Then, when flight became more common, topographical services began the practice of photogrammetry, where stereo images - image pairs that are slightly offset from each other - were compared using a stereoscope which then informed the creation of a topographic map. In a pre-digital era, map surveyors would have been out on foot taking measurements and recording where things were in particular locations, which were then relayed to a map draughtsperson - usually part of a state topographical service.
To begin to understand what prompts people to hide things on digital maps, it pays to take a step back to understand how cartography happens today. Prior to GPS, illustration and aerial photography provided the bulk of cartographic information.
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