Overlay

Definition

In Geographic Information System (GIS), an overlay is an essential operation used to merge geographic and attribute data from two or more geographic layers to create a new map layer.

What is Overlay?

Overlay refers to the process of taking multiple map layers and 'overlaying' them onto one another to create a composite map. Each map layer corresponds to a specific theme (such as roads, land use, physical features, administrative boundaries, etc.) and overlay operations enable the integration of these diverse geographic and non-spatial attribute datasets.

The resulting layer incorporates information from both input layers, allowing for an in-depth analysis that considers multiple factors. For example, an overlay operation could be used to determine locations within a flood plane (one layer) that are also within a certain city limits (a second layer).

There are primarily two types of overlay operations in GIS: intersect and union. An intersect operation produces a map layer that includes only areas where both input layers coincide, while a union operation generates a map layer covering all areas from both input layers.

FAQs

What is the purpose of an overlay in GIS?

The principal purpose of an overlay in GIS is to combine information from multiple map layers into a composite map to enable comprehensive analysis or to enhance the understanding of spatial relationships and patterns. By overlaying various map layers, it is possible to analyze different geographic aspects of an area concurrently.

In what industries is the overlay technique in GIS widely used?

The overlay technique in GIS is used in a wide range of industries including urban planning, transportation, environmental management, healthcare, telecommunications, and military. In these industries, overlay operations provide significant insights about spatial relationships and patterns that influence decision-making processes.

What type of data can be used in an overlay operation in GIS?

In overlay operations, both vector and raster data can be used. Vector data includes points, lines and polygons while raster data consists of grid cells or pixels. The types of data to be used depend on the nature of the study or analysis, and the type of GIS software used.

What is the difference between the intersect and union overlay operations?

The intersect overlay operation generates an output layer that includes only areas where both input layers agree or overlap. The output layer only includes features that exist in both input layers. On the other hand, the union overlay operation creates an output layer that includes all areas from both input layers, whether they overlap or not. Features from either of the input layers or both are included in the output layer.