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[Info-gama] Effect of Free Gama Adjustment

From: Jim Sutherland
Subject: [Info-gama] Effect of Free Gama Adjustment
Date: Sat, 27 Mar 2004 11:19:24 -0800

I am trying to derive the most reliabile absolute accuracy solution for a local survey network using Gama adjustment.  The local survey network is a small survey network (4 to 6 points 40 to 60 metres apart) with direction and distance observations derived from photographic image.  The directions have a standard deviation of about 8 minutes and the distances 5 centimetres.  The Gama adjustment works well with a one "fix" and "Adj" point.
I observed multiple autonomous GPS positions and meaned them for each point.  The autonomous GPS coordinates showed an average of 2.2 metres off the known (correct) absolute coordinates from the points - the range differences 1.0 to 4.0 metres.  I seeded the Gama adjustment with these autonomous GPS coordinates as "free" and ran Gama.  I understand that this treats the coordinates as unknown parameters and adjusts the free seeded coordinates.  As a least square adjustment is done I presume this will minimize the residuals for these autonomous coordinates.
I did a compare of the adjusted Gama coordinates with the true coordinates.  The mean of the comapred adjusted to true coordintes is 1.7 metres - the range differences 1.6m to 1.9m.  This seems to be a good improvement.
My goal is to determine the best procedure for using/integrating the autonomous GPS positioning observations of multiple points of a local survey network along with the relative survey measurements of the local network.  Has anyone else done any research similar to this?  If so, I would be interested to see  your summary results. 
I will do some more testing.  The automous GPS unit is only $150.  I am looking for a way to optimize the global positioning and reliabitlity in using the autonmous receiver.  I will also test this network using a WADGPS system that was recently rolled out to cover all of Canada.  The receiver/communications device will be ported with a PDA device and the positions for the network will be meaned.  I expect to be able to obtain metre level global positions for the local survey network points.
Any comments will be appreciated.
Jim Sutherland
Victoria Canada

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