcuts that can introduce errors. Kind of like those situations where computing from the results of a computation introduces error. Another thing to research. This is part of my problem with this. If we need to find the height of MHW and our tide datum is based on MLLW, I'm not sure how we can directly convert with any certainty from the info given. Also: Diurnal - Single high and low Semi Diurnal - two high and low Mixed - Variations/inequalities in highs and lows .... what we have on the West Coast, with variations in local This is one area I've always been weak on, so BG hopefully this old dog can learn some new tricks. There is no need to convert anything. They are different measurements. On US charts use MWH to deal with clearances and heights. The clearance is normally the minimum clearance available under a bridge, overhead lines, etc.. Use your tide tables to determine if you have additional clearance. Mean High Water (MHW): A tidal datum. The average of all the high water heights observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mhw.html) Use MLLW to deal with depths on US charts. MLLW will normally be the shallowest that the water will be. Use your tide tables to determine how much water you have under you on that day at that time. Also this information will let know how much additional depth you have over underwater rocks that are a danger to navigation, how much water is over rocks awash and whether or not drying rocks are visible. Mean Lower Low Water (MLLW): A tidal datum. The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mllw.html) Always read the title block to establish datum for clearances and depths, and ensure that you use the appropriate tide tables. Canadian datum is based on Lowest Normal Tide, clearances are based on Higher High Water, Large Tides. For US charts use US tide tables, use Canadian tide tables for Canadian charts. BTW - the space between MWH and WLLW on US charts is the green stuff (foreshore). A couple of years I attempted to create an online lesson for reading tide tables (http://www.acs.ucalgary.ca/~jodale/e...t/content.html). Jack __________________________________________________ Jack Dale Swiftsure Sailing Academy Director/ISPA and CYA Instructor http://www.swiftsuresailing.com Phone: 1 (800) 470-SAIL (toll free) __________________________________________________ |
Vertical clearance ??
First off, I must confess, that out on the West Coast, I've just used
the existing tide tables and bridge clearance numbers and compared the two to find my clearance and MLW/MLLW be damned, since most of the bridges that I've passed under, had enough clearance, that it normally wasn't a concern. However, a couple things on Jack's post: 1. MHW is used to discuss bridge clearances in the US, unless otherwise noted (we noted some differences). 2. In dealing with charted depths and tide tables, MLW is the datum for the East (and I believe Gulf - correction any one?) Coast (with exceptions), while MLLW is the datum for the West Coast. 3. This causes the problem (and it may or may not be). If your tide tables are based on MLLW, how do you apply those readings to MLW/MHW (used for bridge clearance) to get the closest possible reading? (realizing that all of these readings are subject to error due to many factors of weather, etc.. ) otn Jack Dale wrote: On Fri, 08 Aug 2003 18:07:42 GMT, otnmbrd wrote: Chuck Bollinger wrote: But something bothers me about Mean Tidal Level being half way between MHW and MLW, especially where there are two diurnal highs and lows. Can't put my finger on it, but that seems like one of those shortcuts that can introduce errors. Kind of like those situations where computing from the results of a computation introduces error. Another thing to research. This is part of my problem with this. If we need to find the height of MHW and our tide datum is based on MLLW, I'm not sure how we can directly convert with any certainty from the info given. Also: Diurnal - Single high and low Semi Diurnal - two high and low Mixed - Variations/inequalities in highs and lows .... what we have on the West Coast, with variations in local This is one area I've always been weak on, so BG hopefully this old dog can learn some new tricks. There is no need to convert anything. They are different measurements. (?) On US charts use MWH to deal with clearances and heights. The clearance is normally the minimum clearance available under a bridge, overhead lines, etc.. Use your tide tables to determine if you have additional clearance. Understood, however, see above Mean High Water (MHW): A tidal datum. The average of all the high water heights observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mhw.html) Again, understood Use MLLW to deal with depths on US charts. MLLW will normally be the shallowest that the water will be. Use your tide tables to determine how much water you have under you on that day at that time. Also this information will let know how much additional depth you have over underwater rocks that are a danger to navigation, how much water is over rocks awash and whether or not drying rocks are visible. Disagree with using MLLW for all US Charts as the datum, the rest understood. Mean Lower Low Water (MLLW): A tidal datum. The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mllw.html) Understood Always read the title block to establish datum for clearances and depths, and ensure that you use the appropriate tide tables. Canadian datum is based on Lowest Normal Tide, clearances are based on Higher High Water, Large Tides. For US charts use US tide tables, use Canadian tide tables for Canadian charts. BTW - the space between MWH and WLLW on US charts is the green stuff (foreshore). A couple of years I attempted to create an online lesson for reading tide tables (http://www.acs.ucalgary.ca/~jodale/e...t/content.html). Jack __________________________________________________ Jack Dale Swiftsure Sailing Academy Director/ISPA and CYA Instructor http://www.swiftsuresailing.com Phone: 1 (800) 470-SAIL (toll free) __________________________________________________ |
Vertical clearance ??
