The story
eTrex inside
Map datum / user datum

        Map datum could be described like matematical model of conversion bettween Earth sphere (shape, dimensions....) and the "flat world" of map. For detailed description follow read this article or search some other resources related to GPS and geography.
        For the GPS user is important, to know, what datum (there are many of them:) is used by his map and setup the unit to work with it. Inside eTrex there are many datums (coordinate systems) predefined, but some are missing. From firmware versiom 2.5, user could define it's own coordinate system. The "user date" is defined by a difference from WGS84 containning 5 parameters DA,DF + DX,DY,DZ.

DA,DF - are core values, defining the elipsoid (aproximation of Earth's shape) used by a coordinate system. Elipsoid has two parameters: equatorial radius and flattering. WGS84 is based on elipsoid:

A=6 378 137m,    F=1/298.25722 (cca 3 per mile)
So the DA, DF values inside eTrex are differences from them (be careful ... is's often to find DF values multiplied by some factor - usefully x10000).

DX,DY,DZ - defines reference point (it's "shift" on the map). Entered in meters if metric system is used by eTrex.

See ... tables ... for search your coordinate system - map datum.

Made by DaTrex 2000, Contact: datrex@atlas.cz