One of the most basic questions asked of a GIS is "What's on top of what?" For example:
- What land use is on top of what soil type?
- What parcels are within the 100-year floodplain? ("Within" is just another way of saying "on top of.")
- What roads are within what counties?
- What wells are within abandoned military bases?
In GIS, these What's on top of what? questions are answered with the use of overlay tools.
The Overlay toolset contains tools to overlay multiple feature classes to combine, erase, modify, or update spatial features, resulting in a new feature class. New information is created when overlaying one set of features with another. There are six types of overlay operations; all involve joining two existing sets of features into a single set of features to identify spatial relationships between the input features.
| Tool | Description |
|---|---|
Generates planarized overlapping features from the input features. The count of overlapping features is written to the output features. | |
Creates a feature class by overlaying the input features with the polygons of the erase features. Only those portions of the input features falling outside the erase features outside boundaries are copied to the output feature class.  | |
Computes a geometric intersection of the input features and identity features. The input features or portions thereof that overlap identity features will get the attributes of those identity features.  | |
Computes a geometric intersection of the input features. Features or portions of features which overlap in all layers and/or feature classes will be written to the output feature class.  | |
Joins attributes from one feature to another based on the spatial relationship. The target features and the joined attributes from the join features are written to the output feature class. | |
Features or portions of features in the input and update features that do not overlap will be written to the output feature class.  | |
Computes a geometric union of the input features. All features and their attributes will be written to the output feature class.  | |
Computes the geometric intersection of the Input Features and Update Features. The attributes and geometry of the input features are updated by the update features in the output feature class.  |