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ACRO E-mail Archive Thread: GPS markers


Thread: GPS markers

Message: RE: GPS markers

Follow-Up To: ACRO Email list (for List Members only)

From: Eric Rood <ericrood at>

Date: Tue, 15 Apr 1997 05:00:48 UTC


  Thank you to all those who responded to my posting regarding the use of
handheld gps for locating box points.  As I did not group the respondees
email addresses, I will shotgun this explaination to the group.
Yes, selective availability does MAY create a circular error of 150m. 
BUT, as I wrote in my posting, you AVERAGE THE DITHER.  This may be done
is one of several ways.  The simplest is to obtain a software program for
a laptop computer and take a picnic lunch and some snacks out to the site
some nice hangar flying type day, connect the GPS output to the computer
input and let the numbers flys for a couple hours.
SA is not constant, hence the term dither.  If you stand in one place and
watch the GPS location readout over time, you will observe the position
where you are standing stationary will change.  What nonsense is this you
ask?  Simply SA at work.  The satellite is constantly altering the timing
signal the GPS is receiving.  By averaging the changing positions it is
possible to determine the exact location.  Some software will even compute
the average and plot the data points for you.  The time consuming solution
is to record the dithering positions by hand, calulate the average by
hand, and plot the true position by hand.
A faster method is to find a cropduster with his own DGPS rig for working
a field or a locale surveyor with a DGPS system.  Ask them to donate their
services/equipment for the project. The worst they can say is no.

Eric Rood
ericrood at


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