Visualizing Hierarchies - Spatial Mathematics: Theory and Practice through Mapping - page 168

that you have no records selected. A “join” in GIS usually operates on only the
selected records, and if none are selected, the join works on all of the records.
Therefore, it is important not to have any selected before the join is performed.
Right click on the tgr08031grp00 map layer in the table of contents and use
the “Joins and Relates” function. Join from the data table to the map attribute
table so you can map the demographic data for Denver County block groups.
Refer to ArcGIS Help if you need to. After joining your tables, right click on
your tgr08031grp00 map layer again, and using Data → Export Data, export
your joined data to a permanent data file named DenverCountyBlockGroups.
Save your ArcMap document.
For your new DenverCountyBlockGroups layer, symbolize it as a graduated
color thematic map of population 18 to 21 years old. Next, symbolize the
block groups as a graduated color thematic map of population 18 to 21 years
old normalized by the 2000 population of the block group.
What does normalizing the data do? Normalizing divides one variable by
another; in this case, normalizing in effect creates a map showing 18 to 21
year-olds as a percentage of the total population of the block group.
Describe the resulting geographic pattern of percentage of 18 to 21 year-olds
by block group in Denver. The pattern shows generally low percentages of
18 to 21 year-olds, except for the west side of Denver, and near the three uni-
versity campuses. It will be much easier to solve your problem if you create a
field that contains the actual percentage of 18 to 21 year-olds by block group.
To do this, you will need to add a field and calculate the value of the new field
that represents the percentage 18 to 21 year-olds in each block group. Name
the new field p1821, make it a float (floating point) data type, and give it a
precision (width) of 5 and a scale (number of decimal places) of 2.
You may receive a VBA (Visual Basic for Applications) overflow error. If so,
cancel the calculation and examine the table at the row where the percentage
vacant field stopped calculating. Why did the calculation stop working at the
point where it did? Hint: Is it possible to divide by zero? The calculation may
stop when it encounters a division by zero. One of the block groups has zero
population (an industrial or military block group), and at this point where
0 was in the denominator, the calculation stopped. How can you select cer-
tain records in the table so this problem will not occur again? You can select
block groups where the total population was greater than zero. After select-
ing only those records where the block group population is greater than zero,
recalculate.
Make a thematic map of your new field p1821. Indicate the sections of Denver
where the highest percentage of 18 to 21 year-olds are found. Next, make a
pie chart map where the two parts of each pie represent the following: In
the first part, the percentage of 18 to 21 year-olds, and in the second part,
the total population in each block group. Experiment with the size and the
overlap functions.