Bob, Could be your GPS uses a different datum for its altitude determination than what is used for the tide book's "sea level." The tide book uses MLLW (= Mean Lower Low Water) as its zero point, while USGS topos use a National Geodetic Vertical Datum, usually the 1929 datum, which uses MHHW, I think, as "zero altitude." Around here, the difference is about 7 ft (2.1 meters). If the tide range where you are is 20 some-odd feet, that would account for most of the discrepancy. -- Dave Kruger Astoria, OR ----- Original Message ----- From: "Bob Carter" <revkayak_at_aptalaska.net> > My problem with GPS (a Garmin 12) is the altitude reads are inaccurate. I > went down to the boat harbor when the tide level was at zero, held the GPS a > couple inches abobe the water and got a reading of 32 +feet. (No wonder > tourists ask how far we are above sea level!) *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************Received on Sat Sep 04 2004 - 12:41:36 PDT
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