What is the definition of 'overlay' in GIS?

The definition of 'overlay' in GIS refers to a technique used to combine multiple layers of spatial data to identify relationships. This process involves superimposing different datasets, each representing a geographic feature or phenomenon, and analyzing their interactions or correlations. Overlay analysis allows GIS professionals to assess how various layers of information relate to one another, such as geographic features like land use, topography, and hydrology. By using overlay techniques, analysts can produce new insights, such as identifying areas of conflict, suitability for development, or environmental impacts by seeing how different datasets intersect.

The process often includes operations like union, intersection, and difference, and it is fundamental in spatial analysis, enabling better decision-making based on the relationships identified among the layers. For instance, through overlay, one can determine how proximity to bodies of water affects land use decisions or how different demographic groups are distributed across an area. This makes overlay a crucial tool in many GIS applications, from urban planning to environmental management and resource allocation.