The concept of geographic information systems (GIS) was introduced to cover all the essential needs of the scientific community to provide spatial information, analyze data, and create digital thematic maps through a computer. A fundamental for GIS is the overlaying of different kinds of information and data. Professor Ian L. McHarg (1920–2001) was one of the first scientists who described the GIS concept and the usefulness of map overlaying. In 1954, he took a position as Professor of Landscape Architecture and Regional Planning at the University of Pennsylvania and during the mid-1960s he came to be recognized worldwide for introducing ecological concerns into land planning (McHarg 1992). He introduced the concept of overlaying (Fig. 1.1) and highlighted the four basic principles (measuring, mapping, monitoring, and modeling) that founded GIS.