Chapter 2: What Crime Maps Do and How They Do It
The field of map design has generated substantial literature in cartography focusing on how people comprehend maps and the impact of various design elements such as symbol size, color, and line thickness. Also of interest is the impact of the arrangement of a map within the map frame as well as the merits and demerits of various types of maps.
Debate continues, for example, over the dominance of choropleth maps to represent numbers. Opponents point to the most obvious choropleth defect: its use of one data value to represent an entire area, an absurdity that becomes acute when most of a geographic subdivision contains virtually no human activity because of the terrain or the existence of bodies of water.6 Muehrcke (1996) quoted Ronald Abler, Executive Director of the Association of American Geographers, who, in 1987, said something to the effect that choropleth maps were an abomination that GIS would soon eliminate through the use of dasymetric mapping.7 (The death of the choropleth map has been slow!) However, the increased use of density surfaces in various desktop GIS programs is a move away from such heavy reliance on either dot or choropleth modes of representation and is consistent with the concept of areal averagingwithout slavish adherence to political or administrative boundaries.
Map design is at once a technical and an artistic effort. Dent (1990) devotes 5 chapters and about 120 text pages to designing thematic maps; we can give only broad consideration to a few issues relating to typical parts of a map and how they should be organized. The reader is referred to the following textbooks for a full explanation, particularly on such details as lettering and labeling: Campbell, 1993; Dent, 1990, 1993; Robinson et al., 1995; and MacEachren, 1994, 1995.
How Much Exploration?|
A histogram of the numerical data, as will as statistics for skewness and kurtosis will help determine what kind of map would be most effective and least misleading. Common statistical packages such as SAS® and SPSS® enable the rapid production of these diagnostics. Also, Microsoft Excel and other spreadsheets yield the following statistics as well as a histogram option. (In Excel, see the Tools menu, then Data Analysis.)
Interpretation: Values of the skewness and kurtosis are centered at zero. If either is relatively large, a nonnormal distribution is likely. See Norcliffe (1977 for more detail on this, and consult figures 2.15 and 2.16 for graphic interpretations.
*HOMRATE is the homicide rate per 100 persons in a selected part of Baltimore, Maryland. These statistics were used to compile figures 2.15, 2.16, and 2.17
Source: Keith Harries
It may be helpful for newcomers to mapmaking to make a flow chart in which the design of the map is adjusted to ensure that the map fulfills its stated purpose. This activity may become less important over time as intuition and experience take over from reliance on a formally structured process. Dent (1990, p. 316) lists the following elements of the thematic map, which could serve as a checklist for inclusion:
Most of these elements are necessary in a typical crime map. The principal exception is the graticule (spherical coordinate grid), which normally serves no useful purpose.8 Also, credits are rarely used because data are likely to be locally derived. However, if data sources are not self-explanatory, credits clarify exactly where the data came from. This information could be listed under the body of the map using the keywords "source" or "data source." We could add author to the list to assist in the process of accountability, inconspicuously noting the name or initials of the analyst-cartographer in a corner of the map.
- Title (or caption).
- Geographic content (showing information that may not necessarily be included in the subject matter of the map, such as orientation or north arrow).
- Graticule (spherical coordinate system: latitude/longitude, State plane).
- Borders and neatlines (the lines that bound the body of a map, usually
parallels and meridians).
A useful approach to learning more about design is to look at examples of crime (and other) maps that have been deemed acceptable by their respective audiences. Fortunately, there is no shortage of maps, whether on crime or on other phenomena. The easiest access is via the World Wide Web, and the appendix of this guide lists some useful Web site addresses. Chapter 3 uses examples to discuss various applications of crime mapping. Here we confine ourselves to outlining principles.
Dent (1990, chapter 13) has noted several elements of map composition: balance, focus of attention, and internal organization.
