how were topographic maps drawn 20 years ago and how were they drawn today?

Answer 1

A friend of mine in a met. office in Africa drew topographic maps as part of his job in the 1970s and showed me how it was done. Two aerial photos are taken with the same camera, about one second apart, so they form a stereoscopic pair. The camera has some optics which correct for distortion at the edges of the field of view due to the fact that you're looking at it obliquely. Then the stereoscopic pair of photos is placed under a viewer with two eyepieces, so he sees a single 3D image. There's a transparent plastic sheet in the system somewhere. He manually traces contour lines of constant elevation with a felt pen on the plastic sheet. I tried it, and it tires you out very quickly. I don't know how it's done today.