Chapter 4: Mapping Crime and Geographic Information Systems

Although GIS makes mapmaking relatively easy, it is not necessarily easy to make good maps. The fundamental problem is that fancy fonts, tables, maps, or diagrams can dress up almost any data. However, just because data look good does not mean they are.

As a rule of thumb, do not blindly accept the default settings in GIS programs. Defaults apply to such steps as the selection of class intervals in choropleth maps (see "Classifying map information" in chapter 2), as well as the colors, symbol types, and sizes.

Rule of Thumb

Default settings in GIS programs should not be accepted blindly.

Boilerplate maps, produced regularly to show specific needs such as weekly precinct or division crime patterns, can be fine tuned so they consistently convey the intended message. Problems are more likely to arise with specialized maps. A checklist may be a useful reminder of the most important map elements and criteria:

  • Need. Is a map needed for this message or analysis? Could the job be done as well or better with another approach, such as a table, a narrative, a chart, or conversation?

  • Data source. Are the data reliable? If there are questions about data quality, how can the audience be alerted? (By using a map subtitle or map footnote?) Are data so poor that mapping and analysis should not be done? Who decides if this is the case?

  • Scale. Is the appropriate area shown? Can the map be enlarged without compromising the message?

  • Scope. Is the map trying to show too much? Too little? Can more context be added to better inform the reader?

  • Symbols. Would icons convey the message more convincingly than abstract default symbols? Note that some icons are awkward shapes and may have a minimum size, below which they become meaningless.

  • Color. Misused color detracts from an otherwise excellent map. Excessive use of bright colors may hurt the eye and repel the reader. Illogical color gradations may be confusing. Think in terms of drawing the eye to important areas (normally higher data values) by using more intense color tones. Think about how the map will be used. Color may be irrelevant if the map is to be distributed by fax or printed in a document that will not be reproduced in color. In such cases, color can be counterproductive. Even black-and-white (gray-scale) shades should be chosen carefully if the document is to be faxed. Gray-scales should not be too subtle (variations will be lost), and cross-hatched shading should be coarse (lines relatively far apart) or they will not hold up through the fax process.

  • Lettering. Are the default font style and size appropriate? Consider, for example, whether the user may decide to make the map into an overhead transparency. Will the lettering be legible in that medium?

  • Methodology. What opportunities does your software offer? (Hot spot identification? Buffers? Filtering?) Have those opportunities been taken advantage of? Or have unnecessarily glitzy methods only created confusion?

  • Privacy. Will this map reveal information about individuals who may be subject to privacy restrictions? Most data are in the public domain, including arrest records and court documents. Exceptions to this include the practice of protecting the identities of rape or sexual assault victims, and the identities of juveniles.
Chapter 4: Mapping Crime and Geographic Information Systems
Previous Contents Next
Return to Home Page

Mapping Crime: Principle and Practice, by Keith Harries, Ph.D., December 1999