Overlay is a GIS operation that superimposes multiple data sets (representing different themes) together for the purpose of identifying relationships between them.[2]. An overlay creates a composite map by combining the geometry and attributes of the input data sets. Tools are available in most GIS software for overlaying both Vector or raster data.
Explanation:
Before the use of computers, a similar effect was develioped by Ian McHarg and others by drawing maps of the same area at the same scale on clear plastic and actually laying them on top of each other.
