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Old 01-03-2009, 00:12   #1
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mean high water and bluechart

Maybe it's there and I haven't found it, is mean high water data available on my GPS with bluechart? Seems like this is an interesting number to know for getting under bridges. If you're navigating by electronic charting it would be nice not to have to go dig out the paper charts.



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Old 02-03-2009, 07:06   #2
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Try Reeds

John--

I don't believe that information is available in the Garmin Charts. For what it's worth, when we're traveling we keep a current Reed's Almanac at the chart table with a page marker on the tides/current page for the area we're in. It usually only takes a minute or so to find what one needs.

FWIW...

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Old 02-03-2009, 08:09   #3
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On my Garmin 440 you simply go to the 'TIDES' screen and a strip chart is shown with 24 hour tide levels.
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Old 02-03-2009, 09:23   #4
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I'm not looking for tide information, I'm looking for tide datum information.

For example: bridge in my area is listed as 43 feet clearance. This is based on mean high water, which is listed in the tide datum table on the NOAA paper chart as 10 feet. So if the current tide is at 0 feet I have 53 feet of clearance, at a plus 12 tide I have 41 feet of clearance.

So far I have never seen MHW listed in tide tables. I was going to drop by WM today to double check Reeds.

On my GPS, Bluechart has a circle with a tilde symbol in it to indicate a place to click on to get daily tides.

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Old 02-03-2009, 10:53   #5
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John--

Reeds lists tables of corrections for time and height of tides at various locations related to each location's nearest tide station, e.g. tides at Ana Maria Island related to the Tide Station at St. Petersberg, Florida. The correction tables also include the tidal range for each given location. If one selects the tidal location closest to the point one is concerned with, one can see the range of tide at that point. For example, if a referenced bridge has a clearance of 53', and the range of tide for the nearest location is 3'; with a "zero", tide at that location, the bridge clearance should be approximately 56', at least in theory and assuming the location is reasonably close. (On the other hand, if the listed clearance for a given bridge isn't greater than our mast height--and the clearance boards on the dolphins on either side of the bridge indicating that or more--we don't go there!)

FWIW...

s/v HyLyte
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Old 02-03-2009, 15:12   #6
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Originally Posted by svHyLyte View Post
John--

Reeds lists tables of corrections for time and height of tides at various locations related to each location's nearest tide station, e.g. tides at Ana Maria Island related to the Tide Station at St. Petersberg, Florida. The correction tables also include the tidal range for each given location. If one selects the tidal location closest to the point one is concerned with, one can see the range of tide at that point. For example, if a referenced bridge has a clearance of 53', and the range of tide for the nearest location is 3'; with a "zero", tide at that location, the bridge clearance should be approximately 56', at least in theory and assuming the location is reasonably close. (On the other hand, if the listed clearance for a given bridge isn't greater than our mast height--and the clearance boards on the dolphins on either side of the bridge indicating that or more--we don't go there!)



FWIW...

s/v HyLyte
That's close enough to work. Range is the difference between mean high water and mean low water. U.S. chart datum is mean lower low, so range doesn't give you exactly mean high water.

It just seems odd to me that moving into the newer better digital age of charting requires you to look up a useful if not critical number in a paper reference, and if you don't want to have to have the appropriate paper chart to find the correct number, you use a close enough number found in a tide guide. Now that's progress!

John
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