Crime incidents: Measuring time and space|
Chapter 1: Context and Concepts
Crimes happeneverything happensin both time and space. Vasiliev (1996) has suggested that time is a more difficult
concept than space. Spaces and locations can be seen and measured quite easily by means of simple reference systems, such as x-y coordinates. Time, however, is harder to grasp, and maps have represented time in multiple ways (Vasiliev, 1996, p. 138), including:
- Moments. Providing times of events in geographic space. When did crime incidents occur and where?
- Duration. How long did an event or process continue in a specific space? For example, how long did a crime rate remain above or below a certain level in a particular area? How long did a hot spot (an area of high crime) persist?
- Structured time. Space standardized by time (for example, shift-based patrol areas, precincts, and posts).
- Distance as time. We often express distance as time. "How far is it?" "About 20 minutes." More to the point, perhaps, is concern with response times. In practice, a fixed, maximum permissible response time corresponds to the maximum distance feasible for patrol units to drive. Another application would be an investigation to see whether a suspect could have traveled from the last place he was seen to the location of the crime within a certain time period.
Specialized activities, such as policing, have their own unique perspective on time and space; thus, the elements of Vasiliev's typology will vary in their relevance from time to time and place to place.
There is no question that time is an important element in crime mapping owing to the time-structured way in which things are organized in police departmentsby shifts. Patrol area boundaries may differ by shift, commanders may call for maps of crime by shift, and resources may need to be allocated differently by shift. Maps may need to be prepared on a weekly, monthly, quarterly, or annual basis to illustrate trends. Time-based maps may be further refined temporally by adding the shift dimension. Crime data could be mapped vertically, in effect stacking (merging) maps on top of each other, using different symbols for different time periods. Alternatively, a different map could be prepared for each time period. All these modifications can be automated by using GIS.
The importance of selecting appropriate time periods for mapping cannot be overemphasized. For example, a map
covering a month may mask noteworthy week-by-week variations. Or weekly maps could hide day-to-day changes. Mapping intervals selected for administrative convenience may not be the best for analytical purposes. For example, the calendar week may be best for police department convenience, but local events, such as an industrial holiday, a sporting event, a plant closure, or some seasonal phenomenon may cut across administrative time units and may also have relevance to crime frequencies. Related questions are, How much time elapses before a map is out of date? All data are obsolete sooner or later, but when is sooner? When is later? These decisions are quite subjective and call for a "feel" for the data under review.
Another aspect of mapping crime in time and space relates to the representation of change. One of the most crucial questions asked in police departments is, How has crime changed in this neighborhood in the last week or month or year? Maps can help answer this question by symbolizing change in several possible ways, such as by showing crime as a "surface" with peaks representing high levels of occurrence (gray areas) and valleys low levels (red areas) (figure 1.8). This approach borrows from the methodology of the topographic map, on which the land surface is represented by contour lines, each joining points of equal elevation. On the crime surface map, areas of declining crime can be shaded differently from those with increases. Crime mappers are limited only by their imagination when representing time and space in two dimensions or simulated three dimensions.
As usual, we should keep an open mind. Some space-time data may be best represented with a "non-map" graphic, or a combination of map and statistical graphics, such as precinct bar charts or pie charts embedded in a map of precincts. Here we come full circle, back to cartography as art and science and to the abstraction/detail tradeoff. Only crime mappers can decide, probably on the basis of some experimentation, which graphic representations are best for themselves and for their audiences.