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.