Altitude above ground level?

I was visiting a forum a little while ago and someone asked if it's possible to judge your altitude above ground level, now I don't think you can - well i dont know how too anyway, just wondering if this is possible?
I have actually crashed because of this before when I was flying over tenerife in low visibility.
Cheers
I have actually crashed because of this before when I was flying over tenerife in low visibility.

Cheers