Types of RDF-Functions

The basic way in which a RDF is constructed was illustrated above. The diversity of functions used comes from the different ways in which parameters can be measured. The absolute amount of area used in the previous example is only one way to look at the surroundings. Sometimes it is more interesting to use the percentage of a parameter instead (e.g. percentage of woodland). The example from figure 7.2 is treated as such in figure 7.4, showing an RDF-percent-area plot. Here the picture looks quite different from the absolute RDF-area plot shown in figure 7.3. Now there are two equally high modes around 250m and 1000m reaching about 10 percent of the area at the specific distance. The third peak that occurred in the RDF-area plot has almost vanished. In this plot it can also be easily seen that the observation lies in an area with about 30 percent of wood in the closest vicinity.

Figure 7.4: Example RDF percent area plot.

For estimations of resources an animal needs the cumulative amount with respect to the distance is more useful. This is done in figure 7.5.

Figure 7.5: Example RDF cumulative area plot.

The cumulative area plot gives an overview of how much of a resource is available within a certain distance. In figure 7.5 it can be seen that up to a distance of about 700m only little woodland is available. From 700m to about 1200m distance a steep increase is shown and at distances over 1200m the graph again shows a relatively flat slope indicating that travel distances above 1200m provide only a small amount of additional forest areas. In a case where the observation is a nest or sett and the parameter a resource needed by the animal one would expect a large amount of travel distances between 700m and 1200m. As it was the case above with the RDF-area plot, the cumulative versions can also be used in a form using percentage calculations. Figure 7.6 provides an example for the same example area used above. In the previous plots it was becoming obvious that these facts cannot be analyzed and inspected by simply eyeballing the map shown in figure 7.2.

Figure 7.6: Example RDF cumulative percent area plot.

Radial Distance Functions are not limited to polygonal structures. They can also be applied to other elements such as points and linear objects. To provide an overview of possible applications of the concept figure 7.7 shows 23 basic types of RDFs. The figure is divided into three subareas. The upper part contains aspects concerning polygonal structures, the lower right are applications to linear elements and the lower left point objects. The two classes of RDFs mentioned before are drawn in different colors. Black is used for the standard type whereas red is used for the cumulative versions. Green is used for proportional versions. Note that in principle, almost any geometric or thematic variable of relevance for the study of a particular phenomenon could be represented in RDFs.

Figure 7.7: The RDF family. Sample types of RDF plots for polygonal structures, linear elements and point objects. Cumulative functions in red, percentages in green.

From these basic forms of RDFs extensions and variants can easily be derived and used in the same way. I will provide more information on this in sections 7.3 and 7.6.