Cartographic design and the quality of climate change maps

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222 Climatic Change (2009) 95:219–230 2 Selecting a climate change map to evaluate A climate change map from a recent report of the IPCC was selected to demonstrate how climate change maps can be cartographically evaluated. Selecting an IPCC map is appropriate for several reasons. Since 1988, the IPCC has released four comprehensive scientific assessments of climate change. IPCC reports are considered by policymakers and the scientific community to be a definitive source of information on climate change and its impacts (Kintisch and Kerr 2007). The map shown in Fig. 1 was published in the “Summary for Policymakers” completed by Working Group II for Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC 2007b, p.10).Thetitleofthe map is “Changes in physical and biological systems and surface temperature 1970– 2004.” Figure 1 is the only map included in this summary report for policymakers. We consider this map appropriate for a case example given that: (1) the IPCC is a legitimated source of climate change information, (2) the IPCC’s fourth assessment report (AR4) is a major consensus technical effort to summarize current climate change knowledge, (3) the policy summary is an important stand-alone document, and (4) Fig. 1 is the sole map in the policy summary approved by Working Group II. The map published in the report is 184 mm wide by 194 mm tall, accompanied by a caption 184 mm wide by 41 mm tall, and printed in color on a page 215 mm wide by 279 mm tall. The map was also made available as a separate color graphic (though without its title or caption) on the IPCC website (IPCC 2007b). Since its original publication, the map has been distributed through other reports of the IPCC and in the public media in both its original and modified form. The map appears in the draft copy of the “Summary for Policymakers of the Synthesis Report of the IPCC Fourth Assessment Report”—the overall summary of the IPCC’s work (IPCC 2007c). It was published in modified form in the Washington Post (print and online) on 18 November 2007, page A10 with the caption excluded, title changed, and legend edited and rearranged (Struck 2007). The map was adapted and split into two separate maps by the Organization of Ibero-American States (Organización de Estados Iberoamericanos 2007). The resulting two maps were posted on the organization’s website with substantial changes made to the legend (see http://www.oei.es/decada/presentacioneurop.htm). The color version of the map published in the final summary report of Working Group II and shown in Fig. 1 is evaluated in this demonstration, downloaded from the IPCC website 21 November 2007 (IPCC 2007b, p. 10). 3 Selected principles of cartography to use in evaluating climate change maps Research and practice in the field of cartography have produced a set of fundamental principles that are essential to good cartographic design—principles that deal with map layout and symbolizing data and features. Cartographic design principles guide decisions about how to represent locations on a map and attributes of those locations, using graphic symbols such as color, size, shape, typography, and other symbols. Principles describe how points, lines, and areas should be symbolized based on the underlying data (i.e., attributes of features). For example, the selection of
Climatic Change (2009) 95:219–230 223 Table 1 Selected principles for cartographic design and layout with descriptions (adapted from Brewer 2005 and Krygier and Wood 2005) Cartographic principle Description of cartographic principle Map projection All map projections have distortions (distance, area, direction, and/or shape). An equal-area map projection is a good selection for most small-scale maps (e.g., many world maps) and should be used for maps showing data distributions. Generalization Small-scale maps should show more area, less detail, and more generalization of features. Large-scale maps should show less area, more detail, and less generalization of features. The coastline of a country on a large-scale map is symbolized by a line with more detailed curves than the same coastline on a small-scale map. Data classification Qualitative data show differences in kind (e.g., forest versus urban land cover). Qualitative data should be grouped so that features in the same group are more similar than dissimilar and features in different groups are more dissimilar than similar. Quantitative data show differences in amount (e.g., population density). Quantitative data should be grouped by specific external criteria (e.g., quantiles) or by the characteristics of the data (e.g., natural breaks). Map layouta Focus Map layout should guide readers through the map elements and help them focus on the most important parts of the map. Visual variablesb Color hue Color hue (such as red, green, blue) should be used to categorize features that are qualitatively different, such as a river and a road. Color value Color value (or lightness of hue) should be used to represent quantitatively different data (either rank-ordered data or numerical amounts), such as population density. Value is typically light for low numbers (e.g., light green) and dark for high numbers (e.g., dark green) in sequential datasets, such as a dataset of population change from 0% to 100%. For diverging data sets with an important midpoint (between negative and positive values), such as population change from −50% to +50%, hue and value can vary to show the two directions in the data set. The midpoint from 0% to −50% can be symbolized using a light to dark color hue. The midpoint from 0% to +50% can be symbolized using a different, complementary light to dark color hue. Color saturation Color saturation (or intensity of hue, such as bright red compared with a dull, gray red) can be used for qualitative or quantitative data. It is difficult to use on its own to symbolize data. Saturation is typically used to reinforce changes in value for quantitative data or to reinforce changes in hue for small areas on a map that are qualitatively different. Size Size should be used to represent quantitatively different data (either rank-ordered data or numerical amounts). A larger square signifies a greater quantity than a smaller square. Shape Shape should be used to categorize features that are qualitatively different. A square is not more or less than a circle, but is different in kind. Visual hierarchy Visual hierarchy should emphasize the most important map elements. Less important elements should be less noticeable. Visual hierarchy should clearly communicate the intellectual hierarchy and purpose(s) of the map. aFor Krygier and Wood (2005) ‘focus’ is one element of map layout that also includes map pieces (title, legend, border, etc.), balance (stability), and the grid (underlying grid of vertical and horizontal lines that helps with balance). bBertin (1981) initially described seven visual variables. Cartographers have adapted and modified this list. Five visual variables are presented here.
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Cartographic design and the quality of climate change maps

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