GPSS fuzziness...

Southwestern Field Biologists swfbtucs at INDIRECT.COM
Sat Aug 27 21:35:38 CDT 1994


On Fri, 26 Aug 1994, Tom Moritz wrote:

> Can someone explain what the current situation is with respect to
> the attainable resolution of GPSS?  I have heard from people on
> 4 continents that at the time of Desert Storm the Defense Department
> obscured the highest possible resolution.  There is considerable
> confusion about what the reliable precision of GPSS currently is...?
> -- Tom Moritz
>

We are using two hand-held portable GPS units to sample about 2,000 20 x
20 m plots on a 200 x 200 m grid. Although the grid was set up in advance,
we could not use the GPS units to navigate to the sites. This is because
of "selective availability" (sa). Sa randomly confuses the satellite
signals, resulting in a meandering inaccuracy of up to 100m. The units are
great for air and ocean navigation and other applications where pinpoint
accuracy is not needed. We used 1:24000 scale USGS topo maps to get us to
the grid points and then took our samples and GPS readings. Back in the
office we downloaded base station files from a U.S. Forest Service
computer and worked with them to correct the hand-held GPS unit accuracy.
This is "differential correction" and brought our accuracy into the 1 m
range.

The labor required to accomplish all of this was definitely not worth it.
We could have done everything quite easily with topo maps.

Frank Reichenbacher

Southwestern Field Biologists
Tucson, Arizona




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