Instead two questions I have made this picture.

It will simply show the altitude with a minus sign.  As far as using the Auto-Land in that condition, follow what the manual says:

gil Posted at 2015-11-30 14:22
It will simply show the altitude with a minus sign.  As far as using the Auto-Land in that condition ...

Yes a negative altitude.
A similar question which each of us has asked before flying the first time from a bridge or a cliff is “what happens when I hit RTH?” If you have not set a new Homepoint after takeoff, the Phantom will rise to the RTH altitude (above the homepoint “0”) and return. So beware when under a bridge. Another point, if you’re exploring a narrow canyon, you might lose satellites and pass on P-Atti mode. Last, I personally find it difficult to judge relative heights of the bird and obstacles when they are well below. Have fun!

Hi ,
In this case , the altitude should be shown -10 m in the APP . And don't worry about the auto landing , it would not  drop at 0 m .

Take it. Thank you to all