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

To: Michael Tope <>
Subject: Re: [TowerTalk] RE: Downloading terrain data for use with HFTA
From: "Darrel J. Van Buer" <>
Date: Fri, 14 Jan 2005 04:09:21 -0800
List-post: <>
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


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