If you are afraid of heights or have problems with looking down from elevated positions you have a problem: You miss one point in your photography. Nadir is a word people rarely use. Maybe because they do not know it. Why should they? Even if some people reach their zenith, nobody would boast with his travels to the nadir. Well, maybe if you are Orpheus…
So there are these to opposites: zenith and nadir. I am sure there is a very clever explanation in the wikipedia. I also remember that nadir is necessary if you want to paint the neverending stairs like M.C.Escher did – if you try nadir is to be added to a two point perspective.
Coming back to the problem of being afraid of heights I have to say: Photography cured a big bit of it. The urge to take a picture from the highest point and maybe stretching the perspective by using that nasty 10mm glass.
When shooting from the nadir to the zenith make sure to try to compensate for the light from above. Depending on the lighting conditions that can be 2-4 stops or even more. You can consider taking a HDR when shooting against the light, but I recommend shooting in RAW and some good postprocessing.