Kenelm Philip fnkwp at AURORA.ALASKA.EDU
Thu Jan 29 13:49:58 CST 1998

        Jeff Skevington asked: "Do all GPS units work off of the same model
of the earth?"

        GPS units work off a number of different models--which are referred
to as 'map datums'. For example, my Magellan unit (Trailblazer XL) has 73
datums stored internally, and can be set to use any one. In theory, one
ascertains the datum used for the maps you have at hand, and set the GPS
unit to the same datum--after which coordinates from the GPS unit should
match (within the deliberately introduced 'Selective Availability' errors)
coordinates scaled from the maps. The datum is usually printed on the map.

        By the way, I checked with Magellan, and the Trailblazer XL is
not affected by the week number rollover problem.

                                                        Ken Philip
fnkwp at

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