In computer mapping, the cartographic convention of “layering” is an essential metaphor—but how did this practice work before computers, and how did it change when we started turning layers into digital data?
“Layering” data—that is, creating multiple maps, placing them on top of one another, and then making analytic conclusions based on how they overlap—long predates computer cartography. Some of the earliest data layering techniques were pioneered by landscape architects and regional planners who needed to compare different variables, like population density and land use, in the same area. The arrival of the computer into cartography introduced significant changes into how mapmakers conceptualized layers. As a group of geographers and cartographers wrote in 1976:
The computer data file process encodes and provides separable access to each subvariable. Thus, in effect, one makes not one data map, but a separable, mappable file component for each subvariable. The computer allows the selective recall and recoding of these subvariables, as required by the criteria in any particular analysis.