The traditional base for geologic mapping is the topographic map. Topographic maps show details of physiography and culture, which permit accurate positioning of contacts and structures.
In some cases, topographic maps must be prepared as part of the gological mapping process. This is a time-consuming process, warranted only by the need to carry out large-scale mapping of some small area containing structures for which elevational/topographic control is necessary to unravel the three-dimensional structural forms.
In many instances, the most valuable base for geologic mapping is a set of aerial photographs, preferably with enough overlap to afford stereoscopic coverage.