Maps of crime: Thematic maps
Chapter 1: Context and Concepts

What is a thematic map? Quite simply, as the label suggests, it is a map of a theme or topic. Thematic maps have almost infinite variety and include most of the maps in the media showing, for example, the spread of fire ants, the status of sales taxes by State, or world population density. Thematic maps are like a comprehensive toolkit—we can select a topic and then choose from many possible ways of converting the data into a legible map that effectively communicates with the intended audience. Thematic maps may be quantitative or qualitative.

  • Quantitative maps portray numerical information, such as numbers of crimes in an area or crime rates.

  • Qualitative maps show nonnumerical data like land use types or victim/ offender characteristics, such as male or female, juvenile or adult.

Crime analysts use both quantitative and qualitative maps. Thematic maps can include four kinds of measurement data: nominal, ordinal, ratio, and interval. (For a more detailed explanation, see any basic statistics textbook, such as Burt and Barber, 1996.)

  • Nominal measurement names or labels items in unordered categories, such as race. If a map shows homicide victims by race, it is a qualitative thematic map. Mapping by race, age group, or marital status puts labels on groups without ranking them as higher or lower or better or worse. (Quantitative information can also be inferred from this type of map. For example, the number of incidents affecting racial groups by areas can be counted, thus combining qualitative and quantitative interpretations. Types of measurements are often mixed on maps.)

  • Ordinal measurement classifies incidents, victim or offender characteristics, or some other attributes (perhaps areas) according to rank. Thus patrol areas or precincts might be ranked according to their crime rates, their incidence of complaints, or the average seniority of officers. This involves only sorting and evaluating the data according to their relative values so that subjects can be ranked. How much the subjects differ is not considered. Thus we can put qualitative characteristics on an ordinal scale, such as a hierarchy of police areas based on size, with districts above precincts, which are in turn above patrol beats.

  • Ratio scales, such as distance in inches, feet, yards, millimeters, meters, and so forth, start at zero and continue indefinitely. Zero means there is none of it and 20 means there is twice as much as 10. For example, the homicide rate is 3 per 10,000 persons in the city. Crime analysts will use nominal, ordinal, and ratio scales for these data but are very unlikely to use the fourth kind of measurement, interval.

  • Interval scales show values but cannot show ratios between values. Temperature is a good example. We can measure it, but we cannot say that 80 degrees is twice as warm as 40 degrees, since the starting points of both the Fahrenheit and Celsius scales are arbitrary—that is, they are not true zeros. A possible exception to the assertion that crime analysts will not use interval scales could be seriousness weighting. (See Wolfgang et al., 1985a and 1985b, for more on this topic.)

Thematic maps come in considerable variety and will be examined in more detail in chapter 2. Each type represents some kind of data best. Information with address-level detail calls for one kind of thematic map, whereas data measured only at the neighborhood, precinct, or census tract level require a different approach. Examples of various ways in which thematic maps can be designed are contained in the following categorization of thematic maps:

  • Statistical maps use proportional symbols, pie charts, or histograms to visualize the quantitative aspects of the data. Typically, the statistical symbols are placed in each subdivision on the map, such as patrol areas, census tracts, neighborhoods, or wards. Such maps can be quite difficult to read if they contain a large amount of detail, particularly when many geographic subdivisions and several attributes of the information are being mapped. Nominal data, such as the race of victims as proportions in precinct-based pie charts, can be represented on a statistical map.

  • Point (pin) maps use points to represent individual incidents or specific numbers, such as when five incidents equal one point. (Aggregating multiple incidents to single points would be done only on a small-scale map.) A map showing locations of drug markets by types of drugs prevalent in each is an example of a point map with nominal scale data. Point maps are probably the most frequently used maps in policing, as they can show incident locations quite precisely if address-level data are used.

  • Choropleth maps show discrete distributions for particular areas such as beats, precincts, districts, counties, or census blocks. Although point data can give us the best detail in terms of where events happen, information may be needed for areas in summary form that has meaning in terms of planning, management, investigation, or politics. Note that point data and choropleth representations can both be put on the same map, if it is appropriate and the result is not a garbled mess. For example, burglary (point) data could be put over neighborhood boundaries (choropleth data) or any areas representing police geography. Also, choropleth maps can be given a three-dimensional appearance by making each area into a raised block, with the height of the block representing the relevant data value.

  • Isoline is derived from "iso," the Greek prefix for equal, and refers to maps with lines that join points of equal value. Physical geography is replete with isoline maps that use the following: isobars (equal barometric pressure), isohyets (equal rainfall), isotherms (equal temperature), isobaths (equal depth), and, in a rare departure from use of the iso prefix, contour lines to join points of equal elevation. The form most likely to be used in crime analysis is the isopleth (equal crowd), in which data for areas, such as crimes per neighborhood or population density, are calculated and used as control points to determine where the isolines will be drawn.14 (See Curtis, 1974, for an example of an isopleth map of homicide, rape, and assault in Boston.)

  • Surface maps can be regarded conceptually as a special case of an isoline presentation. Such maps add a three-dimensional effect by fitting a raised surface to data values. Typically, an arbitrary grid is placed over the map and the number of incidents per grid cell are counted. These counts form the basis for what is, in effect, an isopleth map that is given its third, or z (vertical), dimension derived from the isoline values. The resulting map is rendered as if it were being viewed from an oblique angle, say 45 degrees. (If it were to be viewed from overhead, like a normal map, the surface would be lost and the map would appear to be a flat contour map.) Such continuous surface maps can make a powerful visual impact, but they have dual disadvantages in that data values are hard to read on the map and detail behind data peaks is lost (see figures 1.13 and 1.14).

  • Linear maps show streets and highways as well as flows using linear symbols, such as lines proportional in thickness to represent flows. Apart from base maps of streets and highways, crime mappers use linear maps infrequently—the most common application is vehicle theft investigation showing connections between the place of theft and place of recovery.
Chapter 1: Context and Concepts
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Mapping Crime: Principle and Practice, by Keith Harries, Ph.D., December 1999