My Boston Harbor chart says the datum for soundings is MLLW. My Reed's (East Coast 2001)
says that the "US Datum" for tides is MLLW, and then all of its US tables list "US Datum." It goes on to say the Canadian tides and charts use Lowest Normal Tides (LNT), which is significantly different from the US, and is usually synonymous with Lowest Low Water, Large Tides (LLWLT) - the average of the lowest water from each of the 19 years of reference. Got that? There will be a quiz on Monday. And yes, everyone should be aware that local weather conditions can add or subtract several feet to the height of the tide. "otnmbrd" wrote in message nk.net... Check your datum. Boston Harbor may use a different datum "Boston Harbor Low water datum" Also, just because you are using a particular datum which states that your high might be, say 6.0', there is nothing that says you will not get higher highs and lower lows, than this, at varying times. Jeff Morris wrote: Most of this is correct. However, I would take issue with the statement Use MLLW to deal with depths on US charts. MLLW will normally be the shallowest that the water will be. Since this is the average of the lowest tide for each day, roughly half the days will have a lower tide. In some locations this might not be significant, but in Boston, for instance, there are several days every month that are more than a foot below MLLW. There are several days a year that are two feet or more below MLLW. "Jack Dale" wrote in message ... On Fri, 08 Aug 2003 18:07:42 GMT, otnmbrd wrote: Chuck Bollinger wrote: But something bothers me about Mean Tidal Level being half way between MHW and MLW, especially where there are two diurnal highs and lows. Can't put my finger on it, but that seems like one of those shortcuts that can introduce errors. Kind of like those situations where computing from the results of a computation introduces error. Another thing to research. This is part of my problem with this. If we need to find the height of MHW and our tide datum is based on MLLW, I'm not sure how we can directly convert with any certainty from the info given. Also: Diurnal - Single high and low Semi Diurnal - two high and low Mixed - Variations/inequalities in highs and lows .... what we have on the West Coast, with variations in local This is one area I've always been weak on, so BG hopefully this old dog can learn some new tricks. There is no need to convert anything. They are different measurements. On US charts use MWH to deal with clearances and heights. The clearance is normally the minimum clearance available under a bridge, overhead lines, etc.. Use your tide tables to determine if you have additional clearance. Mean High Water (MHW): A tidal datum. The average of all the high water heights observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mhw.html) Use MLLW to deal with depths on US charts. MLLW will normally be the shallowest that the water will be. Use your tide tables to determine how much water you have under you on that day at that time. Also this information will let know how much additional depth you have over underwater rocks that are a danger to navigation, how much water is over rocks awash and whether or not drying rocks are visible. Mean Lower Low Water (MLLW): A tidal datum. The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mllw.html) Always read the title block to establish datum for clearances and depths, and ensure that you use the appropriate tide tables. Canadian datum is based on Lowest Normal Tide, clearances are based on Higher High Water, Large Tides. For US charts use US tide tables, use Canadian tide tables for Canadian charts. BTW - the space between MWH and WLLW on US charts is the green stuff (foreshore). A couple of years I attempted to create an online lesson for reading tide tables (http://www.acs.ucalgary.ca/~jodale/e...t/content.html). Jack ________________________________________________ __ Jack Dale Swiftsure Sailing Academy Director/ISPA and CYA Instructor http://www.swiftsuresailing.com Phone: 1 (800) 470-SAIL (toll free) ________________________________________________ __ |
Vertical clearance ??
LOL, relax, Jeff, I'm not Neal. I'm reading some older stuff which
relates it as MLW (with an exception for Boston) (also the reason I asked for confirmation). My particular tide program does not have the East Coast, but again some older stuff I was looking at did, and it used MLW for tide datum. At any rate, you need to be sure which datum (as I think I've been stressing, or at least should have) you are using, but it still doesn't clear up the situation of how you change MLLW to MLW or MHW, from the data given in the particular tide tables, which I think is the jist of the discussion, when looking at bridge clearance, and I think Chuck was having the same problem (although he came up with a workable solution), that I do (although I have basically said "the hell with it" use what you've got and be sure you have a built in safety factor). otn Jeff Morris wrote: My Boston Harbor chart says the datum for soundings is MLLW. My Reed's (East Coast 2001) says that the "US Datum" for tides is MLLW, and then all of its US tables list "US Datum." It goes on to say the Canadian tides and charts use Lowest Normal Tides (LNT), which is significantly different from the US, and is usually synonymous with Lowest Low Water, Large Tides (LLWLT) - the average of the lowest water from each of the 19 years of reference. Got that? There will be a quiz on Monday. And yes, everyone should be aware that local weather conditions can add or subtract several feet to the height of the tide. "otnmbrd" wrote in message nk.net... Check your datum. Boston Harbor may use a different datum "Boston Harbor Low water datum" Also, just because you are using a particular datum which states that your high might be, say 6.0', there is nothing that says you will not get higher highs and lower lows, than this, at varying times. Jeff Morris wrote: Most of this is correct. However, I would take issue with the statement Use MLLW to deal with depths on US charts. MLLW will normally be the shallowest that the water will be. Since this is the average of the lowest tide for each day, roughly half the days will have a lower tide. In some locations this might not be significant, but in Boston, for instance, there are several days every month that are more than a foot below MLLW. There are several days a year that are two feet or more below MLLW. "Jack Dale" wrote in message ... On Fri, 08 Aug 2003 18:07:42 GMT, otnmbrd wrote: Chuck Bollinger wrote: But something bothers me about Mean Tidal Level being half way between MHW and MLW, especially where there are two diurnal highs and lows. Can't put my finger on it, but that seems like one of those shortcuts that can introduce errors. Kind of like those situations where computing from the results of a computation introduces error. Another thing to research. This is part of my problem with this. If we need to find the height of MHW and our tide datum is based on MLLW, I'm not sure how we can directly convert with any certainty from the info given. Also: Diurnal - Single high and low Semi Diurnal - two high and low Mixed - Variations/inequalities in highs and lows .... what we have on the West Coast, with variations in local This is one area I've always been weak on, so BG hopefully this old dog can learn some new tricks. There is no need to convert anything. They are different measurements. On US charts use MWH to deal with clearances and heights. The clearance is normally the minimum clearance available under a bridge, overhead lines, etc.. Use your tide tables to determine if you have additional clearance. Mean High Water (MHW): A tidal datum. The average of all the high water heights observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mhw.html) Use MLLW to deal with depths on US charts. MLLW will normally be the shallowest that the water will be. Use your tide tables to determine how much water you have under you on that day at that time. Also this information will let know how much additional depth you have over underwater rocks that are a danger to navigation, how much water is over rocks awash and whether or not drying rocks are visible. Mean Lower Low Water (MLLW): A tidal datum. The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mllw.html) Always read the title block to establish datum for clearances and depths, and ensure that you use the appropriate tide tables. Canadian datum is based on Lowest Normal Tide, clearances are based on Higher High Water, Large Tides. For US charts use US tide tables, use Canadian tide tables for Canadian charts. BTW - the space between MWH and WLLW on US charts is the green stuff (foreshore). A couple of years I attempted to create an online lesson for reading tide tables (http://www.acs.ucalgary.ca/~jodale/e...t/content.html). Jack ______________________________________________ ____ Jack Dale Swiftsure Sailing Academy Director/ISPA and CYA Instructor http://www.swiftsuresailing.com Phone: 1 (800) 470-SAIL (toll free) ______________________________________________ ____ |
Vertical clearance ??
