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Vector based GIS Overlay
Classification of Vector Overlay Operations

Vector based GIS Overlay

In a vector-based system, overlay operations are much more complex than in a raster-based system. This is because the topological data is stored as points, lines and/or polygons. This requires relatively complex geometrical operations to derive the intersected polygons, and the necessary creation of new nodes (points) and arcs (lines), with their combined attribute values.

In a vector-based system, topological map overlay operations allow the polygon features of one layer to be overlaid on the polygon, point, or line features of another layer. Depending on the objectives of the Overlay operation, different output features can result.

Classification of Vector Overlay Operations
Topological vector overlay operations can be classified via two methods:

1. Through the elements contained in the layers to be overlaid (ie. whether the layers contain point, line or polygon elements), or

2. By operation type (for example; the user wants to generate a layer comprising of the Union, Intersection, or some other boolean operation of the two input layers).

When classifying the vector overlay operation via method one, the element types each layer contains are considered.

The following table identifies which overlay options exist for each possible combination of element types contained in the two input layers.

Complex databases such as GIS classify vector overlay operations via method two, where the particular overlay operation a user wishes to perform defines which element types may be contained in the two input layers.