A place in history: a guide to using GIS in historical research

CHAPTER 4: BASIC GIS FUNCTIONALITY: QUERYING, INTEGRATING AND MANIPULATING SPATIAL DATA

4.4 Buffering, Thiessen polygons and dissolving

There are times when rather than simply being interested in the locations of a type of feature, a user is interested in the locations within a set distance of a feature. Examples of this might include wanting to know all areas within (or outside of) 1km of a hospital, or areas within 10km of a railway line, or within 5km of an urban area. Where information of this type is required a buffering operation is used. Buffering takes a point, line, or polygon layer as input and produces a polygon layer as its output as is shown in Figure 4.2.

Figure 4.2: Buffers around points, lines and a polygon
In all cases the buffer polygons are shaded light grey while the input features are in black.

Figure 4.3: Thiessen polygons
The input layer is the set of points. From these, Thiessen polygons are defined in which each polygon defines the area closest to the point that lies within it. The boundary lines are thus lines that are equidistant from two points.

A user may also want to allocate catchment areas to a point dataset, and this can be done by generating Thiessen polygons (also known as Voronoi polygons). This creates a polygon layer in which the polygon boundaries are lines of equal distance between two points. This means that a polygon is the area that is nearest to the point that generated it, as is shown in Figure 4.3. This is a simple form of interpolation, whereby data are allocated from one set of spatial units to another.

Figure 4.4: Dissolving to aggregate polygons

Another option occurs where a user wants to create aggregate polygons from a more detailed layer. An example of this might be where a user has a polygon layer where each polygon represents a farmer's field with attribute data that includes crop type. If the user is only interested in where particular crops are grown then many field boundaries represent redundant information that can be removed. This is done by what is called a dissolve operation whereby the boundaries of adjacent polygons with the same crop type are removed to form aggregate polygons. This is shown in Figure 4.4.