[Alberg30] GPS Question

[Bob Chambers]

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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