On 4 Sep 2004 at 12:41, Dave Kruger wrote: > 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." Further to that, remember that elevation errors in GPS are roughly 50% greater than horizontal errors. A good spread of satellites for horizontal determination is not necessarily a good spread for vertical. For horizontal you can get satellites all around you, but for vertical, you never get satellites below you. Also remember that as with vertical, there are several horizontal datum standards that can fool folks when they compare GPS to maps. If the GPS uses a different standard than the map, it won't show the same location as the map. GPS usage is not as simple as just turning it on. Not everyone realizes that. You don't just read the numbers, you have to interpret them sometimes. Mike *************************************************************************** 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 - 15:50:09 PDT
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