Mapping Crime: Principle And Practice

ion and translates this into a map that is read by the map user and transferred to the user’s mind. Choosing a crime map Chapter 1 characterized thematic maps as falling into the following broad categories: statistical, point, choropleth, isoline, surface, and linear. How do we choose the most appropriate type for mapping crime and crime-related phenomena? Some decisions jump out at us while others are open to interpretation. For example, if we want to see the precise locations of burglaries for the last month, then we use a point map of addresses of incidents. Or perhaps a city council member has asked the police department for a map summarizing the number of incidents of graffiti per structure by city neighborhoods. This calls for a choropleth map, with neighborhood boundaries making up the geographic units. Links between victim and offender residences demand a linear representation. A generalized picture of crime risk or incidents is seen best with an isoline or surface map, and census information depicting the relationship between poverty and race can be shown using either a statistical or choropleth map. Because of the infinite potential combinations of crime-related conditions that can be depicted on maps, we can combine map types to put more information on the same map. For example, we can combine nominal and ratio data, such as a choropleth map of drug-related crime by patrol beats and add the locations of drug markets on the same map. Crime mappers should be aware of the potential for combining thematic map types, provided that the result is not overloaded with information—or just plain incomprehensible. An overloaded map will have so much information that the eye is unable to take it all in. It will prevent the reader from discriminating between what is important and what is not. Examples of thematic maps 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/cmrc), which provides links to police departments across the United States. Another useful Web site is maintained by Hunter College in New York (http://everest.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.