Thus, a GIS can use combinations of mapped variables to build and analyze new variables. Using GIS technology and water company billing information, it is possible to simulate the discharge of materials in the septic systems in a neighborhood upstream from a wetland. The bills show how much water is used at each address. A property ownership map might be at a different scale from a soils map. Map information in a GIS must be manipulated so that it registers, or fits, with information gathered from other maps.
Projection is a fundamental component of mapmaking.
Different projections are used for different types of maps because each projection is particularly appropriate to certain uses. For example, a projection that accurately represents the shapes of the continents will distort their relative sizes. Since much of the information in a GIS comes from existing maps, a GIS uses the processing power of the computer to transform digital information, gathered from sources with different projections to a common projection. So a GIS must be able to convert data from one structure to another. Raster data files consist of rows of uniform cells coded according to data values.