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Map OverlayContents |
Background
Overlay operations are some of the most fundamental and most used processes in GIS applications. Overlay operations provide the ability to integrate separate feature classes together into new feature classes that represent the combination of two inputs. There are two types of overlay operators:
- Arithmetic operators are used to add, subtract divide or multiply values in one layer by values in the corresponding location in the second layer.
- Logical operators are used are used to identify a specific set of conditions in one layer with a feature or condition in another layer. Logical operators use Boolean functions to operate on the spatial and attribute properties of the features. The four types of Boolean algebra operators uses in GIS are AND, OR, XOR, NOT to see whether a particular condition is true or false.
Vector overlay can be performed on point line or polygon features.
Point in Polygon Overlay
This method is used to find which points fall inside which polygons. The attributes from the polygon overlay are added to the point attribute table of the output layer. In this example the Boolean operator NOT is used to find the point features that are not in the polygon layer.
Line in Polygon Overlay
This method is used to find the common space between a line feature class and a polygon feature class. The arcs from the input polygon coverage are split at the overlay coverage polygon boundaries and new lengths are calculated for these arcs. Attributes from the polygon overlay are added to the attribute table for the new arcs. In this example the Boolean operator AND is used to find features that common to both layers.
Polygon on Polygon Overlay
This method is used to find the common area between two polygon feature classes. The polygons of the input coverage are split at the overlay polygon boundaries and new arcs are calculated for the polygons of the input coverage. Attributes from the polygon overlay are added to the polygon attribute table. In this example the Boolean operator OR is used to find features that are in both layers.
Buffering
The buffer operation is a frequently used spatial analysis tools. A buffer is useful for determining whether a point/line/polygon falls within a certain distance of another point/line/polygon. The buffer operation creates a new polygon layer, where a specified distance is drawn around specific features within a layer. The buffer is used to identify and quantify areas within a specified distance of existing features. The distances can either be constant or can vary depending upon attribute values. In the example below, a buffer layer has been created 200 nautical miles from the coastline to identify the EEZ boundary.
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Information about this article
Short title: Overlay
Description: Overlay operations are some of the most fundamental and most used processes in GIS applications
Expertise level: beginner
Author: Greg Reed
Approval status: approved
Approved by: Greg Reed
Last change: 2010-4-22
Subsection of: Spatial Analysis
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