[Alberg30] GPS Question

[Fritz]

Hi Bob,
There are calibration issues with most GPS units. They must be set at a
known height, or known barometric pressure. Even with one of the newer
models I have, I find it needs to be recalibrated every couple of months.
I don't see why the GPS companies don't put a elevation marker on their
maps so that the GPS auto calibrates when you cross over on of those
marks. But that's the future I guess. I hope this helps.
-Fritz

On Thu, 12 Sep 2002, Bob Chambers wrote:

> I wanted to find the height of a particular land mass over sea level and
> thought I would use my two (identical) GPSs to do it.  It is my
> understanding that the +- 30 foot error we see is due to atmospheric induced
> errors, relatively constant for quite a large area, and that differential
> GPS simply broadcast the difference between a known location and the
> measured location (including altitude) so other units could apply this
> correction and get very accurate positions.  I thought the resulting
> accuracy was in the neighborhood of inches.  Based on this assumption, I
> looked at my two GPSs and was surprised to find the altitide and position of
> each was significantly different even with the antennas only inches apart.
> I expected them to be the same since they would experience the same
> atmospheric error and I could use the difference between them at any instant
> to be my measured altitude.  I look forward to any comment or info from the
> electronic gurus on this site as to where my thought process is flawed.
>
> Thanks
>
> Bob Chambers
>
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