Recently, Ben Jackson posted:

In article ,
Phil Verghese wrote:

So it looks like the GPS is showing True Altitude (which makes a lot
of sense). True altitude is the actual height above MSL,

More precisely it's showing the height above the datum, which is the
mathematical model of the earth ellipsoid the GPS uses to approximate
the actual Earth.

The Garmin GPS receivers that I've used all report elevation above the
geoid which models Mean Sea Level (MSL), not the ellipsoid. Internally
they initially calculate elevation above the ellipsoid, but then they apply
values from a lookup table that has corrections for the geoid - ellipsoid
difference based on the geographic position. I believe Magellan units and
some newer Lowrance units also correct for the geoid-ellipsoid difference
so the reported elevations represent height above MSL rather than the
ellipsoid.