Mapping as art and science
Chapter 1: Context and Concepts

Like other forms of visualization, maps are the outcome of scientific activity: hypothesis formulation, data gathering, analysis, review of results, and evaluation of whether the initial hypothesis should be accepted or rejected in favor of a modified version. This cycle, known as the hypothetic-deductive process, is used throughout science as a fundamental tool. It is a universal paradigm, or model, for scientific investigation.

Constructing a map involves taking a set of data and making decisions consistent with the hypothetic-deductive process. Decisions need to be made about the kind of map to be prepared, how symbols or shading will look, how statistical information will be treated, and so forth. These decisions must be based on the objective to be achieved, including consideration of the target audience. Certain scientific principles can be applied. For example, if we are preparing a shaded map with a range of statistical data to be divided equally, then a simple formula can guide us:

Range in data values Number of classes.

Thus, if the range (difference between maximum value and minimum value) of data values is 50 and it has been determined that 4 classes, or "subranges," of data would be appropriate, each will span 12.5 units. This simple example demonstrates an elementary cartographic principle.

However, this science provides no help with elements of map design, such as choice of colors, symbols, or lettering, and the arrangement of map elements on the page or within a frame. These elements are pieces of the art of cartography and are just as important to the overall purpose and effectiveness of the map as the scientific elements. A map that is scientifically perfect may be ineffective if it is an artistic disaster.

Art and science may merge imperceptibly and confound one another in unfortunate ways. For example, oversized symbols or lettering may draw attention to parts of the map that should be understated or less emphasized from a purely scientific perspective. A poor choice of colors or intervals for statistics can also make a big difference. Thus the scientific mission of the map can be subverted through inappropriate artistic choices, and poor choices on the science side can similarly affect the artistic elements. In cartography, as in medicine, art and science are inseparable. The perfect map blends art and science into an effective tool of visual communication. Figure 1.6 shows elements of weaker (middle figure) and stronger (bottom figure) map design. The chart (top figure) known as a histogram accompanying the maps shows an approximately normal (bell-shaped) distribution of scores on an index representing the association between violence and poverty in a neighborhood of Baltimore. The scores have a mean, or average, of zero and a measure of spread around the mean (standard deviation) of 1.0. Technically, this means that the census block groups in the range -1.0 to +1.0 are not significantly different from the average. Those with scores lower than -1.0 or higher than +1.0 are considered relatively extreme. We may wish to map the extremes because they could convey information of value in community policing-areas with a strong link between poverty and violence may warrant special allocations of crime prevention and social service resources.

Figure 1.6

If the objective of the map is to represent the high and low extremes, then the bottom figure does a much better job than the middle one, both in terms of how the data are divided and how they are symbolized with colors. The bottom figure divides the data into standard deviation units such that the blue and red values are the focus of attention. In the middle figure, the visual message is almost lost in a combination of a poor choice of colors and shading symbols and a confusing division of the data into classes. The visual messages conveyed by the maps are different, yet the underlying geographies and statistics are identical. This example emphasizes the importance of the map as a medium for the interpretation of information. Subjective choices relating to both the art and science content of the map are critical.

Figure 1.6 also reminds us that it is just as easy to lie with maps as it is to lie with statistics. (Remember the three kinds of lies: lies, damn lies, and statistics—and maps, perhaps?) "Lying" may be a bit strong. It is more likely that the compiler of the map misleads readers by choosing inappropriate designs rather than by intentionally falsifying information. This may happen on several levels, the most basic being that the author of the map created it to achieve an objective different from that experienced by the readers. Next are issues related to data manipulation and cartographic art. Ultimately, the possibilities for misrepresentation and misinterpretation are virtually infinite. The key question is whether these problems are fatal flaws with respect to specific maps.

Chapter 1: Context and Concepts
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Mapping Crime: Principle and Practice, by Keith Harries, Ph.D., December 1999