Ogive-based Legends for Choropleth Mapping

Abstract

Exploratory data analysis and geovisualization have opened new paths for displaying and analyzing data. Alternate views of the same data set provide more insight into the nature of the data. The choropleth map has been the primary display device for portraying the spatial distribution of statistical data aggregated over areal units. However, the standard choropleth map is not as informative with respect to the statistical distribution of the data. Data classification for classed choropleth maps, as well as the transformation of numeric values into lightness values, filters out much of the information regarding the underlying statistical distribution of the data. Traditionally, the map legend has the role of connecting lightness values to their numeric counterparts and providing some information regarding the statistical properties of the data. It is proposed here that a more versatile legend can be constructed based on the ogive cumulative frequency diagram. Ogive legends can present the overall statistical distribution of data in its natural state without resorting to data aggregation, as do histograms and other frequency diagrams. Furthermore, ogive legends can display the distribution of data within any individual class interval.