You can't get elevation from Orthorectified photography.
To get elevation data from airphotos you need special photos, hardware and software:
Photos:
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).
Hardware:
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).
Software:
Specialist plotting software like Socet Set to drive the special hardware and capture the 3d input.