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post #7 of Old 01-14-2011
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"the Fathometer, showed me to be in the center of the bayou."
That's interesting. Here I thought a GPS showed *position* while a fathometer only showed depth of water under you.

It isn't that one or the other is necessarily wrong, they can both be "right" and still be in total disagreement. The question is, what is the GPS position based on (datum and accuracy) and what is the fathometer based on? (Presumably "showing" your position by depth matchig a surveyed depth on some chart, again with datum and age to be considered.

The GPS accuracy is usually excellent, the problem is the geocoding, matching up that position to where things have been put on a chart. In an area where I haven't actually been there and "done the ground conformation" I have to allow for the GPS position possibly being off 1/2 mile or more, because I've experienced that and I know the limits of the MAPPING as well as the GPS.

I've also been diving in a channel well surveyed and marked at 40' to bottom, and seen 80+ feet and descending on a depth gauge. Depths change too!

So like a good pilot I trust my instruments--but as Bush and Mao and many others have said, "Trust but verify". I look up, I see the sun against a blue-ish sky and a certain size, so my circle of position starts as "probably on earth" and gets refined from there. Although some days....

Last edited by hellosailor; 01-14-2011 at 01:26 PM.
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