Method permitting drawing contour lines by using two overlapping air photos under a stereoscopic instrument.

• A. Pull-up
• B. Stereoscopic Vision
• C. Photolithography
• D. Oblique Photography

Answer: (B)

Viewing two overlapping air photos through a stereoscopic instrument lets you see terrain in three dimensions. The slight differences in position of features between the left and right images create parallax, which your eyes fuse into a single 3D impression. With that 3D effect, a cartographer can trace where elevations rise and fall to produce contour lines. This is the essence of stereoscopic vision (stereoscopy) in photogrammetry. The other options don’t involve using a stereo pair to interpret depth for contouring: a single method like pull-up isn’t about stereo 3D interpretation, photolithography is a printing process, and oblique photography refers to angled shots without the depth perception provided by two overlapping photos viewed in a stereo instrument.