Hey Tom!
Here is a test I did a few years ago (see attached image). It is a rough test, but gives clues to precision. A word commonly used in high accuracy gps circles is "repeatability". GPS drift a key aspect of repeatability, which is simply variation of indicated position (vs. actual position).
Initially when folks see this kind of thing, they are rather disappointed. The key thing is to note the scale bar. Now imagine the average length boat overlayed on the plots. That makes things seem better and reinforce why the gps is useful. It is common to get within a boat length or slightly more of a previously saved waypoint. If one can get that close, the object of interest can usually be found quickly with the depth sounder.
Users of GPS units for agriculture were commonly interested in the "relative" accuracy of their hardware. A quality submeter gps would only drift 1 foot in a 10-15 minute period. If a farmer was spraying a field he might be getting <1 foot pass to pass accuracy as long as he didn't take longer than 10-15 minutes to make another swath. Now if he wanted to run the same recorded path again an hour (or hours) later, it is likely he might be off a few feet, even with a submeter gps.
Pat H.
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