Here's a thought, Jeff. Check your chart and latest Coast Pilot for
Boston Harbor. What is the "datum" being used for bridge clearance? MHW? MHHW? I'm trying to get us all on the same reference plain. otn otnmbrd wrote: LOL, relax, Jeff, I'm not Neal. I'm reading some older stuff which relates it as MLW (with an exception for Boston) (also the reason I asked for confirmation). My particular tide program does not have the East Coast, but again some older stuff I was looking at did, and it used MLW for tide datum. At any rate, you need to be sure which datum (as I think I've been stressing, or at least should have) you are using, but it still doesn't clear up the situation of how you change MLLW to MLW or MHW, from the data given in the particular tide tables, which I think is the jist of the discussion, when looking at bridge clearance, and I think Chuck was having the same problem (although he came up with a workable solution), that I do (although I have basically said "the hell with it" use what you've got and be sure you have a built in safety factor). otn Jeff Morris wrote: My Boston Harbor chart says the datum for soundings is MLLW. My Reed's (East Coast 2001) says that the "US Datum" for tides is MLLW, and then all of its US tables list "US Datum." It goes on to say the Canadian tides and charts use Lowest Normal Tides (LNT), which is significantly different from the US, and is usually synonymous with Lowest Low Water, Large Tides (LLWLT) - the average of the lowest water from each of the 19 years of reference. Got that? There will be a quiz on Monday. And yes, everyone should be aware that local weather conditions can add or subtract several feet to the height of the tide. "otnmbrd" wrote in message nk.net... Check your datum. Boston Harbor may use a different datum "Boston Harbor Low water datum" Also, just because you are using a particular datum which states that your high might be, say 6.0', there is nothing that says you will not get higher highs and lower lows, than this, at varying times. Jeff Morris wrote: Most of this is correct. However, I would take issue with the statement Use MLLW to deal with depths on US charts. MLLW will normally be the shallowest that the water will be. Since this is the average of the lowest tide for each day, roughly half the days will have a lower tide. In some locations this might not be significant, but in Boston, for instance, there are several days every month that are more than a foot below MLLW. There are several days a year that are two feet or more below MLLW. "Jack Dale" wrote in message ... On Fri, 08 Aug 2003 18:07:42 GMT, otnmbrd wrote: Chuck Bollinger wrote: But something bothers me about Mean Tidal Level being half way between MHW and MLW, especially where there are two diurnal highs and lows. Can't put my finger on it, but that seems like one of those shortcuts that can introduce errors. Kind of like those situations where computing from the results of a computation introduces error. Another thing to research. This is part of my problem with this. If we need to find the height of MHW and our tide datum is based on MLLW, I'm not sure how we can directly convert with any certainty from the info given. Also: Diurnal - Single high and low Semi Diurnal - two high and low Mixed - Variations/inequalities in highs and lows .... what we have on the West Coast, with variations in local This is one area I've always been weak on, so BG hopefully this old dog can learn some new tricks. There is no need to convert anything. They are different measurements. On US charts use MWH to deal with clearances and heights. The clearance is normally the minimum clearance available under a bridge, overhead lines, etc.. Use your tide tables to determine if you have additional clearance. Mean High Water (MHW): A tidal datum. The average of all the high water heights observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mhw.html) Use MLLW to deal with depths on US charts. MLLW will normally be the shallowest that the water will be. Use your tide tables to determine how much water you have under you on that day at that time. Also this information will let know how much additional depth you have over underwater rocks that are a danger to navigation, how much water is over rocks awash and whether or not drying rocks are visible. Mean Lower Low Water (MLLW): A tidal datum. The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mllw.html) Always read the title block to establish datum for clearances and depths, and ensure that you use the appropriate tide tables. Canadian datum is based on Lowest Normal Tide, clearances are based on Higher High Water, Large Tides. For US charts use US tide tables, use Canadian tide tables for Canadian charts. BTW - the space between MWH and WLLW on US charts is the green stuff (foreshore). A couple of years I attempted to create an online lesson for reading tide tables (http://www.acs.ucalgary.ca/~jodale/e...t/content.html). Jack __________________________________________________ Jack Dale Swiftsure Sailing Academy Director/ISPA and CYA Instructor http://www.swiftsuresailing.com Phone: 1 (800) 470-SAIL (toll free) __________________________________________________ |
Vertical clearance ??
