Geographic information systems (GIS) are software tools for storing, retrieving, mapping, and analyzing geographic data. Because paper maps depict geographic phenomena in two dimensions (2D), GIS were originally designed to handle 2D data and present visual images in 2D. The digital maps created and visualized with early GIS are thus basically similar to conventional paper maps. For several reasons, there has been dramatic development in three-dimensional (3D) GIS in the past 15 years or so.