wrote:

You can try to improve vertical accuracy using calculations from one
of the following websites but you need to know if your GPS is already
correcting or not!

AFAIK, all the Garmin portable GPS units include geoid vs. ellipsoid
corrections.
Set the unit to output NMEA data from its serial port and connect it to
a PC running Hyperterminal or similar program that captures the output
stream. The $GPGGA sentence will show the value being used by the GPS
for the geoid-ellipsoid correction as the 8th data field.

http://sps.unavco.org/geoid/

http://earth-info.nga.mil/GandG/wgs8...0/intptW.html?