It is anticipated that digital cartographers will be continually challenged to develop new techniques as software evolves and as geologists and users demand more com‑ plex and informative products. (See Figure 6.104 for color suggestions.) U.S. Geological Survey, National Cooperative Geologic Mapping Program8
GIS maps of geologic features illustrate the location of rocks and unconsolidated deposits (structures and materials) at the Earth’s surface. These data are often in three or four dimensions, so they include the height of the rocks and the time at which the rocks were formed (measured in eras, periods, and epochs). These data add to our understanding of geologic hazards like volcanic activity and landslides, aid in visualizations of general Earth resources like oil and ore, and contribute to land use planning like site development and construction potential. The data are presented in various ways, including any of the three vector formats: points, lines, and polygons, with the most typical being polygons that represent map units. Geologic map units are areas of similar rock type and age. Less common but no less important geologic map feature types are points that represent such things as bedding attitudes, fold orientations, and sample locations. Line format data are also included in geologic mapping, representing such things as fault lines and glacial moraines. Another geologic line feature is the isograd, which is the geological equivalent of an isoline. To sum up, a great variety of data items fall into the geology feature category. As mentioned above, rock height and age are often included in geologic spatial data. These enable GISers to create 3D geologic maps and, indeed, they are increas‑ ingly being visualized in 3D because it is much more helpful to the end goals of the scientific audience for this feature type than a simple flat map. If you are, however, displaying the geologic information in traditional 2D you may want to supplement this with one or more cross sections to illustrate subsurface feature trends. These cross sections, which are common on 2D geology maps, are always referenced in the main map via some sort of graphical construct, typically a dashed or solid line. Symbols for such geologic mapping mainstays as landslide locations, faults, volcanic features, and land subsidence are somewhat standardized. The Federal Geographic Data Committee (FGDC) Digital Cartographic Standard for Geologic Map Symbolization contains a cartographic specification that you can use when you are putting together your geologic map.9 The standard describes what symbols to use for certain features and contains color charts and other items to get you started. The FGDC standard is known as the American standard. There is also an international