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| GeoCommunity Mailing List |
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| Mailing List Archives |
| Subject: | RE: GISList: Handheld GIS - WAAS and othe ACCURACIES |
| Date: |
08/22/2001 01:46:46 PM |
| From: |
Pat Waggaman |
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At 17:35 08/21/2001 -0600, you wrote:
>A point on this accuracy thing.
>
>With SA off the best "any" autonomous GPS receiver will navigate is by
>definition 30 meters depending smoothness of the receiver - you may get=
like
>15-20 meters some days.
In our experience, 15 meters is the "normal outside accuracy" of GPS with=20
SA turned off.
Furthermore, if you average GPS readings, as is typical in cartography and=
=20
bathymetry, the error drops significantly: left in a fixed position for a=20
few hours the error drops well into the sub-metric range.
Test it by leaving a GPS in a fixed position and capturing the data for a=20
couple of hours. Display it, you'll get a good idea of median accuracy=20
it'll be better than 15 meters.
Or, walk a straight line slowly with a 12 channel handheld (saving the data=
=20
into a notebook in hyperterminal via the NMEA 0183 interface to a serial=20
port if that's the only way) without too much foliage and buildings=20
blocking the sky, taking readings every 5 seconds. Then, clean up the=20
excess data and plot the readings on something as simple as Excel. Look at=
=20
the path, the points will be regularly spaced, oriented in the right=20
direction. You'll have to do some number-crunching to reduct to meters of=
=20
error, but this is a GIS list :-) That test alone, however crude, is=20
sufficient to debunk the 30 meter accuracy.
Cheers,
Pat
Thos. E. Waggaman, III (Pat)
Director
Marine Imaging Systems S.A.
Ave Borgo=F1o 22.090
Caleta Higuerillas
Vina del Mar
Chile
waggaman@marimsys.com
Tel: 56 32 815005
FAX: 56 32 815243
http://www.marimsys.com
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