I just learned that E911 Phase II requires us to express our antenna heights in meters above the ellipsoid. This means that we have to determine the difference between the AMSL and the ellipsoid for each XY tower coordinate. Which led me to realize that my GPS receiver must report elevation relative to the ellipsoid and not relative to mean sea level (MSL.) At a particular set of coordinates in TN, the difference (N) between MSL and the ellipsoid is 28 meters! This probably explains why GPS elevations seem to vary so much from published elevations.

We learn something new every day in spite of ourselves.