TM 5-230 Topographic Drafting

The advent of aerial photography in the early twentieth century brought with it large-scale surveying of landforms and land use. To capture aerial photographs, photographers first affixed special cameras to the bottom of in-flight aircrafts. The photographs they took were then developed and aligned to one another, creating an overlapping grid or index mosaic that covered the entire surveyed area (as shown in Figure 141). Mapmakers traced over these “base layers” to create the geographic features, like rivers and mountains, that would eventually appear in other cartographic materials including U.S. Geographical Survey topographic sheets.