Color distance and legend position

The first goal of this study was to explore the influence of colour distance and legend position on map users’ ability to correctly interpret choropleth maps. The second goal was to analyze optimised sequential color schemes of green and red hue.

Methods

Procedure

We systematically evaluated 5 sequential and 5 qualitative colour schemes in a two-stage user experiment. At first, we conducted an online study to obtain data of large variety of heterogeneous population. Following this, in a controlled lab study with eye-tracking, we re-examined the findings from the online study for a subset of experimental stimuli and further assessed the user experience through an analysis of their visual behaviour.

Respondents were asked to find an area in the map depicted with a dot, match it with one of the six classes of the legend and mark their answer via questionnaire following the experimental stimulus. The order of stimuli was randomized for each single participant in order to avoid the effect of the eye adaptation to handle such tasks.

Independent variables

Three independent variables were studied: colour distance, order of the interval in the color scheme and legend position.

Stimuli

The experiment consists of 64 stimuli which are characterized by 6 different legend positions I to VI  and 9 colour schemes with different colour distance step between classes. 

Five schemes have equal steps of colour distance between their classes – the lightness decreases by the same amount of colour distance (∆E00 = 2, 4, 6, 8 or 10) through the whole scheme. Another four schemes are designed so the steps between classes are not equal. The sequence of colour distance steps conform to values ∆E00 4-8-10-8-4, 6-8-10-8-6 and 4-6-8-10-12 . Each choropleth colour scheme is compounded by 6 classes A to F (A refers to the lightest, F to the darkest shade of the scheme).

 Participants

The experiment was attended by 35 respondents (19-35 years; 19 males and 16 females; 18 experts in the field of cartography and 17 novices (based on self-evaluation)). Prior to projecting experimental stimuli, participants were subjected by the test of colour vision with pseudoisochromatic plates. All participants succeeded in this test.

Apparatus

For the eye-tracking study the remote eye-tracking device SMI RED 250, developed by SensoMotoric Instruments, was used. 

 Analyses

We used eye-tracking to determine the efficiency and strategy of stimulus reading. Our dependent variables were accuracy, speed, and a selected set of eye movement metrics, namely fixation frequency, fixation duration and scanpath speed, to interpret users’ strategies, as well as an area of interest (AOI) analysis.

Conclusion

Results of the experiment didn’t prove any influence of legend position on the map sheet on the effectiveness of the map reading process. The effect of colour distance on the accuracy of reading the choropleth maps was confirmed. The higher is the colour distance between classes of the colour scheme the more accurate are map-users while estimating the value of concrete area comparing it with the map legend. Another interesting finding is that equal colour distance steps are not the best solution in designing choropleth scales. Map users perform better when determining minimal and maximal values, which are plotted by the lightest, respectively darkest colour shade.

Outputs

Brychtova, A. (2014). Exploring the influence of colour distance and legend position on choropleth maps readability. In J. Brus, A. Vondráková, & V. Voženílek (Eds.), Modern Trends in Cartography: Selected Papers of CARTOCON 2014 (Lecture No., pp. 315–326). doi:978-3-319-07926-4

Brychtová, A. and Vondráková, A. (2014). Green versus Red: Eye-tracking evaluation of sequential colour schemes. In SGEM 2014 Informatics, Geoinformatics and Remote Sensing Proceedings Volume III (p. 8). Sofia, Bulgaria: STEF92 Technology Ltd. doi:10.5593/SGEM2014/B23/S11.082

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