> How accurate and repeatable are currently available GPS devices?
It's a bit like building a race car : "Accuracy costs money, how accurate do
you want to be?"
The cheapest handhelds (roughly $150 and under) are only good to about 10
meters (30 feet). Adding WAAS capability roughly doubles the price, but
gets accuracy and repeatability down to around 2 meters.
Higher accuracy is very possible, but gets real expensive real fast. You'll
need to either buy real-time correction information from a commercial
service, or set up a second unit of your own to serve as a reference point.
AFAIK units with these capabilities aren't directed to the consumer market,
you're talking about professional units with corresponding prices. I
believe the cheapest unit my employer sells is around $6k, plus some amount
per month for the correction service. (It's actually priced by expected
accuracy, let me know if you'd like me to find some prices for you.)
For $20-30k, you can get a pair of units that offer relative accuracy well
under an inch, but you have to provide the communications link between them.
Put one of them down on a "first order" surveyed point, and the second unit
will give you absolute positions as good as the surveyed point up to several
hundred miles (limited by the communication link of course) away.
If you only need relative accuracy (eg pick some starting point and then use
the GPS to measure distance/direction from there); there is a trick that can
be played with the less expensive (non-WAAS) units. They actually do very
well, as long as the set of satellites they are using doesn't change. If
you have some way to monitor the sats used, and don't mind starting over if
it changes, you can expect relative accuracies of just a few feet. It
drifts with time of course, and it's probably tough to keep the same
collection of sats used for more than 10 or 15 minutes, but it's one
possibility.
Randall
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