Another thing to watch for when comparing GPS and maps is the "datum".
This defines the reference system for coordinates. Most topo maps in the US
are NAD-27, but the default for most GPS and very recent maps is WGS-84. Here
in California, there's about a 100 meter displacement between these two, but
the amount and direction of the difference varies throughout the US. Many GPS
can change the datum used on the display (buried in some setup menu).
The underlying cause of the different datums are various approaches to fudge
factors for the fact that the Earth is not a sphere, or even uniform in shape.
Darrel J. Van Buer, AK6I
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
TowerTalk mailing list