Analysis of GIS Data by Simple Examination - Introducing Geographic Information Systems with ArcGIS - page 377

FIGURE 6-15
5. Look at the histogram (the graph with the gray columns). These columns represent the number
of values that are in the vicinity of abscissa (x-coordinate) values. Change the number of
columns to 20. By looking at the vertical axis, indicate how many values (around 1003) are
represented by the first column on the left. ________ How many by rightmost column? ________
(Notice the tick marks and reference numbers at the bottom of the histogram. The range of the
data set is 24031 (that's 25034 minus 1003). (Regardless of how many classes you asked
for the numbers and tic marks divide the range of the data set up into intervals of fourths.)
Concentrate on the numbers at the top of the histogram; those are the numbers where the
class on the left ends and the next one begins.
6. Examine the Break Values box. These numbers—4436, 7869, . . ., 25034—represent the
top end of each category. They divide the range of the data set (that's 24031) up into seven
equal subranges. If you click one of these numbers, the break line separating its class and the
class above it, turns red. At the bottom of the window, you can read the number of numbers
(elements, as they are called) in that class. How many numbers are in the class whose top
value is 7869? ________
By clicking the % button (next to Break Values), determine the percentage of elements that fall
in the first four categories. ________. Press % again to return to the numerical break values.
7. Make sure the Classification Method is still Equal Interval. Change the number of
classes to 5.
User Selection of Classes
8. Notice each vertical blue line. The value at its top end indicates the break value as well. So, the
first category consists of 23 values, 19 of which are at the lower end (look at the height of the
column), and 4 of which are toward the upper end of the category. The range of this category is
from 1003 to 5809.