Answering the question in your number 3, on US domestic charts* there
is a box labeled "Tidal Information", which shows tide heights at several important points one the chart. On a random East Coast chart, it shows MHW, Mean Tide Level, MLW, and Extreme Low Water for each point, which allows you to figure bridge clearance = stated clearance above MHW plus difference between MHW and MLW less present height of tide referenced to MLW I put an asterisk on my generalization in the first sentence -- although we own about 600 charts (did a circumnav a while ago), we don't have any for the US West Coast, so it's just an informed guess that on West Coast charts the box shows both the depth datum and the height datum. Remember, too, that this kind of calculation has a lot of room for error, particularly with local wind conditions, which can change the water height by several feet, and with local error -- clearance numbers aren't always right. If I were going through a bridge for the first time and was within three feet of the calculated clearance, and didn't have good local knowledge available, I'd absolutely send someone up the mast to watch. This assumes conditions under which you have complete control of the boat, preferably with a small current against you, as the worst possible outcome would be to be forced under the bridge and lose the stick with a person at the top. An alternative might be to put someone ashore and have him or her watch from the bridge. Jim Woodward www.mvfintry.com otnmbrd wrote in message ink.net... First off, I must confess, that out on the West Coast, I've just used the existing tide tables and bridge clearance numbers and compared the two to find my clearance and MLW/MLLW be damned, since most of the bridges that I've passed under, had enough clearance, that it normally wasn't a concern. However, a couple things on Jack's post: 1. MHW is used to discuss bridge clearances in the US, unless otherwise noted (we noted some differences). 2. In dealing with charted depths and tide tables, MLW is the datum for the East (and I believe Gulf - correction any one?) Coast (with exceptions), while MLLW is the datum for the West Coast. 3. This causes the problem (and it may or may not be). If your tide tables are based on MLLW, how do you apply those readings to MLW/MHW (used for bridge clearance) to get the closest possible reading? (realizing that all of these readings are subject to error due to many factors of weather, etc.. ) otn Jack Dale wrote: On Fri, 08 Aug 2003 18:07:42 GMT, otnmbrd wrote: Chuck Bollinger wrote: But something bothers me about Mean Tidal Level being half way between MHW and MLW, especially where there are two diurnal highs and lows. Can't put my finger on it, but that seems like one of those shortcuts that can introduce errors. Kind of like those situations where computing from the results of a computation introduces error. Another thing to research. This is part of my problem with this. If we need to find the height of MHW and our tide datum is based on MLLW, I'm not sure how we can directly convert with any certainty from the info given. Also: Diurnal - Single high and low Semi Diurnal - two high and low Mixed - Variations/inequalities in highs and lows .... what we have on the West Coast, with variations in local This is one area I've always been weak on, so BG hopefully this old dog can learn some new tricks. There is no need to convert anything. They are different measurements. (?) On US charts use MWH to deal with clearances and heights. The clearance is normally the minimum clearance available under a bridge, overhead lines, etc.. Use your tide tables to determine if you have additional clearance. Understood, however, see above Mean High Water (MHW): A tidal datum. The average of all the high water heights observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mhw.html) Again, understood Use MLLW to deal with depths on US charts. MLLW will normally be the shallowest that the water will be. Use your tide tables to determine how much water you have under you on that day at that time. Also this information will let know how much additional depth you have over underwater rocks that are a danger to navigation, how much water is over rocks awash and whether or not drying rocks are visible. Disagree with using MLLW for all US Charts as the datum, the rest understood. Mean Lower Low Water (MLLW): A tidal datum. The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mllw.html) Understood Always read the title block to establish datum for clearances and depths, and ensure that you use the appropriate tide tables. Canadian datum is based on Lowest Normal Tide, clearances are based on Higher High Water, Large Tides. For US charts use US tide tables, use Canadian tide tables for Canadian charts. BTW - the space between MWH and WLLW on US charts is the green stuff (foreshore). A couple of years I attempted to create an online lesson for reading tide tables (http://www.acs.ucalgary.ca/~jodale/e...t/content.html). Jack __________________________________________________ Jack Dale Swiftsure Sailing Academy Director/ISPA and CYA Instructor http://www.swiftsuresailing.com Phone: 1 (800) 470-SAIL (toll free) __________________________________________________ |
Vertical clearance ??
OK, I don't have the latest stuff at home - its on the boat. However, from a 8 year old
chart 13270 of Boston harbor there is a table that lists various heights for "Boston Light": Height referred to datum of soundings (MLLW) Mean Higher High Water 9.7 feet Mean High Water 9.3 feet Mean Low Water 0.3 feet Extreme Low Water -3.0 feet elsewhere it says: HEIGHT Heights in feet above Mean High Water The tables that convert between the various heights is on a number of charts in my BBS ChartKit, but the comment on bridge heights I couldn't find without going to an actual chart. I also have the same chart from 1867. It lists the minimum and maximum observed tides from the "reference plane," plus the mean spring and neap low tides from the reference plane, plus the mean range of the spring and neap tides. It doesn't list what the "reference plane" is, nor does it have any bridge heights. It does have the "Corrected Establishment" for determining the state of the tide relative to the full moon, and lists longitude relative to the State House on Beacon Hill. "otnmbrd" wrote in message k.net... Here's a thought, Jeff. Check your chart and latest Coast Pilot for Boston Harbor. What is the "datum" being used for bridge clearance? MHW? MHHW? I'm trying to get us all on the same reference plain. otn otnmbrd wrote: LOL, relax, Jeff, I'm not Neal. I'm reading some older stuff which relates it as MLW (with an exception for Boston) (also the reason I asked for confirmation). My particular tide program does not have the East Coast, but again some older stuff I was looking at did, and it used MLW for tide datum. At any rate, you need to be sure which datum (as I think I've been stressing, or at least should have) you are using, but it still doesn't clear up the situation of how you change MLLW to MLW or MHW, from the data given in the particular tide tables, which I think is the jist of the discussion, when looking at bridge clearance, and I think Chuck was having the same problem (although he came up with a workable solution), that I do (although I have