Photogrammetry is the science of making measurements and maps using photographs. A technician aboard an aircraft takes a series of overlapping photographs of an area to be mapped. Adjacent images are placed in a stereo-plotter,and calibrated using ground truth data obtained by surveyors in the field. The final map shows the features and elevations of the surface.
New technologies allow contour information to be overlayed on ortho photography.
The NRC now uses a digital stereoplotters to produce topographic maps and cross-sections of river valleys. The floodplain division uses this data to determine the 100 year floodplain.