A few thoughts on the GPS accuracy --

The following URL
http://www.ngs.noaa.gov/cgi-bin/ds_radius.prl
has a search engine to find survey monuments within a specified radius of a 
particular lat & long.  I found one near me that had been surveyed to 
within fractions of an inch and took my GPS there.  It was consistently 
about 15 feet off in the same direction.

How about the following poor-man's DGPS.
1.  Lock the satellites in use by the GPS so they don't change.
2.  Make an offset measurement at such a benchmark.
3.  Travel to another close-by location.
4.  Could the offset be applied to improve location accuracy?

I suppose the real test of this is to get an offset from one surveyed 
benchmark and then testing the offset at another surveyed benchmark.  I 
realize there are errors due to reflections in GPS signals off buildings, 
mountains, etc., but I would  bet some improvement could be made over the 
uncorrected position.

Karl

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