What crime maps do
Chapter 2: What Crime Maps Do and How They Do It

Maps are often thought of solely as display tools. In fact, maps have a wide-ranging role in the process of research, analysis, and presentation. Mapping is most effective when those broad capabilities are recognized and used to their fullest extent. The map is the end product of a process that starts with the first-responding officer's report that is processed by data entry personnel, entered into a database, and transformed into a symbol on paper. In this narrow interpretation, a map is merely a picture or part of a database. But maps can be useful in other ways. MacEachren (1994) and MacEachren and Taylor (1994), following DiBiase (1990), noted the distinction between visual thinking and visual communication in the use of maps and graphics (figure 2.1).

Figure 2.1

Visual thinking

In visual thinking, the map is used to generate ideas and hypotheses about the problem under investigation. By inspecting a map, for example, we may notice a relationship, or correlation, between environmental factors that otherwise might have gone unnoticed. This correlation may be vertical in the sense that we see connections between different phenomena, such as crimes, land uses, and demographics. Alternatively, we may see a horizontal relationship in which we recognize a common factor across a particular crime type, such as graffiti in similar types of crime locations. Visual thinking is a private activity involving exploration and confirmation.

In the exploratory phase, maps may be crude and are not intended for display or publication. A computer-printed map of burglary patterns for the most recent week might be marked with handwritten information provided by investigators or with other data not in digital form. Information might be transcribed from a mental map to a paper map. Another possibility is that the tools of exploratory spatial data analysis (ESDA) are used to find anomalies in data, such as an unexpected cluster of incidents, that could point to unexpected relationships.

Visual Thinking versus Visual Communication

  • Visual thinking is abstract and internal. Some ideas for putting data into maps, charts, or other media may never see the light of day.
  • Visual communication is a tangible expression of visual thinking. It is putting thoughts about data and processes into a format others can see and understand with minimal effort.

At this stage the analyst may generate a formal hypothesis, or educated guess, to explain the process producing the observed crime pattern. Did the observed cluster of burglaries pop up by chance? Is there some recognizable cause? Is a serial burglar operating in the area? Do officers in the field have insight to offer? By developing a hypothesis, the analyst is in the mainstream of scientific research, using a venerable methodology—the so-called (and awkwardly called) hypothetic-deductive method.

Maps and other graphics are integral tools for exploration and hypothesis testing. Do preliminary maps confirm the hunch that a burglary pattern is likely the product of a repeat offender who is using a bus route, and apparently a specific bus stop, to visit a neighborhood and commit his offenses? If so, the preliminary information will help the hypothesis gel into something useful.

At the core of this method is a potentially repetitive process involving:

  • Development of a hypothesis on the basis of the best available information derived from both theory and field data.

  • Development of a method for testing the hypothesis, perhaps involving statistical and graphic testing or modeling.

  • Analysis of the data.

  • Evaluation of the results.

  • A decision to accept or reject the original hypothesis.

  • Reevaluation of the original hypothesis, if it was unsatisfactory. It may be modified to take into account new knowledge. If so, the process begins anew.

The confirmation stage tells the analyst whether the hypothesis does indeed have a factual basis that will withstand scrutiny. If it does not, we reevaluate and make necessary adjustments, perhaps gathering more information to add depth to what is already known and to shore up the hypothesis, which itself may now have been modified to take new data into account.

What's Hypothesis Got to Do With It?

Will the typical crime analyst go through the rigmarole of visual thinking and visual communication—plus the process of hypothesis testing? It's not likely, since most analysts work under though deadlines with inadequate resources. (Just like everyone else!) Also, much of the work is prescribed, routine, and repetitive, leaving little flexibility for research.

So, what's the point? The formal structure outlined here is an ideal model for map-related research—a paradigm or modus operandi. Thus it is unlikely to be replicated often in practice. Like other models, it provides an ideal guide and enables the analyst to apply whatever parts of the process can be applied in the time available.

MacEachren (1994) cautions that investigators should realize that maps and other graphics are prone to error resulting from their underlying data, inappropriate design, or even the margin of error introduced by the normal process of abstraction. If possible, the analyst should not rely on any one data source, whether it be a map, field observation, or survey, if other sources can be used to complement each other.

Visual communication

As we move from visual thinking to visual communication, we go from the private realm to the public activities of synthesis and presentation. Synthesis implies merging various types of information-in this case, geographic information system (GIS) layers-into a coherent final product. Although synthesizing is essentially scientific, human judgment is at the core of this filtering and refining process.

Synthesis is assisted by the ability to find overlaps (intersections) between layers in a GIS. But even then decisions have to be made about what to put in, what to leave out, and what importance to attach to each layer. A presentation puts all the relevant pieces together in a map. The map can be highly persuasive if it provides information germane to the question at hand and is well designed. As MacEachren (1994, p. 9) noted, "People believe maps."

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