Scale - Spatial Mathematics: Theory and Practice through Mapping - page 120

with an aura of uncertainty as some cartographers have proposed (Clarke
and Teague 1998).
5.4.2 Dot density maps: Investigating population change
Examine the map of future population growth and decline ( http://esriurl.
com/popchangedotmap ) and view the legend while looking at Denver,
Colorado. Note that the dot color depends on whether the area is expected
to experience high growth, normal growth, or population decline through
2015. Do you find this map easy or difficult to interpret? Why? The numerous
dots may make it difficult to interpret, but the dots also show trends that
may be hidden by a standard choropleth map. How many persons does each
dot, at this scale, represent? At this scale, one dot represents 180 people.
Change the scale. How does the number of persons per dot change at dif-
ferent scales, and why? As you zoom in to larger scales, each dot represents
fewer people, because you are now examining a smaller area. Click any
area on the map and observe the growth or decline for the area you have
selected from 1990 through 2015 as a bar chart. What patterns do you notice,
and what are some reasons for those patterns? What side of Denver (north,
east, south, or west) is projected to grow the most rapidly through 2015? It
appears that the south and eastern sides of Denver are projected to grow
most rapidly. The west side of Denver's growth is hindered by steep moun-
tain terrain. Pan the map to Detroit. What do you notice about population
change in Detroit versus in Denver? Much of Detroit is losing population,
unlike Denver.
This tool also illustrates fundamental concepts of interpreting dot density
maps that were discussed in this chapter. Reload the dot density population
growth and decline map, which resets to Denver ( Figure 5.8 ). Zoom in to the
text on the map identifying the city of Lakewood, just west of Denver. Zoom
in until you find some lakes just south of Lakewood; for example, Marston
Lake, shown in the following figure. If you zoom in to a certain scale, you will
see dots in the lake. Do these dots represent people living on houseboats?
Why are dots shown in the lake? The dots are there not because people are
living in or on the lake, but because the dots are randomly assigned to the
statistical area that was used, such as census tracts or block groups. The dots
do not represent the actual location where people live.
Examine the essay “Better Census Maps with Dot Density” on http://thewhyaxis.
info/census/. The author reinforces some of the points we are raising in this
chapter about the care that needs to be taken in generating any map, includ-
ing dot density maps. Which of the dot density maps by race and ethnicity do
you find most visually appealing? Why? How might these maps be misleading
in terms of the amount of segregation by neighborhood in the cities depicted?
Great care must be used in interpreting any map. What data were used to color
code the dots? The maps give the appearance of more segregation than what
might actually exist. Can you suggest ways to improve these maps?