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.

General question: is there are practical difference between the datum and
MSL? Neither have any local relevance, AFAICT.

Neil