Here is a statement from Garmin Engineering on how the selected DATUM affects serial port (NMEA, GARM, and PVT) information in Garmin GPS units.
"In the NMEA output, the positions are output in the selected Datum, and the selected Datum is indicated in the $PGRMM sentence.
In the GARMIN (upload/download) protocol, waypoints (and in the new Garmin PVT protocol tracking positions, waypoints, routes and tracklogs) are downloaded in WGS84 regardless of the GPS internal DATUM setting.
A consistent way to look at Garmin's handling of Datums is:
NMEA content is always relative to the selected datum. Reason
is that the primary consumer of NMEA information are chart plotters which
do not have any ability to select map datums. What you see on the
GARMIN screen is what you get on your plotter.
Garmin I/O (either GARM/GARM or the new PVT protocol) is always relative to WGS 84. It is up to the external application to convert the WGS-84 to other datums properly if required."
Note: eMap, eTrex and eTrex Summit presently DO NOT have the $PGRMM NMEA sentence to indicate selected datum when in NMEA mode. Garmin advises that they are planning to correct this oversight in a future firmware update.
Joe Mehaffey