Overlay (in GIS)

Overlay is a general method for analysis of co-occurring geographic phenomena. Traditional maps are in a sense a product of an overlay process where separate information layers such as roads, houses, and types of land have been put together on top of each other. In GIS the overlay process has been developed into an analytical tool where various forms of descriptive, deductive, and inductive analyses can be performed on the data layers. As a noun the term overlay also often refers to the individual layers in such analyses. Important applications of overlay analysis include change analysis, spatial data accuracy assessment, and various forms of multicriteria evaluations.