Georeferencing is the process of taking a digital image, it could be an airphoto, a scanned geologic map, or a picture of a topographic map, and adding geographic information to the image so that GIS or mapping software can 'place' the image in its appropriate real world location. This process is completed by selecting pixels in the digital image and assigning them geographic coordinates. In rare instances, one may already know the geographic coordinates of certain pixels in an image; more frequently, a non-georeferenced image is georeferenced to an existing image that already has embedded geographic information, such as a DRG, DLG, or DEM (see Data Resources).Specialized software is generally necessary to complete the process of geocoding. ArcGIS has functionality for this procedure, as do many remote sensing software packages, such as ENVI. For the purposes of GeoPads, the most pressing need for georeferencing are the digital images of geologic maps available from the National Geologic Map Database. The USGS provides a comprehensive tutorial on georeferencing in ArcGIS. Other programs will follow a similar procedure.