On Fri, 04 Apr 2008 21:50:07 -0500, Gordon Burditt <gordonb.00byx@burditt.org> wrote:
>>GPS, or at least the civilian version of it, is only accurate to within
>>about 300 feet or 100 meters. I once did a "site survey" using a
>>Motorola M12+T GPS timing receiver. The software I used plotted
>>something like 10,000 position readings on the map. The result was a
>>strip about 10 meters wide and 100 meters long and oriented ENE-SSW. My
>>antenna was more or less in the middle of this mess.

>There's also something called Differential GPS, which uses a transmitter
>on the ground to broadcast corrections. That supposedly improves
>the reading, if and where a DGPS transmitter is available.

It was a bigger deal when consumer GPS had a built in 200' error.

I worked on a differential GPS system used by a local phone company to track
their service trucks. The error from a fixed unit was subtracted from the
mobile units to remove their error. Apparantently, the 200' error was the
same over a large region.