Multivariate map: Difference between revisions

Revision as of 00:09, 9 November 2020

Revision as of 00:11, 9 November 2020

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* A ''[[small multiple]]'' is a series of small maps, arranged in a grid or array, each of which shows a different (but possibly related) variable over the same space.<ref name="Tufte-EI">{{cite book |last1=Tufte |first1=Edward |title=Envisioning Information |date=1990 |publisher=Graphics Press |isbn=978-0961392116 |page=[https://archive.org/details/envisioninginfor0000tuft/page/67 67] |url=https://archive.org/details/envisioninginfor0000tuft/page/67 }}</ref> It has been argued that this is not technically a multivariate map because it is a set of separate maps,<ref name="gistbok" /> but it is included here because it is intended to accomplish the same purpose.

== Advantages and criticisms ==

[[File:2016 US Presidential Election Pie Charts.png|thumb|right|300px|A multivariate symbol map of the 2016 U.S. presidential election, using a combination proportional and chart symbol]]

[[File:Dot map black hispanic.png|thumb|left|A bivariate dot density map showing the distribution of the African American (blue) and Latino (red) populations in the contiguous United States in 2010.]]

Multivariate thematic maps can be a very effective tool for discovering intricate geographic patterns in complex data.<ref name="gistbok" /> If executed well, related patterns between variables can be recognized easier in a multivariate map than by comparing separate thematic maps.

The technique works best when the variables happen to have a clear geographic pattern, such as a high degree of [[spatial autocorrelation]], so that there are large regions of similar appearance with gradual changes between them, or a generally strong correlation between the two variables. If there is no clear pattern, the map can become an overwhelming mix of random symbols.

A second problem occurs when the symbols do not harmonize well. In keeping with [[Gestalt psychology]], a multivariate map will work best when map readers can isolate patterns in each variable independently, as well as comparing them to each other. This occurs when the [[Map symbol | map symbols]] follow the gestalt [[principles of grouping]]. Conversely, it is possible to select thematic symbol strategies that are effective on their own, but do not work together well, such as a proportional point symbol that obscures the choropleth map underneath, or a bivariate choropleth map using base colors that create unintuitive mixed colors.