Mapping altitude from stereo aerial photos?

How can I turn two high resolution aerial photographs of the same area from different angles into a map of altitude?

More specifically, is there a free or low cost software tool that facilitates this?

I can see how to work it out the hard way. In fact I’ve done the opposite, turning three dimensional representations into two dimensional perspective views, in programming several times. I’m wondering if there is a tool already available.

In particular, one requirement would be to measure the relative shift between the two images that makes a particular region line up, and this would be done best if both images were visible at the same time in a sort of semitransparent overlay. This would be so hard for me to do from scratch that I’d probably trace features and measure offsets with the feature tracings. That’s also alot of work! I suppose there are photo treatment programs that do this superposition but don’t know if any of them give a measurement of the shift.

What term can I use in web searches? All the searches I have thought of produce mainly various imagine methods you look at with both eyes to produce the effect of depth perception, which is obviously related but not much use.

“Obviously related but not much use”? Actually, that’s exactly how it’s done. The photogrammetry technician looks at the stereo pair and draws lines of equal elevation simply using the visual cue of apparent height. Of course, you won’t get a map of absolute altitude unless you know the elevation above sea lavel of some object visible in the photographs.