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There is a way to coax somewhat more accuracy out of a handheld GPS by using a "do it yourself" differential correction. Basically, circumscribe the area in question with a series of line segments. Stake out the endpoints of each line. Position the GPS over one end of a line, and allow it to "average" over a period of 30 seconds or so (if it has this capability). Record the position on paper, or save it as a waypoint. It is easiest to use the UTM system; this will give you your coordinates in meters -- you really only need about the last four digits. Now walk to the other end of the line and allow it to collect the position in the same fashion. Record this position, keeping track of the order you make the measurements. Go all the way around the loop and "close out" the traverse at the original first point. After you are done, take all the raw data back to your office. Starting from the first point, subtract the coordinates of each end of each line segment to compute a "delta East" and "delta North" for that segment. Graph it on paper or use Autocad or whatever to create the set of line segments. Then measure the area enclosed by the line segments. You can measure the difference in position between two points more accurately than you can measure the absolute position in space of those two points, simply because some of the sources of error (atmospheric propagation and satellite geometry) cancel out when you do the subtraction.
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