Maps and aerial photographs in hard copy have a lot of valuable data on them. When this data needs to be brought into a GIS, they are digitally scanned to produce raster imagery. The output of a digital scanner has a coordinate system, but it is a local coordinate system created by the scanning process. The scanned imagery needs to be georeferenced to a real-world coordinate system before it can be used in a GIS.
Georeferencing is the process of transforming the coordinate reference system (CRS) of a raster dataset into a new coordinate reference system. Often, the process transforms the CRS of a spatial dataset from a local coordinate system to a real-world coordinate system. Regardless of the coordinate systems involved, we'll call the coordinate system of the raster to be georeferenced the source CRS, and the coordinate system of the output the destination CRS. The transformation may involve shifting, rotating, skewing, or scaling the input raster from the source...