Height difference between earth models

Hello, is it possible to calculate the height difference between earth models at specific points of lat and long? For example let's say I have sphere model and an ellipsoidal model. Now I want to have for specific points of lat and long the following: Length of vector that is normal to my sphere model at lat/long, starts on the sphere model, and ends at the intersection with the ellipsoid.

So the same way as you can calculate a geodetic height as difference between an ellipsoid and the geoid, I want to have the height difference between different earth models.

Does anyone have an idea? Thanks!

[Chad Greene]

What do you mean by sphere model and ellipsoid model? Are these gridded DEMs or simply equations of a sphere versus ellipsoid?

[Joseph Cheng]

Chad,there are different earth models out there. The latitudes on the ellipsoid earth are not geocentric so center to 60 lat point on the sphere doesn't intersect the 60 lat point on the ellipsoid.

T, However since you know the angle of the sphere you can calculate the point at that angle for the ellipsoid then knowing the point on the ellipsoid calculate either the geocentric or geodetic "radius".

if you check out the wikipedia entry on latitude there should be some equations to get you where you are looking for