You can't get elevation from Orthorectified photography.
To get elevation data from airphotos you need special photos, hardware and software:
Sterio pairs, a set of overlapping images with an accompanying file detailing the location of the aircraft at the time of capture (x,y and z).
3d monitor, with glasses. Nowdays you can get cheap 3d LCD monitors with glasses, previously only CRT monitors had a refresh rate suitable and LCD polarization interfered with the special glasses.
A special box attaches to the computer via USB and Video output to control the flicker rate.
Also a 3d input device is needed, either wheels and pedals or a rat (a mouse with wheels to zoom in/out).
Specialist plotting software like Socet Set to drive the special hardware and capture the 3d input.