As a qualified PI I was using very similar stereoscopes in the 1980's. There has to be an overlap between photos to get stereo (3D). Some can squint and see 3D without the stereoscope. It doesn't allow you to measure height though. That it is done by knowing the focal length of the camera and the height it was flown at. 3D's advantage is that it can allow you to determine what is a real and what is a fake target.
Nowadays they are Imagery Interpretors viewing digital imagery.