Exploring the Influence of Colour Distance and Legend Position on Choropleth Maps Readability

Abstract

This paper describes design and results of an experiment, which was aimed at exploring the influence of colour distance and legend position on map users’ ability to correctly interpret choropleth maps. Participants of the experiment were asked to find depicted area on the choropleth map and match it with the corresponding legend class by its colour. Experimental stimuli cover five levels of colour distance between neighbouring classes of choropleth maps (ΔE00 = 2, 4, 6, 8 and 10) and six different legend positions within the map sheet. The colour distance was determined by the method CIEDE2000. Results of the experiment were based primarily on analysing the accuracy of answers, and analysing duration of fixations in defined areas of interest. Research proved that increasing colour distance has a positive influence on the ability of users to interpret choropleth maps correctly. Legend position was not found to be significantly important factor of map readability. It was also proved, that sequential colour schemes with visually equal steps between classes are not appropriate, because map users have problems to interpret classes in the middle of the colour scheme. Based on these observations three optimised colour schemes were designed and evaluated. The highest accuracy of answers were observed for the colour scheme in which the lightness of classes if graduated by values of colour distance ΔE00 = 4, 8, 10, 8 and 4.