Choosing a crime map
Chapter 2: What Crime Maps Do and How They Do It

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.

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