According to Dent, contrast also is important to visual perception. Line, texture, value, detail, and color are powerful tools because they allow map elements to be differentiated from one another. More contrast makes objects stand out, less allows them to fade into the background. Line thickness, or weight, can assist in this process, and using more than one line weight on the map can add interest. Texture can add variety and draw attention to an important part of the map. Value refers to the use of lighter or darker shades of color, and detail draws the eye in. As noted elsewhere, however, detail is a two-sided coin. It adds interest, but when used to excess it can cause clutter and make the map illegible. If a map is
to be reduced for publication, fine detail may be completely lost in the reduction process. Experiment with enlarging and reducing on a photocopier to learn more about how this works in practice.
- Balance refers to the need to arrange parts of the map in a way that enhances its visual symmetry. However, the crime cartographer may have little flexibility with respect to balance owing to inherent content limitations. For example, the jurisdiction may be extremely asymmetrical, making it
difficult, if not impossible, to map without leaving considerable white space on the paper. Cities with long "shoestring" annexations, like Los Angeles, or States with long panhandles, like Oklahoma, are good examples of difficult map shapes. This problem sometimes can be solved by chopping the city or other area of interest into its component parts. An inset, or miniature map of the whole, is used to show how the pieces fit back together. Another solution is to routinely map individual precincts or districts under the assumption that the managers of those areas are first and foremost interested in seeing patterns in their areas of responsibility. The drawback to this is that crime patterns do not pay much attention to administrative or political boundaries, so that looking at individual subdivisions in isolation from the rest of the area may cause someone to miss hot spots or other useful patterns by fragmenting them.
- Focus of attention is a concept based on the assumption that people read maps like they read the printed page, by moving their attention from upper left to lower right.9 Hence the optical center of a map is somewhat above the geometric center suggesting that, ideally, the most significant information should be closer to the optical center. Again, this is easier to manage in theory than in practice. Still, it is a useful concept to bear in mind because crime analysts will sometimes have enough discretion in design that the focus of attention can be exploited to
- Internal organization refers to the alignment of the parts of a map or individual maps on a page or within a frame. Map elements should be arranged in a logical way rather than placed haphazardly on the page. The core contents of the map, for example, should dominate the space, and other elements should be secondary.
Color is extremely important in the process of area differentiation. It is also a complex issue owing to the physiological, psychological, and physical processes involved. Dent (1990, chapter 16) notes that color has three dimensions: hue, value, and chroma.
Choices of color in maps need to be made quite carefully because color may have strong emotional connotations for some readers. For example, should red be used for a map of violent crime, given the symbolic connection to blood? It is tempting to overload crime maps with warm colors, such as red and orange, but the analyst should be mindful of the symbolic effect and the impact this may have on the intended audience.
- Hue is the term given to color labels-red, yellow, and blue, the primary colors-and the millions of permutations derived from them.
- Value refers to the degree of lightness or darkness of a color. GIS programs can help you select color values by providing color "ramps" (or series of related shades or values of a hue) in a visually logical sequence ranging along an intensity spectrum. Colors vary along a continuum from light to dark. For example, reds may range from light pink to deep red, and blues may range from sky blue to navy blue.
- Chroma is understood through the concept of color saturation. A less saturated color appears to contain more grays, and a saturated color has no gray and appears as the "pure" color. In photography, some films have a reputation for conveying more saturation than appears in natural scenes (bluer blues, greener greens), "larger than life" color that is pleasing to some viewers but excessive to others.
Just as color makes maps and other graphics come alive, color also enhances our ability to mislead people with maps through the use of inappropriate hues and values. For example, a crime category that is a local political "hot potato" could be visually minimized through the use of cool colors in subtle shades lacking saturation. The use of color in maps and graphics is complicated by the fact that a significant portion (8 percent of males and 0.5 percent of females) of the population is at least partially colorblind.
Using Colors and Shades|
- Use darker colors or gray shades for more or higher values.
- Use lighter colors for less or lower values.
Crime mappers can take advantage of various models of color sequencing. GIS software typically defaults to a part-spectral plan with shades from yellow to brown. In a full-spectral plan, colors range from warm to cool, and in a double-ended plan, data values representing an increase (or above average) are in one color and a decrease (or below average) in another. Increases (or higher values) are typically shown in warmer colors, decreases (or lower values) in cooler (see Dent, 1990, p. 387). GIS software normally permits the customizing of colors to fit your purpose.