https://www.usa.gov/copyrighted-government-workshttps://www.rawpixel.com/image/4051431
Austin Photo Interpretometer (Stereoscope)
This device enabled 3-D viewing of stereo pairs of aerial photography. It has a height finder and scale for making linear measurements. Original public domain image from Flickr
Public DomainFree CCO U.S. Government image for Personal and Business use