basically said "the hell with it" use what you've got and be sure you have a built in safety factor). otn Jeff Morris wrote: My Boston Harbor chart says the datum for soundings is MLLW. My Reed's (East Coast 2001) says that the "US Datum" for tides is MLLW, and then all of its US tables list "US Datum." It goes on to say the Canadian tides and charts use Lowest Normal Tides (LNT), which is significantly different from the US, and is usually synonymous with Lowest Low Water, Large Tides (LLWLT) - the average of the lowest water from each of the 19 years of reference. Got that? There will be a quiz on Monday. And yes, everyone should be aware that local weather conditions can add or subtract several feet to the height of the tide. "otnmbrd" wrote in message nk.net... Check your datum. Boston Harbor may use a different datum "Boston Harbor Low water datum" Also, just because you are using a particular datum which states that your high might be, say 6.0', there is nothing that says you will not get higher highs and lower lows, than this, at varying times. Jeff Morris wrote: Most of this is correct. However, I would take issue with the statement Use MLLW to deal with depths on US charts. MLLW will normally be the shallowest that the water will be. Since this is the average of the lowest tide for each day, roughly half the days will have a lower tide. In some locations this might not be significant, but in Boston, for instance, there are several days every month that are more than a foot below MLLW. There are several days a year that are two feet or more below MLLW. "Jack Dale" wrote in message ... On Fri, 08 Aug 2003 18:07:42 GMT, otnmbrd wrote: Chuck Bollinger wrote: But something bothers me about Mean Tidal Level being half way between MHW and MLW, especially where there are two diurnal highs and lows. Can't put my finger on it, but that seems like one of those shortcuts that can introduce errors. Kind of like those situations where computing from the results of a computation introduces error. Another thing to research. This is part of my problem with this. If we need to find the height of MHW and our tide datum is based on MLLW, I'm not sure how we can directly convert with any certainty from the info given. Also: Diurnal - Single high and low Semi Diurnal - two high and low Mixed - Variations/inequalities in highs and lows .... what we have on the West Coast, with variations in local This is one area I've always been weak on, so BG hopefully this old dog can learn some new tricks. There is no need to convert anything. They are different measurements. On US charts use MWH to deal with clearances and heights. The clearance is normally the minimum clearance available under a bridge, overhead lines, etc.. Use your tide tables to determine if you have additional clearance. Mean High Water (MHW): A tidal datum. The average of all the high water heights observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mhw.html) Use MLLW to deal with depths on US charts. MLLW will normally be the shallowest that the water will be. Use your tide tables to determine how much water you have under you on that day at that time. Also this information will let know how much additional depth you have over underwater rocks that are a danger to navigation, how much water is over rocks awash and whether or not drying rocks are visible. Mean Lower Low Water (MLLW): A tidal datum. The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mllw.html) Always read the title block to establish datum for clearances and depths, and ensure that you use the appropriate tide tables. Canadian datum is based on Lowest Normal Tide, clearances are based on Higher High Water, Large Tides. For US charts use US tide tables, use Canadian tide tables for Canadian charts. BTW - the space between MWH and WLLW on US charts is the green stuff (foreshore). A couple of years I attempted to create an online lesson for reading tide tables (http://www.acs.ucalgary.ca/~jodale/e...t/content.html). Jack __________________________________________________ Jack Dale Swiftsure Sailing Academy Director/ISPA and CYA Instructor http://www.swiftsuresailing.com Phone: 1 (800) 470-SAIL (toll free) __________________________________________________ |
Vertical clearance ??
There we go ...... My error ( BG My wife's loving this) was in dealing
with older publications and not having any charts here, which gave me that info..... LOL shoulda known better. At any rate, now we have a way to get MHW and can go back to an earlier post for determining clearance, based on using the tide tables. I haven't got one here .... anyone check a "Light List" for the info Jeff has given? (I'm asking, to see if it's there, also). otn Jeff Morris wrote: OK, I don't have the latest stuff at home - its on the boat. However, from a 8 year old chart 13270 of Boston harbor there is a table that lists various heights for "Boston Light": Height referred to datum of soundings (MLLW) Mean Higher High Water 9.7 feet Mean High Water 9.3 feet Mean Low Water 0.3 feet Extreme Low Water -3.0 feet elsewhere it says: HEIGHT Heights in feet above Mean High Water The tables that convert between the various heights is on a number of charts in my BBS ChartKit, but the comment on bridge heights I couldn't find without going to an actual chart. I also have the same chart from 1867. It lists the minimum and maximum observed tides from the "reference plane," plus the mean spring and neap low tides from the reference plane, plus the mean range of the spring and neap tides. It doesn't list what the "reference plane" is, nor does it have any bridge heights. It does have the "Corrected Establishment" for determining the state of the tide relative to the full moon, and lists longitude relative to the State House on Beacon Hill. "otnmbrd" wrote in message k.net... Here's a thought, Jeff. Check your chart and latest Coast Pilot for Boston Harbor. What is the "datum" being used for bridge clearance? MHW? MHHW? I'm trying to get us all on the same reference plain. otn otnmbrd wrote: LOL, relax, Jeff, I'm not Neal. I'm reading some older stuff which relates it as MLW (with an exception for Boston) (also the reason I asked for confirmation). My particular tide program does not have the East Coast, but again some older stuff I was looking at did, and it used MLW for tide datum. At any rate, you need to be sure which datum (as I think I've been stressing, or at least should have) you are using, but it still doesn't clear up the situation of how you change MLLW to MLW or MHW, from the data given in the particular tide tables, which I think is the jist of the discussion, when looking at bridge clearance, and I think Chuck was having the same problem (although he came up with a workable solution), that I do (although I have basically said "the hell with it" use what you've got and be sure you have a built in safety factor). otn Jeff Morris wrote: My Boston Harbor chart says the datum for soundings is MLLW. My Reed's (East Coast 2001) says that the "US Datum" for tides is MLLW, and then all of its US tables list "US Datum." It goes on to say the Canadian tides and charts use Lowest Normal Tides (LNT), which is significantly different from the US, and is usually synonymous with Lowest Low Water, Large Tides (LLWLT) - the average of the lowest water from each of the 19 