davevoith,
I am baffled that there are so many surveyors who seem to have such a lack of understanding about all the horizontal and verticle datums that are used in the GPS they turn on today, so it is no wonder that you would have a hard time with this. I may be saying some of the stuff Jesse already said, but I want to make this complete. Orthometric heights are heights that are referenced to a geiod, an equipotential surface closely identified with mean sea level. And this geoid is modeled by the verticle datum know as NAVD88. These height were done with leveling. Then came along good old GPS. Now GPS is actually referenced to the WGS84 reference frame. From the earths XYZ coordinate system (WGS84) we obtain LAT, LONG and an ellipsoidal height. Now this is confusing, but take my word here, anyone at NGS would (or should) tell you the same thing. All of our control is based on NAD83, and since NAD83 and WGS84 are so close, and since we GPS only figures out baselines in reference to other known control (when doing highly accurate work) all coordinates obtained are in NAD83. The only way to obtain true WGS94 coords is to autonomous positioning (+- 6-10m) or tie a project in to WGS84 control points obtaind from the Department of Defense (let me know when you get those too). Any height referenced to the ellipsoid is said to be an ellipsoidal height. Now, we still need to get to an orthometric height, so NGS came up with a model to closely represent MSL, and hopefully give you heights that are supposed to be Orthometric heights. So along come GEIOD03 (and others before that). Your software, I have no doubt, has this loaded so it is figuring out real time the difference between the ellipsoid and geiod. And the height from the Geiod to where you are on the ground would be considered an "Orthometric" height, supposedly heights representing NAVD88. I put orthometric in quotes for a variety of reasons, but for your purposes, you are using orthometric heights from GPS or NAVD88 heights, though I would not use these for any highly accurate work, unless you are running Static sessions to BM's close by, even then, leveling is the only true way to be sure if you ask me.