Examples of thematic maps
Chapter 2: What Crime Maps Do and How They Do It

Perhaps the best way to get a feel for the kinds of maps used to display crime data is to look at examples and to think about why each type of map was selected. A good place to start is the Web site of the National Institute of Justice Crime Mapping Research Center (http://www.ojp.usdoj.gov/nij/maps/), which provides links to police departments across the United States. Another useful Web site is maintained by Hunter College in New York (http://www.geography.hunter.cuny.edu/capse/projects/nij/crime.html). (See the appendix for additional information.)

Thematic maps using point symbols: The dot map

When should point symbols be used? The first prerequisite is that you have locational detail—information specific to your points, such as street addresses or coordinates in latitude/longitude or some other system, such as State Plane (explained in chapter 1). The second prerequisite is that the audience needs locational detail. If you have point data, but the audience wants information summarized by patrol areas or neighborhoods, then the point data can be added up, or aggregated, to the areas of interest. Examples of point, or dot, maps are shown in figures 2.3 and 2.4.

Figure 2.3

Figure 2.4

When there are too many points to be mapped, using point data may result in a mess of superimposed points that have little or no meaning. This could happen if calls for service are mapped using addresses in a large city. The point data may need to be summarized by areas to make the data legible. Point maps also get too crowded if long time periods are summarized for more frequent crime categories. Thus, even though you have reasonably precise locational information, aggregation by areas in the form of a choropleth map may yield a more legible map than the presentation of each individual point.

Thematic maps using statistical symbols

At its most primitive, a statistical map consists of raw numbers written in the subdivisions of the map. The advantage is that the reader can see exactly what the statistic is. The downside is that maps designed in this way are difficult to read quickly. It could be argued that, in effect, they defeat the purpose of the map, which is to facilitate visualization of the data. Admittedly, this form of map does put data in its geographic context, but in an inconvenient format. Cartographers argue that if you want to see only the raw numbers, then a table, not a map, is needed (see next section, "Thematic maps using area symbols").

Statistical symbols commonly take the form of pie charts, bar charts, graduated circles, or dots representing incident counts (dot density) placed in the relevant map subdivisions (figure 2.5). This allows multiple variables to be mapped at the same time. Examples could include bar charts with bars representing both crime and poverty or graduated circles like those in figure 2.6, showing the U.S. House of Representatives vote on an Omnibus Drug Bill provision requiring a 7-day waiting period for the purchase of handguns. At first glance, the symbols in figure 2.6 look like pies, but circle segments are all 90 degrees. This is actually a graduated circle map, in which the area of the 90-degree segments is proportional to the number of yes votes (top part of the circle) or no votes (bottom part of the circle), with the left side of the circle representing votes by Democrats, the right by Republicans.1 The map shows both nominal data (party affiliation and yes/no votes) and quantitative data (the number of votes), as well as location of votes by State. Although reading this map takes some effort, it is rich in information and gives that information a clear geographic context.

Figure 2.5

Figure 2.6

More typical graduated symbol maps used in crime analysis applications are shown in figures 2.7, 2.8, 2.9, and 2.10. Note that points and proportional circles can be combined if this helps convey the essential information to readers and avoids overloading the map. Note also that the size range for symbols is a judgment call. If the size range is too small, readers will have difficulty extracting meaning from the map. Also, some symbols are more effective than others in conveying the message. Solid symbols probably work best in most cases because they engage the eye more effectively.

Figure 2.7

Figure 2.8

Figure 2.9

Figure 2.10

A disadvantage of using statistical symbols on maps is that they may overlap one another and result in an illegible mess. Map design must take into account the final size of the map, the scale to be used, and the possibility of overcrowding.

The use of statistical devices of various kinds on maps is limited only by the analyst's imagination. For example, it may be useful to accompany a map with a scatter diagram showing a collateral relationship, such as calls for service by time of day (chart) and calls for service by location (map). Mapping software offers numerous possibilities because the programs usually can make both charts and maps and combine them in layouts in useful ways.

Chapter 2: What Crime Maps Do and How They Do It
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Mapping Crime: Principle and Practice, by Keith Harries, Ph.D., December 1999