Introduction to Overlay: Overlay with Vector Data (CAD and Topological) (Especially for GATE-Geospatial 2022)

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Overlay is a GIS operation that superimposes multiple data sets 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 and raster data.

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An overlay operation combines the geometries and attributes of two feature layers to create the output. Feature layers to be overlaid must be spatially registered and based on the same coordinate systems.

Overlay with Vector Data

Feature overlays from vector data are created when one vector layer is merged with one or more other layers covering the same area with points, lines, and 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.

Types of Vector Overlays

There are mainly two types of vector overlay:

Diagram Shows Two Types of Vector Overlay

1. CAD – Type Overlay

CAD type overlay is first and simplest method of vector overlay. To perform simple operations of overlay on vector data, the computer requires only that the software be able to keep track of the location of each point, line and polygon to ensure that the entire coordinate can be represented on the screen. The result of this operation is a visual graphic and not a GIS layer.

2. Topological Vector Overlay

Topological overlay is also known as Least Common Geographic Unit (LCGU) , specifically to show how the changes in polygonal entities can eventually reach a point at which no further divisions are possible. It is also divided into three categories:

  • Polygon-on-polygon overlay,
  • Line-on-polygon overlay,
  • Point-on-polygon overlay

Topological overlay is a process whereby separate layers sharing the same spatial extent are merges in different ways. In topological overlay, we need software that is able to locate the coordinate of each point, line and polygon. In short, the program must be able to determine whether this point is located within the polygon.

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