Overlay operations

Overlay is defined as a GIS operation that combines the geometries and attributes of input layers to create the output.
The geometry of the output represents the geometric intersection of features from the input layers.
Each feature on the output contains a combination of attributes from the input layers and this combination differs from its neighbours.
Features to be overlaid must be spatially registered and be based on the same coordinate system. In addition, the layers must be in the same zone and should have the same datum.

Feature type and Overlay
Considering the feature type, there are two groups of overlay operations.

1. Using two polygon layers as inputs
2. Using one polygon layer and another layer that contains either lines or points

Hence, overlay operations can be classified as either:

1. point-in-polygon
2. line-in-polygon or
3. polygon-in-polygon

The layer that may be either a point, line or polygon layer is called the input layer and the layer which is a polygon layer is called the overlay layer.

Point-in-polygon overlay operation, the same point features in the input layer are included in the output and each point is assigned the attributes of the polygon within which it falls.

In a line-in-polygon overlay operation, output contains the same line features as the input but each line feature is dissected by polygon boundaries on the overlay layer. This implies that the output layer has more line segments than the input layer.

Each line segment on the output layer combines attributes from the input layer and the underlying polygon.

The most common overlay operation is the polygon-on-polygon overlay operation. This operation involves two polygon layers and the output combines the polygon boundaries from the input and overlay layers to create a new set of polygons.

Each polygon carries attributes from both layers and and these attributes differ from those of adjacent polygons.

Overlay methods are based on boolean connectors (AND, OR and XOR)

An overlay operation is called:

• intersect if it uses AND connector
• is called union if it uses OR connector and
• is called difference or symmetrical difference if it uses the XOR connector
• is called identity or minus if it uses a combination of AND and OR connectors

Union preserves ALL features from the inputs. The area extent of the output combines the area extents of both input layers. The Union overlay method requires that both input layers be polygon layers.

Intersect preserves only those features that fall within the extent common to both input layers. Input layers may contain different feature types. It is a preferred method of overlay as any feature on its output has attribute data from both the input layers.

Symmetrical difference preserves features not common to either of the input layers. It is opposite to the OR connector in terms of output area extent.

Identity preserves only features that fall within the area extent of the layer defined as the input layer.

The overlay operations combine features and attributes from input layers and the output is useful for query and modeling purposes.