Editing Geographic information system (section)

===Map overlay===
{{main | Vector overlay | Map algebra }}
The combination of several spatial datasets (points, lines, or [[polygons]]) creates a new output vector dataset, visually similar to stacking several maps of the same region. These overlays are similar to mathematical [[Venn diagram]] overlays. A [[union (set theory)|union]] overlay combines the geographic features and attribute tables of both inputs into a single new output. An [[intersection (set theory)|intersect]] overlay defines the area where both inputs overlap and retains a set of attribute fields for each. A [[symmetric difference]] overlay defines an output area that includes the total area of both inputs except for the overlapping area.

Data extraction is a GIS process similar to vector overlay, though it can be used in either vector or raster data analysis. Rather than combining the properties and features of both datasets, data extraction involves using a "clip" or "mask" to extract the features of one data set that fall within the spatial extent of another dataset.

In raster data analysis, the overlay of datasets is accomplished through a process known as "local operation on multiple rasters" or "[[map algebra]]", through a function that combines the values of each raster's [[matrix (mathematics)|matrix]]. This function may weigh some inputs more than others through use of an "index model" that reflects the influence of various factors upon a geographic phenomenon.