Abstract. This paper deals with the extension of internet-based geographic information systems with functionality for exploratory spatial data analysis (ESDA). The specific focus is on methods to identify and visualize outliers in maps for rates or proportions. Three sets of methods are included: extreme value maps, smoothed rate maps and the Moran scatterplot. The imple-mentation is carried out by means of a collection of Java classes to extend the Geotools open source mapping software toolkit. The web based spatial analysis tools are illustrated with applications to the study of homicide rates and cancer rates in U.S. counties. Key words: internet GIS, exploratory spatial data analysis, spatial outliers
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