Meaning of "ground altitude"

[spiegel]

The reference pane, reference settings has a field

"Ground altitude [m]"

What does this mean?
The altitude of the UAV over the starting point?
The altitude of the ground at the starting point?
The middle, lowest or highest altitude of the terrain?
Or what else?

[Alexey Pasumansky]

Hello spiegel,

Ground Altitude parameter defines the average altitude of the ground level above ellipsoid (or above geoid in case of compound coordinate systems).

It is only used for the Reference pair preselection, when camera orientation angles are loaded. So it may be useful for the datasets that contain many oblique images.