Which device uses stereo photographs to determine elevations and is the primary method for plotting contours on large topographic maps since the 1930s?

The device that uses stereo photographs to determine elevations and has been the primary method for plotting contours on large topographic maps since the 1930s is the stereoplotter. This instrument allows surveyors and cartographers to view two overlapping aerial photographs taken from slightly different angles. By using these photographs, the stereoplotter enables the viewer to perceive depth and relief, which is crucial for accurately determining the elevation of terrain features.

The stereoplotter works by converting the three-dimensional visual information captured in stereo pairs of photographs into two-dimensional contour lines. This capability has made it invaluable for creating detailed topographic maps that represent variations in elevation across large areas. Its design and function specifically support the analysis of terrain, making it an essential tool in the fields of surveying and geographic information systems.

In contrast, while a stereoscope also allows for the viewing of stereo photographs, it does not quantify elevations or aid in contouring. A photometer is primarily used for measuring light intensity, and an altimeter is designed to measure altitude or elevation, but it does not plot contour lines based on stereo imagery. Thus, the stereoplotter stands out as the most suitable answer for this question, given its specific application and functionality in topographic mapping.