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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) __________________________________________________ |
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