3D thematic maps – Areals

Although many 3D thematic cartography methods exist, the effectiveness of their use is not known. The described experiment comprised two parts focusing on the evaluation of two 3D thematic cartography methods (Prism Map and Illuminated Choropleth Map) compared to a simple choropleth map. The task in both parts of the experiment was to determine which of the marked areas showed a higher value of the displayed phenomenon. The correctness of answers, response time and selected eye-tracking metrics were analysed. In the first part of the experiment, a higher number of correct answers was found for Prism Maps than for simple choropleth maps, but it required more time to solve the task. The Illuminated Choropleth Map showed a higher proportion of correct answers than a simple choropleth map. During evaluation of the eye-tracking metrics, a statistically significant difference was not found in most cases.

Methods

Procedure

The described eye-tracking experiment comprised two parts. In the first, a simple choropleth map and Prism Map were compared. In the second, an Illuminated Choropleth Map was compared with a simple choropleth map.

Stimuli

The first part of the experiment looked at the differences between simple choropleth maps and Prism Maps. It aimed to verify the following hypotheses:

• H1: A higher number of correct answers will be recorded for the Prism Map
• H2: Work with the Prism Map will be more efficient (in the case of Trial Duration, Scanpath Length and TimeLine visualization

Eight stimuli pairs depicting municipalities in the Czech Republic were created as simple choropleth maps and Prism Maps. The task of the respondents was to determine which of the two marked areas contained a higher value of the displayed (fictitious) phenomenon. Red was used for both visualization methods. 

The stimuli in the second part of the experiment contained simple choropleth maps and Illuminated Choropleth Maps. Their use can eliminate problems with overlapping elevated areas and perspective distortion. The same dataset from the previous part was used and the eight stimuli pairs were again created. The task was also identical — determine which of the two marked areas contained a higher value of the phenomenon.

The second part of the experiment aimed to verify the following hypotheses:

• H3: A higher number of correct answers will be recorded for the Illuminated Choropleth Map
• H4: Work with Illuminated Choropleth Map will be more efficient (in case of Trial Duration, Scanpath Length and TimeLine visualization)

Participants

The experiment was attended by 40 respondents.

Apparatus

For the study, remote eye-tracking device SMI RED 250, developed by SensoMotoric Instruments, was used. This device was operating at frequency of 120 Hz. 

 Analyses

The correctness of answers, response time and selected eye-tracking metrics were analysed in both parts of the experiment.

The TimeLine method introduced by Andrienko, Andrienko, Burch, and Weiskopf (2012) was used to visualize recorded data analytics for the Prism Map variant of pair Q4. The colour of the line represents the distance between the gaze and the line connecting both target areas. For most of the time, participants’ gazes were less than 100 px from the line. Only a few participants looked further than 300 px from the line.

In the next step, five eye-tracking metrics (Trial Duration, Fixation Count, Fixation Frequency, Fixation Duration and Scanpath Length) were analysed.

For the Scanpath Length metric, a statistically significant difference between the simple and Illuminated Choropleth Map was found in four pairs of tasks (Q3, Q5, Q7 and Q8). Higher values of the Scanpath Length metric were found for the simple choropleth map (2D) in Q1, Q5, Q7 and Q8. For pairs Q2, Q3, Q4 and Q6, higher values were found for the Illuminated Choropleth Map.

Conclusion

In the two parts of the experiment, a simple choropleth map was compared with a Prism Map and an Illuminated Choropleth Map.

When comparing a simple choropleth map and Prism Map, a higher number of correct answers was observed for the Prism Map (H1 verified). A statistically significant difference was found in three pairs out of eight. The analysis of Dwell Time in Areas of Interest showed that respondents predominantly compared the colours of the marked areas and did not pay attention to surrounding ones. A statistically significant difference was found in four of the five analysed eye-tracking metrics. Higher values were observed in for the Prism Map (H2 not confirmed). Respondents selected the answer more quickly in simple choropleth maps, but better accuracy was seen for Prism Maps.

Comparison of simple and Illuminated Choropleth Maps revealed a significantly higher number of correct answers than in the first part of the experiment. A higher number of correct answers was found for the Illuminated Choropleth Map, with a statistically significant difference in three pairs of eight (H3 confirmed). The exception was task Q4 where 2D outperformed 3D. No statistically significant difference in all tasks when analysed together between both variants for all five, analysed eye-tracking metrics was found. When testing each pair of tasks separately, a statistically significant difference in most metrics was found only in few pairs (H4 not confirmed).

The experiment concludes that Prism Maps provide conditions for a higher accuracy of responses, but respondents need more time to solve the task. The hypothesis that the Illuminated Choropleth Map is more appropriate than a simple choropleth map has not been confirmed. This finding agrees with the results of the comparison between both analysed 3D variants, where better results were obtained with Prism Maps than Illuminated Choropleth Maps.

Outputs

Stanislav Popelka. 2018. Eye-tracking evaluation of 3D thematic map: Extended Abstract. In ETVIS’18: 3rd Workshop on Eye Tracking and Visualization, June 14–17, 2018, Warsaw, Poland. ACM, New York, NY, USA, 5 pages. https://doi.org/10.1145/3205929.3205932

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