How to use Negative Distances in your Geospatial Buffer Analysis

Geospatial buffer analysis involves generating a polygon at a specified distance from a proposed feature to determine proximity. For example, a 100m buffer around a point feature creates a circle with a 100m radius. Similarly, a 50m buffer around a polygon feature creates a larger polygon – a polygon that encompasses the source feature.

However, did you know that you can also use a negative distance to create a buffer? That’s right! When you use a negative distance, such as -25m, to buffer a polygon feature, the result is a smaller polygon – a polygon that is inside the source feature.

Consider the following example where the objective is to determine the area of shoreline habitat available for a certain species of wildlife. The species in question has been observed not to forage beyond 100m from shore and includes a riparian buffer of 25m as part of its habitat. To compute the total area of habitat available to this species requires the creation of two buffers and a polygon overlay operation. The first buffer (ie BufferOutside25) is created at a distance of +25m and represents the riparian buffer. The second buffer (ie BufferInside100) is created at a distance of -100m and represents the forage extent. A polygon overlay analysis of type subtract is then used to determine the zone of available habitat where:

HabitatZone = BufferOutside25 – BufferInside100

The total area of habitat available is equal to the area of the zone of available habitat (ie HabitatZone).

Check out the video to see the process in action with AutoCAD Map 3D.