Low cost method to increase the accuracy of GPS data

[LASHLEYT@detroitedison.com]

There has been a lot of information presented on RadSafe concerning
improving 
the accuracy of GPS data.  Almost all of the suggestions have been to
purchase 
additional equipment that range from $1,000 to $10,000.  If you are working 
with a small budget, like myself, and can't afford the additional equipment, 
there is a manual method to improve the accuracy of GPS data.  This method
was 
used to perform geological surveys (gravity and magnetic) before the days of 
GPS technology.  The only additional equipment that you need to purchase is
a 
7.5 minute U. S. Geological Survey topographical map of the area you want to 
survey.  The USGS maps show the locations of USGS benchmarks.  These 
benchmarks are brass plates attached to cement markers that have the
latitude, 
longitude and altitude of that point on the ground.  When you start your 
survey use the USGS map and locate the nearest benchmark to the area you
want 
to survey.  At the benchmark take a GPS reading (I suggest an average of
three 
readings) and calculate a correction factor for the latitude and longitude
and 
note the time.  At this point you can start collecting you survey data and 
recording the time of each data point.  Periodically and at the end of your 
survey return to the benchmark and take another correction reading.  At the 
end of the day, determine the equations for the lines that represent
latitude 
and longitude correction factors vs. Time and apply these equations to you
GPS 
survey data.  This technique is not as accurate as real-time differential
GPS, 
but gives you improved data at a fraction of the cost. 
 
 
================================ 
Thomas M. Lashley 
Radiological Environmental Engineer 
Fermi 2  Nuclear Power Plant 
LashleyT@DetroitEdison.com 
(734)-586-1697