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#21
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Vertical clearance ??
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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 |
#22
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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 |
#23
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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) __________________________________________________ |
#24
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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) __________________________________________________ |
#25
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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) __________________________________________________ |
#26
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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) ________________________________________________ __ |
#27
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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) ______________________________________________ ____ |
#28
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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) __________________________________________________ |
#29
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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) __________________________________________________ |
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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) __________________________________________________ |
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