In this 1971 clip, an operator from the Department of Lands and Survey demonstrates how she uses a stereoscope to create a topographical map – a technique called photogrammetry. Looking through the stereoscope at the overlapping aerial photographs gives a three-dimensional view of the land, which enables her to trace the contours. Other details, such as rivers, are also recorded. Once checked against the land itself, a stencil of the contours is created. This is then printed with other stencils to create a topographical map.
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Reference: An inch to a mile. National Film Unit, 1971
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