Analysis of GIS Data by Simple Examination - Introducing Geographic Information Systems with ArcGIS - page 380

from one end to the other. Click OK, then Apply, then OK. Observe. You might find the
display in the T/C confusing. That's because the “-” sign is used both as a minus and as a
separator.
Natural Breaks 5
To understand “natural breaks” (also known as the Jenks method), you need to comprehend the
idea of “variance from a mean.” If you have a cluster of values, they have a mean, or average, value.
If you take each value and subtract the mean from it, you get another set of numbers. The variance
used by the Jenks approach is calculated by squaring each of these new numbers, taking their sum,
and dividing by the number of numbers. You can see that the variance would be larger the further the
original values were from the mean. So the variance is an indication of the dispersal from the mean
of the set of values. If we had, say, five clusters of numbers, each with its own variance, we could
add those five variances together to get a total variance. The idea behind the Jenks approach is to
minimize the total variance by moving values around from cluster to cluster. At the end of the process,
it should be the case that no value can be moved from one class to another without raising the total
variance. This method works so well that it is the default that ArcMap uses. It is illustrated by using a
different data set.
17. In Layer Properties > Labels, change Label Field to WOMBATS (click Apply) and, under the
Symbology tab change Value Field to WOMBATS. Change the Show box to Categories-Unique
Values. Click Add All Values. (Make sure WOMBATS is chosen in the Value Field) Click Apply,
then click OK. Look at the quilt. Open the attribute table and sort the WOMBATS field into
ascending order. Note that there are seven groups of values (100s, 200s, and so on).
Each group has five values, except for the first group, which has six, and the second group,
which has four.
18. Go to the Layer Properties window and pick Quantities > Graduated Colors. Now classify areas
according to the number of WOMBATS contained in each, using seven categories and the
equal interval method. Looking at the Classification window, you can note that in two
categories two groups are placed together. Also note that there are two empty classes.
Display the labeled map.
19. Classify according to septiles (quantiles, seven classes), showing class ranges using feature
values. Show the labeled map. Look at the ranges shown in the Table of Contents. What is the
problem with this method? _________________________________________________________
_________________________________________________________________________________
20. Classify according to Natural Breaks, using seven classes. Problem of inappropriate class
ranges solved? _____
5 A method first published by Professor George Jenks, Department of Geography, University of Kansas. If you use
the Natural Breaks method in ArcGIS check your answers carefully. There are differences between the results
produced by Natural Breaks and other Jenks programs.