years of reference. Got that? There will be a quiz on Monday. And yes, everyone should be aware that local weather conditions can add or subtract several feet to the height of the tide. "otnmbrd" wrote in message thlink.net... Check your datum. Boston Harbor may use a different datum "Boston Harbor Low water datum" Also, just because you are using a particular datum which states that your high might be, say 6.0', there is nothing that says you will not get higher highs and lower lows, than this, at varying times. Jeff Morris wrote: Most of this is correct. However, I would take issue with the statement Use MLLW to deal with depths on US charts. MLLW will normally be the shallowest that the water will be. Since this is the average of the lowest tide for each day, roughly half the days will have a lower tide. In some locations this might not be significant, but in Boston, for instance, there are several days every month that are more than a foot below MLLW. There are several days a year that are two feet or more below MLLW. "Jack Dale" wrote in message news:fc08jv0v8rr0u209jo82bch0ok1lm1hnvb@4ax. com... On Fri, 08 Aug 2003 18:07:42 GMT, otnmbrd wrote: Chuck Bollinger wrote: But something bothers me about Mean Tidal Level being half way between MHW and MLW, especially where there are two diurnal highs and lows. Can't put my finger on it, but that seems like one of those shortcuts that can introduce errors. Kind of like those situations where computing from the results of a computation introduces error. Another thing to research. This is part of my problem with this. If we need to find the height of MHW and our tide datum is based on MLLW, I'm not sure how we can directly convert with any certainty from the info given. Also: Diurnal - Single high and low Semi Diurnal - two high and low Mixed - Variations/inequalities in highs and lows .... what we have on the West Coast, with variations in local This is one area I've always been weak on, so BG hopefully this old dog can learn some new tricks. There is no need to convert anything. They are different measurements. On US charts use MWH to deal with clearances and heights. The clearance is normally the minimum clearance available under a bridge, overhead lines, etc.. Use your tide tables to determine if you have additional clearance. Mean High Water (MHW): A tidal datum. The average of all the high water heights observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mhw.html) Use MLLW to deal with depths on US charts. MLLW will normally be the shallowest that the water will be. Use your tide tables to determine how much water you have under you on that day at that time. Also this information will let know how much additional depth you have over underwater rocks that are a danger to navigation, how much water is over rocks awash and whether or not drying rocks are visible. Mean Lower Low Water (MLLW): A tidal datum. The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mllw.html) Always read the title block to establish datum for clearances and depths, and ensure that you use the appropriate tide tables. Canadian datum is based on Lowest Normal Tide, clearances are based on Higher High Water, Large Tides. For US charts use US tide tables, use Canadian tide tables for Canadian charts. BTW - the space between MWH and WLLW on US charts is the green stuff (foreshore). A couple of years I attempted to create an online lesson for reading tide tables (http://www.acs.ucalgary.ca/~jodale/e...t/content.html). Jack ___________________________________________ _______ Jack Dale Swiftsure Sailing Academy Director/ISPA and CYA Instructor http://www.swiftsuresailing.com Phone: 1 (800) 470-SAIL (toll free) ___________________________________________ _______ |
Vertical clearance ??
Since I'm the one what caused most of the confusion and gave the most
bad info, let me try an example to hopefully clear up the mess. Your tide datum is MLLW (also called Zero tide) On your chart you have a box showing tidal data. In this box, MHW is listed as +6.0'; MHHW is listed at +7.0'. You are on a sailboat approaching a bridge. The height at your present draft to the tip of an antenna at the masthead, is 50'. The clearance of the bridge shown on the chart, is 50' at MHW. AT Zero tide (MLLW), you will have 6' clearance between your antenna and the bridge. At +6.0' tide (MHW), you will hear a slight "Ping" as the tip of your antenna touches the bridge. At any stage of the tide which your tables show to be greater than this (+7.0,8.0,9.0, etc.) your next stop will be the nearest boatyard for repairs. At any stage of the tide where the numbers show a minus reading (below MLLW) you will gain additional clearance. All warnings about accuracy of tables and predictions based on possible weather, etc. conditions, still apply. BG otn PS got my hands on a light list .....no tidal info in there for particular lights, that I could see. |
Vertical clearance ??
I hope I didn't send you astray by using "Boston Light" as a reference - the table lists
several other points that are not lights and I just used Boston Light as a well known reference point in Boston. I could have picked the Chelsea Bridge, but few people would have recognized it. Many of the charts in my ChartKit have similar tables, listing 3 or 4 reference points. "otnmbrd" wrote in message nk.net... There we go ...... My error ( BG My wife's loving this) was in dealing with older publications and not having any charts here, which gave me that info..... LOL shoulda known better. At any rate, now we have a way to get MHW and can go back to an earlier post for determining clearance, based on using the tide tables. I haven't got one here .... anyone check a "Light List" for the info Jeff has given? (I'm asking, to see if it's there, also). otn Jeff Morris wrote: OK, I don't have the latest stuff at home - its on the boat. However, from a 8 year old chart 13270 of Boston harbor there is a table that lists various heights for "Boston Light": Height referred to datum of soundings (MLLW) Mean Higher High Water 9.7 feet Mean High Water 9.3 feet Mean Low Water 0.3 feet Extreme Low Water -3.0 feet elsewhere it says: HEIGHT Heights in feet above Mean High Water The tables that convert between the various heights is on a number of charts in my BBS ChartKit, but the comment on bridge heights I couldn't find without going to an actual chart. I also have the same chart from 1867. It lists the minimum and maximum observed tides from the "reference plane," plus the mean spring and neap low tides from the reference plane, plus the mean range of the spring and neap tides. It doesn't list what the "reference plane" is, nor does it have any bridge heights. It does have the "Corrected Establishment" for determining the state of the tide relative to the full moon, and lists longitude relative to the State House on Beacon Hill. "otnmbrd" wrote in message k.net... Here's a thought, Jeff. Check your chart and latest Coast Pilot for Boston Harbor. What is the "datum" being used for bridge clearance? MHW? MHHW? I'm trying to get us all on the same reference plain. otn otnmbrd wrote: LOL, relax, Jeff, I'm not Neal. I'm reading some older stuff which relates it as MLW (with an exception for Boston) (also the reason I asked for confirmation). My particular tide program does not have the East Coast, but