What's the advantage of each?

I use Orthometric heights since it gives me the elevations I use, similar to spirit-leveling . . . sea level.

BUT, evidently some surveyors use ellipsoid heights.

Is it an issue of the quality of the geoid in a particular area, or what?

Can't grade a drainage to drain on ellipsoid heights, need ortho heights.

GPS can't measure ortho heights, it can only measure ellip heights. ortho height is ellipsoid height with the geiod separation applied to it. geoid separation can come from two places, either a geoid model or derived from observations on points with known elevations.

The geoid and the ellipsoid are not parallel to one another.
Modified By David Karoly on 1/18/2006 at 9:14 PM

But in reading posts here, I get the impression that some surveyors like to "actively" work with ellipsoid" elevations and, evidently, manually add the difference.

I might be wrong on this observation, but I like for the A.S. or GNSS software to give me orthometric heights.

sorry I misinterpreted your post.

Like you, I prefer ortho heights because they compare to my conventional work. You allow AS or GNSS to convert your GPS position to grid coords and ortho height.

You could export LLH and run it through Corpscon if you wanted too.

I suppose if you're doing GPS work with no conventional ellipsoid height would be a more pure expression of the results although it isn't much use in the real world.