Overlay

Overlay is a GIS operation that superimposes multiple data sets (representing different themes) together for the purpose of identifying relationships between them. . 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.

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.

Overlay with Vector Data
Feature overlays from vector data are created when one vector layer (points, lines, or polygons) is merged with one or more other vector layers covering the same area with points, lines, and/or polygons. A resultant new layer is created that combines the geometry and the attributes of the input layers.

An example of overlay with vector data would be taking a watershed layer and laying over it a layer of counties. The result would show which parts of each watershed are in each county.

Polygon Overlay Functions


Various GIS software packages offer a variety of polygon overlay tools, often with differing names. Of these, the following three are used most commonly for the widest variety of purposes:


 * Intersection, where the result includes all those polygon parts that occur in both input layers and all other parts are excluded. It is roughly analogous to AND in logic and multiplication in arithmetic.


 * Union, where the result includes all those polygon parts that occur in either A or B (or both), so is the sum of all the parts of both A and B. Different from identify in that individual layers are no longer identifiable. It is roughly analogous to OR in logic and addition in arithmetic.


 * Subtract, also known as Difference or Erase, where the result includes only those polygon parts that occur in one layer but not in another. It is roughly analogous to AND NOT in logic and subtraction in arithmetic.

The remainder are used less often, and in a narrower range of applications. If a tool is not available, all of these could be derived from the first three in two or three steps.


 * Symmetric Difference, also known as Exclusive Or, which includes polygons that occur in one of the layers but not both. It can be derived as either (A union B) subtract (A intersect B), or (A subtract B) union (B subtract A). It is roughly analogous to XOR in logic.


 * Identity covers the extent of one of the two layers, with the geometery and attributes merged in the area where they overlap. It can be derived as (A subtract B) union (A intersect B).


 * Cover, also known as Update, is similar to union in extent, but in the area where the two layers overlap, only the geometry and attributes of one of the layers is retained. It is called "cover" because it looks like one layer is covering the other; it is called "update" because its most common usage is when the covering layer represents recent changes that need to replace polygons in the original layer, such as new zoning districts. It can be derived as A union (B subtract A).


 * Clip contains the same overall extent as the intersection, but only retains the geometry and attributes of one of the input layers. It is most commonly used to trim one layer by a polygon represent an area of interest for the task. It can be derived as A subtract (A subtract B).

It is important to note that these functions can change the original polygons and lines into new polygons and lines and their attributes.

Overlay with Raster Data
Raster overlay involves two or more different sets of data that derive from a common grid. The separate sets of data are usually given numerical values. These values then are mathematically merged together to create a new set of values for a single output layer. Raster overlay is often used to create risk surfaces, sustainability assessments, value assessments, and other procedures. An example of raster overlay would be to divide the habitat of an endangered species into a grid, and then getting data for multiple factors that have an effect on the habitat and then creating a risk surface to illustrate what sections of the habitat need protecting most.

GIS&T Body of Knowledge Concepts
Overlay is covered in section AM4-2 in the 2006 GIS&T Body of Knowledge.