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[at-l] Re: GPS

I have one the inexpensive Magellan's. It is completely unstatisfactory
under tree cover. Without differential correction or having
the government turn off selective availability, GPS accuracy is
100 meters or so. You can do somewhat better by averaging
over a long period (hours).
Differential correction can be applied in two ways:
1) just record data in the field then call up a service (and pay money)
for the differential data and correct it (requires a computer).
2) on the fly correction requires a radio that can talk to another
GPS, either owned by you and set up as a base station, or one of the
"public" ones available in some areas (near the coast mostly).
Differential correction gets you into the better than 10 meter area on
the cheaper models and under 1cm in the surveyor grade GPSs ($15,000).

Selectively availability off also gets you into the 10 meter area.

Even the most expensive GPSs have problems under tree cover.
Elevation data is no where near as accurate as position data.

There are also good times and bad times to be out.  It depends on how
many satelites are visible from where you are which is a function of
their orbits.  There are programs that can tell you when the good
times are.  If you are on the wrong side of a mountain it may cut off
your view of many satelites.

So it really depends on what you are going to use the GPS for.  Any
cheap GPS will get you within 100 meters which should be sufficient to
help if you are lost.  If you want to use it mapping, as I do, then
only serious money will do.

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