Count overlapping polygons - an ArcView GIS extension

Description

This extension counts the number of polygons in one theme (the polygon distribution theme) that overlap each of the polygons in another theme (the polygon grid theme).

The number of overlapping polygons is added into a new field in the polygon grid theme, which is then displayed in the View with a suitable legend.

This extension was designed to count the number of different animals that are found in a series of polygon grid squares based on home range data. It can also be used to calculate species richness for each grid square based on a series of species distribution polygons.

Click here to download the extension from the ESRI website.

For more information please contact Bob Smith.

Notes

1) The polygons in the polygon grid theme do not need to be squares - they can be any shape that is required. If you need to create a hexagon grid theme then visit the ESRI website to download the Hexsample script. If you need to create a grid square theme then download the Draw Vector Grid extension.

2) Make sure that different polygons describing the distribution of the same element (eg the same species or same individual) are associated with one record in the polygon distribution theme table. Use the "Dissolve" option in ArcView's Geoprocessing Wizard to convert from theme where each polygon is associated with a separate record to a theme where all of the polygons describing the distribution of the same element are associated with the same record.

Examples of applications

This extension has been used to:

• Calculate black rhino densities from home range data collected in Namibia.
  
  
• Map species richness patterns for freshwater taxa in East Africa.