Self-Adjusting Legends for Proportional Symbol Maps

Proportional symbol maps are frequently used to represent numerical data. Commonly used symbols are squares or circles that are area-proportional to the numerical values. Proportional-symbol mapping is included in most geographic information systems (GIS) and is frequently used in digital atlases or Web mapping engines. Graduated symbols are simple for the map author to generate and easy for the reader to understand. However, if the reader is to accurately associate the symbol size on the map with the underlying numerical value, a carefully designed legend is essential. To link a symbol with a numerical value, the map reader mentally positions the symbol in the legend and estimates the value from the symbols shown in the legend. To reduce the associated estimation error, a legend should have the following qualities: