[sac-forum] GPS accuracy

Getting the best accuracy requires (1) a WAAS-enabled receiver (most
higher-end units are WAAS-enabled these days), (2) an external antenna,
and (3) significant "dwell time", i.e. staying in the location to be
measured for several minutes. Under the best of circumstances, with
fixes on >6 satellites spread from horizon-to-horizon, I still bet that
95% accuracy is on the order of magnitude of 1 meter (95% of GPS
measurements will be within 1 meter of "truth"). 
 
Getting sub-meter accuracy requires (4) a dual-frequency GPS receiver
(L1 & L2) and (5) a GPS base station broadcasting from some sort of
known, fixed reference point like a USGS survey marker (this is called
differential GPS). This is the sort of stuff that utilities, surveyors,
etc use. Even then, I'd still be surprised if you could get 1 mm
(1/300th of a foot) 95% accuracy. The commercial systems I've seen
advertise 3 - 10mm RMS accuracy, which means only 67% of GPS readings
are within that distance of truth. To convert that to 95% (2DRMS), the
usual rule of thumb is to multiply the accuracy distance by 2.5, i.e. a
3 mm (1 inch) RMS unit is really a 7.5 mm 95% unit.
 
I think your best bets would be to talk with the folks at Wide World of
Maps here in Phoenix or with a local surveying company.
 
Good luck!
 
Dan 
 
Dan Gruber
dgruber@xxxxxxxxxxxx
home  (480) 951 - 6877

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