[TowerTalk] RE: Downloading terrain data for use with HFTA

[Darrel J. Van Buer]

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
darrel at vanbuer.net