None of the other terrain works.

No, but you can set locations to Air detecting only, and then you can detect when an air unit comes there, and stop them via triggers.

No. You can make it with triggers, though.
Make all of the terrain you want them off a certain elevation, and put a location over the map. Disable the other elevations (uncheck every box but "High Air", for example) for that location and use the move unit trigger to kick them out.

K, thanks.

Has anyone else ever had problems with elevations? Once in a map I made that used a lot of elevation locations I noticed that sometimes it took a moment for SC to differentiate between the elevations. It was part of a spawning system (in RP Lathirion if you're familiar with it) sometimes when you brought a unit to the high elevation beacon of a spawn pair it would spawn the other unit for a second, then switch to the correct unit.

That was probably because the unit entered from another elevation.
Would be interesting to test how long a unit that comes from another elevation keeps his old elevation status.
Maybe that other elevation even overlapped into the location.