Facts
The story
Documentation
eTrex inside
HackTheTrex
Hardware
Waypoints
Linx
eTrex datum notice

The map datum is very important value, when you want to upload waypoints/tracks into eTrex (and send it back). The core information is, that eTrex's firmware is using internal coordinate system for storing these data. So the preset datum (in menu system/units) is used only, when uploading or downloading points. The reason is unknown for me, but it takes advantages and disadvantages together:

PLUS You can use different kinds of coordinate systems during one trip (ex: local maps and waypoints downloaded from Internet). Simply switch eTrex to first coordinate system, upload waypoints, then switch to second (third, ...) and upload other parts. The goal of this is, that eTrex is doing conversion when data are uploaded, so these different kinds of waypoints could coexist in its memory without trouble - their coordinates are transformed to one "global" system used internaly by unit.
MINUS I tried, to use custom datum feature, for in-terrain map calibration corrections. If you load waypoints from bad (or not accurate) calibrated map - the easiest thing, you can do in terrain is - to shift all points and make a simple correction. But it's only a theory - it doesn't work .... If you change map datum and points are already in eTrex's memory, nothing happens. The new values will be used only for new incoming (also outcoming) points.

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