Estimates.
Remember that the GPS works with a mathematical model is the WGS-84 ellipsoid by default, thus capturing the precise ellipsoidal height that gives the team and its technical characteristics.
The issue is that to capture elevations of the mean sea level, we need Geoid models made �??�??with Gravity meters or other more sophisticated tools that not all countries have at their disposition. To calibrate an area in question, I use the model EGM 2008 to Chile, although a global geoid model is the most reliable I think to model and capture orthometric heights. EGM The model 2008 has a 1x1 grid and 2.5 x 2.5 minutes. I use 1x1 minutes for the interpolation is a bit more precise, with the calculator ALLTRANS that distances and flat topography measurements or distances, the value of the geoid undulation becomes almost constant for a sector. This value can be placed in ArcPad and so I worked myself. The interpolation method varies according to the objectives of each user (triangulation, nearest neighbor, bilinear).
If we are talking about geodesic distances where we need precision, not recommended to use ArcPad in my appreciation, therefore, ArcPad need an upgrade to these models as gravimetric.
That's my opinion. Sorry for my bad English.
Greetings to all from Chile!