A bivariate map displays two variables on a single map by combining two different sets of graphic symbols or colors. Bivariate mapping is an important technique in cartography. Given a set of geographic features, a bivariate map displays two variables on a single map by combining two different sets of graphic symbols. It is a variation of simple choropleth map that portrays two separate phenomena simultaneously. The main objective of a bivariate map is to find a simple method for accurately and graphically illustrating the relationship between two spatially distributed variables. A bivariate map has potential to reveal relationships between variables more effectively than a side-by-side comparison of the corresponding univariate maps.
A bivariate map is recent graphical method which is intended to convey the spatial distribution of two variables and the geographical concentration of their relationship. An example of a bivariate map is a bivariate choropleth map. A bivariate choropleth map uses color to solve a problem of representation in four dimensions; two spatial dimensions — longitude and latitude — and two statistical variables. Data classification and graphic representation of the classified data are two important processes involved in constructing a bivariate map. The number of classes should be possible to deal with by the reader. A rectangular legend box is divided into smaller boxes where each box represents a unique relationship of the variables.
In general, bivariate maps are one of the alternatives to the simple univariate choropleth maps, although they are sometimes extremely difficult to understand the distribution of a single variable. Because conventional bivariate maps use two arbitrarily assigned color schemes and generate random color combinations for overlapping sections and users have to refer to the arbitrary legend all the time. Therefore, a very prominent and clear legend is needed so that both the distribution of single variable and the relationship between the two variables could be shown on the bivariate map. A bivariate choropleth map can be difficult for map readers to interpret if it is not made well.
An example of a bivariate map that combines graphic symbols and colors would be a choropleth map combined with a propotional symbols map.
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Four color theorem