Buffering in Geographic Information Systems – draft version1
Nagapramod Mandagere
Buffering in general refers the creation of a zone of a specified width around a point or a
line or a polygon area. If is also referred to as a zone of specified distance around
coverage features. In general there are two types of buffers, namely – constant width
buffers and variable width buffers. Both types can be generated for a set of coverage
features based on each features attribute values. These zones or buffers can be used in
queries to determine which entities occur either within or outside the defined buffer zone.
Analogous to buffering in raster GIS is distance analysis.
Buffering point data is the simplest form of buffering which involves creation of a
circular polygon about each point of radius equal to the buffer distance. In this scheme
the buffer distance or the radius of the circle could be fixed for all points in a layer or the
user could specify it. If multiple points in the same layer are being buffered, then the
buffering algorithms check for overlaps in each point’s buffer and remove the
overlapping sections. This process of removal of overlapping sections involves the use of
intersection and dissolves.
Buffering of lines is a little more involved than buffering point data. This is
complicated even more due to the fact that lines can be made up of multiple segments.
Line segments are handled independently of each other. For each line segment, dx and dy
between the end points are computed. Using these, two new parallel buffer lines are
identified. Once these parallel buffer lines have been identified for the next segment, the
line intersections of the parallel buffer lines of each segment are calculated and new
coordinates are assigned to the common vertices. After that the simplest line intersection
test can be used.
Lastly, the ends of the buffer are defined at the lines start and end points. Once the
polygon is generated, again the overlapping sections must be removed using intersection
and dissolve. Buffering of polygonal surfaces uses most of the same concepts used for
line buffering. The only significant change is that the polygon buffer is created on only
one side of the line that defines the polygon. Most GIS software make use of outside
polygon which surrounds the area under consideration, but some make use of an inner
polygon that lies inside the area under consideration.
Sources:
[1] Tutorials on Topics in GIS:
http://www.sli.unimelb.edu.au/gisweb/BuffersModule/BuffSelect.htm
[2] Glossary and definition of key terms in GIS: http://en.mimi.hu/gis/buffer.html