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Author
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Topic: Testing GPS accuracy
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David Burch
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posted May 07, 2003 01:52 PM
One way to evaluate the accuracy or reproducibility of a GPS unit is to plug it into an e-chart program and simply let it run overnight. Then zoom in and look at the distribution of the track points that have accumulated. You can set a mark in the middle of the distribution, and put a range ring on that to see put a scale on the picture. A sample is shown below.
Notes: it is our experience that this cannot be done by simply setting the GPS outside overnight and then in the morning dowloading the track to a computer and looking at it. We don't know yet why this is the case. Some units also will not let you zoom in to the level needed to see this fluxuation (on the order of feet), but in all three we tried the track could not be later uploaded to a PC program.
One reason this may be is that the GPS units automatically compact the track before downloading it, in which case you will lose the precise info you want.
Also, when setting up the track accumulation in the pc program, it is important to go to set up and set track parameters to be say every 1 sec or every 10 seconds, with no requirement on position or course change.
Ultimately we want to add here a comparison of "normal" GPS with WAAS enabled fix, but have not been able to yet.
This picture was accumulated in nobeltec VNS using a garmin etrex for 90 minutes, recording a position every 1 second.
From: Starpath, Seattle, WA
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