Design, abstraction, and legibility
Chapter 2: What Crime Maps Do and How They Do It

Map design and abstraction are inseparable. The map design defines the level of abstraction to be imposed. "To represent is to abstract," wrote Muehrcke, and "abstraction frees us from the tyranny of our physical existence" (1996, p. 275). He presumably meant that it gives us license to, so to speak, "mess with reality." Many of the issues that concern cartographers, such as the degree of distortion on world maps, are of little concern to crime mappers. Where should we focus our attention when it comes to thinking about abstraction in our maps? What can we afford to ignore? Are any map elements indispensable on most crime maps?

Abstraction, the reduction of detail on maps, permits us to design our maps in ways that make them attractive and effective. Abstraction is like the sculptor's chisel—it determines what remains of the raw material and what form the finished product will take. As noted earlier, most map elements are dispensable at one time or another, depending on the context.

First and foremost, the analyst must consider the audience and the medium of presentation. Will the map go to one person and be seen at arm's length? Will it be a page in a report? (If so, will it be in color? How would the map look if it was converted to gray scale?) Will it become a transparency for an overhead projector or a 35-mm slide? Will it be incorporated into a digital projector production in Microsoft PowerPoint or comparable software?

If a map is to be projected, lettering size and line weight become quite critical. You may have a brilliant map with potentially great visual impact, but if two-thirds of the audience can't read the lettering when it is projected, your creativity is wasted.10 Also consider the "demographics" of the audience to be addressed: are they younger? older? more educated? less educated? predominantly female?

If the audience is not similar to the general population, some adjustments in map design may be needed. Research has shown, for example, that there may be subtle differences in the way men and women read maps (Kumler and Buttenfield, 1996).

This begs the corollary question of exaggeration in maps to gain legibility. Sometimes detail must be retained, but this may result in objects running together owing to the thickness of the lines representing them. Line thickness may need to be adjusted and objects may need to be moved slightly to maintain visual separation. Bear in mind that line work on maps often greatly exaggerates the true dimensions of linear features. A typical State highway map may be used as an example. On this map, interstate highways are 1/16th of an inch wide. The representative fraction (RF) of the map is 1:380,160, or 6 miles to the inch, which represents a width of 660 yards. This is probably, on average, double the width of most interstate highways. By comparison, area features such as a city block, a city, or a county, should be accurately rendered because exaggeration is not needed to make them visible.

Even point data generally exaggerate the size of the location at which a crime incident occurred or the address of the victim or offender. Point symbols are actually markers for general locations and should be interpreted as approximations owing to (a) the size of the point symbol and (b) normal problems with address interpolation touched on in chapter 4 of this guide. It is tempting to see point data as the epitome of accuracy, but this accuracy is relative.

Map Design Questions to Consider

  • Is this the best map for the stated objective?
  • Is the scale appropriate?
  • Does the design account for both data representation and aesthetics?
  • Could a flowchart ensure the inclusion of all necessary elements?
  • Are the sources of data, authors of the map, and date of preparation shown?
  • Are balance, focus of attention, and internal organization considered?
  • What colors work best?
  • Is the map legible in all the contexts in which it will be used (print, slide, fax, PowerPoint®, and overhead transparency)?

Crime mappers might consider using more perspective symbol landmarks or mimetics11 to help readers orient themselves, particularly in metropolitan areas (see figures 1.2, 3.25, and 5.9). These symbols are pictorial and characterize the landmarks they represent, such as the use of an airplane symbol to represent an airport or silhouettes of familiar structures, such as a school, church, ballfield, cathedral, city hall, or highrise tower. Such pictorial devices are more important for lay audiences than for police officers, who are familiar with the area, although the assumption that all cops are equally familiar with entire cities or metro areas is a fallacy. We—even cops—are victims of our daily routines and the neighborhoods they take us through. None of us can comprehend entire metropolises, at least not at the level of street name familiarity.

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