Accuracy of Thematic Maps / Implications of Choropleth Symbolization

Accuracy of thematic maps is identified as having two components: planimetric accuracy and data representation accuracy. The first is of only minor concern for thematic maps. Data representation accuracy, however, is at least as significant to map effectiveness as are the perceptual and cognitive aspects of map reading that have been given so much attention in recent years. The focus of the study presented is on data representation accuracy considering the specific case of choropleth maps. It is demonstrated that three factors, enumeration unit size, enumeration unit compactness, and variability of the distributions mapped are significant in determining enumeration unit aggregate value accuracy and, therefore, map accuracy. Based on experimental results, it is suggested that potential accuracy of choropleth maps could be predicted through measurement of these characteristics. A second experiment is used to demonstrate the applicability of this method to predicting overall choropleth map accuracy as well as the geographic distribution of any error present in the map. In recent years considerable attention has been directed toward the accuracy with which thematic maps communicate spatial information. Less attention, in contrast, has been given to accuracy of the maps themselves. This is a seriousomission. The purpose of the present paper, therefore, is to identify aspects of thematic map accuracy deserving attention and, more specifically, to consider implications of the choropleth technique for accuracy of data representation.