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post #23 of Old 12-12-2012
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Re: Gps navigation advise

It has been some time since my nav training so feel free to correct any inaccuracies I may proffer. Nav 101: Charts are flat, the earth is spheroid. Within a given chart, survey work done on the datum points give good relational accuracy which puts you pretty accurately on the chart given the bearings you take. Unfortunately, the earth is spheroid (not round or oval, but actually a little bit lumpy). GPS calculates where you are on the surface of the spheroid and not necessarily the cardinal coordinates for a given chart. If your chart has been updated to WGS 84, then it reflects this and you should have a pretty accurate fix on your (newer) paper or electronic chart. If the chart has not been updated and is showing something like WGS 72 or older, then the chart may or may not be that accurate depending upon how “lumpy” that particular patch of earth is. In the US (and most of Canada and Mexico) the WAAS system is used to ensure greater accuracy in the timing measurement. However, due to the placement and orbits of the WAAS satellites, places like the arctic, coastal Mexico (and potentially the US coastlines) may have slight variations in the timing signals and hence GPS accuracy.

GPS only “replaces” the celestial part of navigation and there are tons more to learn about navigation than turning on the Garmin and setting the autopilot. How many of us have chart #1 for example? Can you calculate set and drift? Heck, most people can’t even spell “ded reckoning” correctly. Sure, Columbus discovered the New World, but a skilled navigator will make landfall where he originally intended to go.
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