«Ian McHarg is widely credited with inventing the layer model that was adopted in geographic information systems (GIS) (e.g. Goodchild 1992). In his seminal book, “Design With Nature”, McHarg (1992) used semi-transparent map overlays for “suitability selection processes”, including highway route selection. His printed map transparencies represented social and environmental values around potential routes by degrees of grey shades. The overlay resulted in sensitive areas being “masked” through multiple overlaid maps while more suitable areas remained translucent. Subsequently, the computational overlay of maps has been formalized in map algebra and cartographic modeling (Berry 1987, Tomlin 1990).»