again some older stuff I was looking at did, and it used MLW for tide datum. At any rate, you need to be sure which datum (as I think I've been stressing, or at least should have) you are using, but it still doesn't clear up the situation of how you change MLLW to MLW or MHW, from the data given in the particular tide tables, which I think is the jist of the discussion, when looking at bridge clearance, and I think Chuck was having the same problem (although he came up with a workable solution), that I do (although I have basically said "the hell with it" use what you've got and be sure you have a built in safety factor). otn Jeff Morris wrote: My Boston Harbor chart says the datum for soundings is MLLW. My Reed's (East Coast 2001) says that the "US Datum" for tides is MLLW, and then all of its US tables list "US Datum." It goes on to say the Canadian tides and charts use Lowest Normal Tides (LNT), which is significantly different from the US, and is usually synonymous with Lowest Low Water, Large Tides (LLWLT) - the average of the lowest water from each of the 19 years of reference. Got that? There will be a quiz on Monday. And yes, everyone should be aware that local weather conditions can add or subtract several feet to the height of the tide. "otnmbrd" wrote in message thlink.net... Check your datum. Boston Harbor may use a different datum "Boston Harbor Low water datum" Also, just because you are using a particular datum which states that your high might be, say 6.0', there is nothing that says you will not get higher highs and lower lows, than this, at varying times. Jeff Morris wrote: Most of this is correct. However, I would take issue with the statement Use MLLW to deal with depths on US charts. MLLW will normally be the shallowest that the water will be. Since this is the average of the lowest tide for each day, roughly half the days will have a lower tide. In some locations this might not be significant, but in Boston, for instance, there are several days every month that are more than a foot below MLLW. There are several days a year that are two feet or more below MLLW. "Jack Dale" wrote in message news:fc08jv0v8rr0u209jo82bch0ok1lm1hnvb@4ax. com... On Fri, 08 Aug 2003 18:07:42 GMT, otnmbrd wrote: Chuck Bollinger wrote: But something bothers me about Mean Tidal Level being half way between MHW and MLW, especially where there are two diurnal highs and lows. Can't put my finger on it, but that seems like one of those shortcuts that can introduce errors. Kind of like those situations where computing from the results of a computation introduces error. Another thing to research. This is part of my problem with this. If we need to find the height of MHW and our tide datum is based on MLLW, I'm not sure how we can directly convert with any certainty from the info given. Also: Diurnal - Single high and low Semi Diurnal - two high and low Mixed - Variations/inequalities in highs and lows .... what we have on the West Coast, with variations in local This is one area I've always been weak on, so BG hopefully this old dog can learn some new tricks. There is no need to convert anything. They are different measurements. On US charts use MWH to deal with clearances and heights. The clearance is normally the minimum clearance available under a bridge, overhead lines, etc.. Use your tide tables to determine if you have additional clearance. Mean High Water (MHW): A tidal datum. The average of all the high water heights observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mhw.html) Use MLLW to deal with depths on US charts. MLLW will normally be the shallowest that the water will be. Use your tide tables to determine how much water you have under you on that day at that time. Also this information will let know how much additional depth you have over underwater rocks that are a danger to navigation, how much water is over rocks awash and whether or not drying rocks are visible. Mean Lower Low Water (MLLW): A tidal datum. The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mllw.html) Always read the title block to establish datum for clearances and depths, and ensure that you use the appropriate tide tables. Canadian datum is based on Lowest Normal Tide, clearances are based on Higher High Water, Large Tides. For US charts use US tide tables, use Canadian tide tables for Canadian charts. BTW - the space between MWH and WLLW on US charts is the green stuff (foreshore). A couple of years I attempted to create an online lesson for reading tide tables (http://www.acs.ucalgary.ca/~jodale/e...t/content.html). Jack ___________________________________________ _______ Jack Dale Swiftsure Sailing Academy Director/ISPA and CYA Instructor http://www.swiftsuresailing.com Phone: 1 (800) 470-SAIL (toll free) ___________________________________________ _______ |
Vertical clearance ??
On Sat, 09 Aug 2003 22:30:17 GMT, otnmbrd
wrote: PS got my hands on a light list .....no tidal info in there for particular lights, that I could see. The height of the light is from MHW on US charts. That is about all the tidal information in that publication. If you use the "dipping the horizon" technique for establishing a circle of position, the height of the tide at the time of the observation must be taken into consideration. Jack |
Vertical clearance ??
Again, I don't have any charts here, but check ..... where you have
multiple listings, do they relate to a tidal reference station, in some way? otn Jeff Morris wrote: I hope I didn't send you astray by using "Boston Light" as a reference - the table lists several other points that are not lights and I just used Boston Light as a well known reference point in Boston. I could have picked the Chelsea Bridge, but few people would have recognized it. Many of the charts in my ChartKit have similar tables, listing 3 or 4 reference points. "otnmbrd" wrote in message nk.net... There we go ...... My error ( BG My wife's loving this) was in dealing with older publications and not having any charts here, which gave me that info..... LOL shoulda known better. At any rate, now we have a way to get MHW and can go back to an earlier post for determining clearance, based on using the tide tables. I haven't got one here .... anyone check a "Light List" for the info Jeff has given? (I'm asking, to see if it's there, also). otn Jeff Morris wrote: OK, I don't have the latest stuff at home - its on the boat. However, from a 8 year old chart 13270 of Boston harbor there is a table that lists various heights for "Boston Light": Height referred to datum of soundings (MLLW) Mean Higher High Water 9.7 feet Mean High Water 9.3 feet Mean Low Water 0.3 feet Extreme Low Water -3.0 feet elsewhere it says: HEIGHT Heights in feet above Mean High Water The tables that convert between the various heights is on a number of charts in my BBS ChartKit, but the comment on bridge heights I couldn't find without going to an actual chart. I also have the same chart from 1867. It lists the minimum and maximum observed tides from the "reference plane," plus the mean spring and neap low tides from the reference plane, plus the mean range of the spring and neap tides. It doesn't list what the "reference plane" is, nor does it have any bridge heights. It does have the "Corrected Establishment" for determining the state of the tide relative to the full moon, and