Generating Features Based on Proximity: Buffering
Proximity is a word that implies nearness in terms of physical distance. It is not quite the inverse of distance, but greater proximity implies smaller distance. Proximity is a concept that strongly affects our lives and activities. We, along with most of the animals of higher intelligence, innately understand the idea. We want to be close to pleasant and useful things, or to those things we must access on a regular basis, such as friends, places of employment, and shopping. We want to be far from those things that are unpleasant or noxious, like smelly dumps or plants, irritating people, or dangerous environmental conditions. Much of the law regulating land use is written with the concept of proximity in the background—it sets limits of acceptable proximity, usually as a threshold distance. An example would be a law that says a liquor store may not exist within 500 feet of school grounds.
The primary analytical tools of GIS that implement the concepts of proximity create what are called buffers. To create a buffer, a set of features, say X, is specified. A distance, say Y, is specified. The software then generates a buffer: a set of areal features that have the property that every point on or within their borders lies within the specified distance of the original features. Formally, the buffer consists of the locus of points whose distance from each of the features in X is less than or equal to the specified distance Y.