Choropleth Map

Issues in Choropleth Mapping

Implied with this mapping method is that the value assigned is consistent throughout any enumeration unit. While this may be the case with qualitative data, especially when symbolizing jurisdictional units (e.g., school districts or counties), it would rarely be the case with quantitative data (e.g., income or rainfall). A second concern arises when this method is used with enumeration units that vary considerably in size, resulting in the visual dominance of larger areas. This issue is explored further below.

Qualitative Choropleth Mapping

A land use or land cover map is an example of a qualitative choropleth map. Shadings with varying textures or colors are used to symbolize the mapped classes, for example, to differentiate land areas from water areas or urban from rural. In symbolization, care must be taken to not include so many categories that it becomes impossible to distinguish slightly different colors or shades, making it difficult to match the shading in the map with the same shading in the legend. Colors or textures must be chosen to be as different from each other as possible. In interactive mapping environments, the association between the shadings in the map and the legend can be enhanced by highlighting areas in the map as the mouse is passed over the corresponding color or shading in the legend, and vice versa.

Quantitative Choropleth Mapping

By far the most common quantitative choropleth maps are those that involve the progression of gray shadings or sequence of colors to represent interval or ratio data over areas. Usually, the data are reduced to ordinal, or classed, data before mapping through one of many different types of data classification. Since the data classes assigned will have some relative ordering, tinting schemes, referred to as color progressions or sometimes color ramps, that progress from light to dark, with higher values receiving a darker shading or color, should be used. In some cases, the color progression may be bipolar, with two different colors or hues around a zero value. An example of this would be a map of percent population change that includes both positive and negative values. In this case, the negative values might be shown using a red progression that gradually decreases in lightness as the values move toward zero, and the positive values shown with a blue progression that increases in darkness as the values get larger.

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