Review of Unclassed Choropleth Mapping
Keywords:unclassed, choropleth, QGIS, ArcGIS, data classification
Although unclassed choropleth maps lead to a more accurate representation of data, grouping of data into classes is still common. Commonly-used data classification techniques such as equal-interval, quantiles, and natural breaks produce very different and possibly misleading representations. An unclassed map creates a distinct color for each unique value. The method was introduced by Tobler in 1973 using an x, y coordinate plotter that created crossed-line shadings. Tobler’s unclassed proposal used grayscale values because color displays were not yet available. Current color monitors have the ability to display 16.7 million colors, while most GIS software packages have limits to their color ramps. QGIS defines color ramps with up to 999 classes. It is also possible to define up to 1000 classes in ArcMap, and ArcGIS Pro has an “Unclassed” option when styling choropleth maps. Utilizing more color classes results in a more truthful map due to minimizing error from the grouping of data. The unclassed method is examined here along with color ramps and classification schemes in QGIS and Esri’s ArcMap/ArcGIS Pro. It is demonstrated that it is usually impossible to create a truly unclassed choropleth map using the default color schemes in these programs.
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