lists longitude relative to the State House on Beacon Hill. "otnmbrd" wrote in message link.net... Here's a thought, Jeff. Check your chart and latest Coast Pilot for Boston Harbor. What is the "datum" being used for bridge clearance? MHW? MHHW? I'm trying to get us all on the same reference plain. otn otnmbrd wrote: LOL, relax, Jeff, I'm not Neal. I'm reading some older stuff which relates it as MLW (with an exception for Boston) (also the reason I asked for confirmation). My particular tide program does not have the East Coast, but again some older stuff I was looking at did, and it used MLW for tide datum. At any rate, you need to be sure which datum (as I think I've been stressing, or at least should have) you are using, but it still doesn't clear up the situation of how you change MLLW to MLW or MHW, from the data given in the particular tide tables, which I think is the jist of the discussion, when looking at bridge clearance, and I think Chuck was having the same problem (although he came up with a workable solution), that I do (although I have basically said "the hell with it" use what you've got and be sure you have a built in safety factor). otn Jeff Morris wrote: My Boston Harbor chart says the datum for soundings is MLLW. My Reed's (East Coast 2001) says that the "US Datum" for tides is MLLW, and then all of its US tables list "US Datum." It goes on to say the Canadian tides and charts use Lowest Normal Tides (LNT), which is significantly different from the US, and is usually synonymous with Lowest Low Water, Large Tides (LLWLT) - the average of the lowest water from each of the 19 years of reference. Got that? There will be a quiz on Monday. And yes, everyone should be aware that local weather conditions can add or subtract several feet to the height of the tide. "otnmbrd" wrote in message arthlink.net... Check your datum. Boston Harbor may use a different datum "Boston Harbor Low water datum" Also, just because you are using a particular datum which states that your high might be, say 6.0', there is nothing that says you will not get higher highs and lower lows, than this, at varying times. Jeff Morris wrote: Most of this is correct. However, I would take issue with the statement Use MLLW to deal with depths on US charts. MLLW will normally be the shallowest that the water will be. Since this is the average of the lowest tide for each day, roughly half the days will have a lower tide. In some locations this might not be significant, but in Boston, for instance, there are several days every month that are more than a foot below MLLW. There are several days a year that are two feet or more below MLLW. "Jack Dale" wrote in message news:fc08jv0v8rr0u209jo82bch0ok1lm1hnvb@4a x.com... On Fri, 08 Aug 2003 18:07:42 GMT, otnmbrd wrote: Chuck Bollinger wrote: But something bothers me about Mean Tidal Level being half way between MHW and MLW, especially where there are two diurnal highs and lows. Can't put my finger on it, but that seems like one of those shortcuts that can introduce errors. Kind of like those situations where computing from the results of a computation introduces error. Another thing to research. This is part of my problem with this. If we need to find the height of MHW and our tide datum is based on MLLW, I'm not sure how we can directly convert with any certainty from the info given. Also: Diurnal - Single high and low Semi Diurnal - two high and low Mixed - Variations/inequalities in highs and lows .... what we have on the West Coast, with variations in local This is one area I've always been weak on, so BG hopefully this old dog can learn some new tricks. There is no need to convert anything. They are different measurements. On US charts use MWH to deal with clearances and heights. The clearance is normally the minimum clearance available under a bridge, overhead lines, etc.. Use your tide tables to determine if you have additional clearance. Mean High Water (MHW): A tidal datum. The average of all the high water heights observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mhw.html) Use MLLW to deal with depths on US charts. MLLW will normally be the shallowest that the water will be. Use your tide tables to determine how much water you have under you on that day at that time. Also this information will let know how much additional depth you have over underwater rocks that are a danger to navigation, how much water is over rocks awash and whether or not drying rocks are visible. Mean Lower Low Water (MLLW): A tidal datum. The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mllw.html) Always read the title block to establish datum for clearances and depths, and ensure that you use the appropriate tide tables. Canadian datum is based on Lowest Normal Tide, clearances are based on Higher High Water, Large Tides. For US charts use US tide tables, use Canadian tide tables for Canadian charts. BTW - the space between MWH and WLLW on US charts is the green stuff (foreshore). A couple of years I attempted to create an online lesson for reading tide tables (http://www.acs.ucalgary.ca/~jodale/e...t/content.html). Jack _________________________________________ _________ Jack Dale Swiftsure Sailing Academy Director/ISPA and CYA Instructor http://www.swiftsuresailing.com Phone: 1 (800) 470-SAIL (toll free) _________________________________________ _________ |
Vertical clearance ??
BG Phew, that's pushing the envelope of what I'd do for that type of
navigation. otn Jack Dale wrote: On Sat, 09 Aug 2003 22:30:17 GMT, otnmbrd wrote: PS got my hands on a light list .....no tidal info in there for particular lights, that I could see. The height of the light is from MHW on US charts. That is about all the tidal information in that publication. If you use the "dipping the horizon" technique for establishing a circle of position, the height of the tide at the time of the observation must be taken into consideration. Jack |
Vertical clearance ??
I got back and want to answer your note.
otnmbrd wrote: Chuck Bollinger wrote: The vertical clearance is measured from 'Mean High Water' in places with a single diurnal tide. In the Pacific Northwest, it is measured from 'Mean Higher High Water' (MHHW). Interesting ..... Can you show me where this information comes from? In looking at CP 7, it list all heights as above MHW, unless otherwise stated, and the only major change to this I can find is for the Columbia River, which uses MLLW below Harrington Point, and "Columbia River Datum", between there and Bonneville Dam. And you are absolutely right. I cannot find anywhere that says that MHHW is used in areas of semidiurnal tide. And yet, 4 out of 5 quite well educated mariners swear up and down that they were told, or read, that information, but, like me, cannot actually come up with the info. That's great, and I appreciate your head check. Bull**** needs to be stomped, no matter how well-intentioned. I can't see why my tide program give me MHHW which isn't really very useful. To use it risks being off by about a foot or so, in the wrong direction. But Mean Tide and 1/2 the Mean Range isn't rocket science, so I'll continue in that direction. I see the conversation is raging away. Some pretty sharp people involved. I'll take my discredited ass and go sit on the sidelines. {:-D |
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