A subtle source of error in correlating GPS coordinates with maps,
especially USGS topo sheets, is the "Map Datum" used.  Many (most?) USGS
maps use the 1927 North America datum, which is not the current one used by
default in my Garmin GPS III and probably many others.  I found that
discrepancies of similar magnitude between my GPS readings and even late
editions of USGS sheets vanished when I switched datums (readily done
through the set-up menu on the GPS).

P. Toulmin

Original message:

>Date:    Sun, 5 Oct 2003 10:05:37 EDT
>From:    [log in to unmask]
>Subject: Re: VA-ROOTS Digest - 3 Oct 2003 to 4 Oct 2003 (#2003-182)
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>In a message dated 10/5/2003 12:00:07 AM Eastern Daylight Time,
>[log in to unmask] writes:
>I'm not positive, but to my knowledge, this will be based on mapping =
>software.  Find the spot on the map, and then note the coordinates.   I =
>have found that the software can be off as little as 10 feet, and as =
>much as 1,000 feet.
>A GPS receiver will show immediately any error in the map data. As far as
>accuracy is concerned I have tested the Earthmate on a USGS trig station and
>found the data to be accurate to less than a meter. The test was performed
>close
>to a COORS station which accounts for the great accuracy but an error of 1,000
>feet could not be tolerated by the technology of the GPS. Something had to be
>wrong in the use of the receiver or in the application software or data to
>show such a large error. For example, the map data could easily be in error by
>that amount but not the GPS technology.

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