Open Geosci. 2016; 8:337–359 Research Article Open Access Rehmat Ullah*, Eskedar Zelalem Mengistu, C.P.J.M. van Elzakker, and Menno-Jan Kraak Usability evaluation of centered time cartograms DOI 10.1515/geo-2016-0035 Keywords: centered time cartograms; usability; Dutch rail- Received Nov 16, 2015; accepted Mar 21, 2016 way network Abstract: A time cartogram visualizes travelling-times be- tween locations. It replaces the geographic distance by time distance and distorts the underlying map accordingly. 1 Introduction By distorting the map, time cartograms may give a more in- tuitive and clear picture of travelling-times. The distortion A time cartogram visualizes travelling-times between loca- of the map, however, can make time cartograms harder tions. It replaces the geographic distance by time distance to recognize and use. Although cartograms are becom- and distorts the underlying map accordingly. Two types of ing widespread in use, very little is known about their time cartograms exist: centered and non-centered ones. A usability. This study focuses on the usability of centered centered time cartogram visualizes travelling-times from time cartograms: time cartograms that visualize travelling- a fixed starting location to other destinations inare- times from a fixed starting location to other destina- gion [1–4], while a non-centered time cartogram visualizes tions in a region. We created several centered time car- travelling-times between all pairs of locations [5–7]. tograms to answer spatio-temporal questions related to The use of cartograms is growing due to their capti- the Dutch railway network. Two experiments were per- vating design [8] and also due to the availability of meth- formed: a laboratory test and an online survey. In the lab- ods to create them automatically [e.g. 7, 9–12]. The simple oratory test, we used eye-tracking, thinking aloud, and reason is because they show what researchers need to see video-recording to compare four different designs of cen- and researchers outside of cartography do not find these tered time cartograms to find out which one (or com- hard to understand. The United Kingdom’s Chief Medical bination) of these performs better in answering spatio- Officer used cartograms throughout her annual report to temporal questions and thus, to establish a favorable de- show disease patterns across the UK [13] – whereas a nor- sign strategy for these cartograms. In the online survey, mal map would be much less useful for such projections. centered time cartograms were evaluated against a geo- Dent [14] described cartograms as “innovative,” “interest- graphic and schematic map for accuracy, response time, ing,” and “stylish”. According to Dorling [15, p. 53], “What- and preference. The first experiment suggested that among ever you choose to use cartograms for, from studying par- various designs, the centered time cartogram with empha- ticipation in elections, to the spread of a disease, or the sized railroads is the most preferred design and the cen- social structure of a country, the very different perspec- tered time cartogram without railroads is the least pre- tives they show are likely to alter the way you imagine ferred. The second experiment indicated that overall, cen- the processes behind these patterns to be operating.” Koc- tered time cartograms perform better than the two other moud and House [16, p. 236] stated that “The cartogram solutions in performing spatio-temporal tasks, particu- is a useful tool for visualizing the geographical distribu- larly when the task has a dominant time-related compo- tion of ‘routine’ data in a variety of disciplines, includ- nent. ing politics, social demographics, epidemiology, and busi- ness.” Shimizu and Inoue [17] reported that cartograms are a highly effective method for visually representing statis- *Corresponding Author: Rehmat Ullah: Department of Geo- tical data. Similarly, Wu and Hung [18] found cartograms Information Processing, Faculty of Geo-Information Science and very useful for creating strong visual impacts. Earth Observation (ITC), University of Twente, P.O. Box 217, 7500 Despite the many positive claims, some researchers AE, Enschede, the Netherlands; Department of Computer Systems Engineering, University of Engineering and Technology, Peshawar, are doubtful of the effectiveness of cartograms because Pakistan; Email: [email protected]; Tel: +31 53 4874 373 of their distortion in shape [e.g. 19–22]. In the literature, Eskedar Zelalem Mengistu, C.P.J.M. van Elzakker, Menno-Jan we find only a few studies on the usability of areacar- Kraak: Department of Geo-Information Processing, Faculty of Geo- tograms [e.g. 23–25], and only limited research has been Information Science and Earth Observation (ITC), University of reported on the usability of time cartograms [26, 27]. In Twente, P.O. Box 217, 7500 AE, Enschede, the Netherlands © 2016 R. Ullah et al., published by De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. 338 Ë R. Ullah et al. Figure 1: Four alternative designs of the CTC from Enschede. (a) with railroads (WR). (b) without railroads (NR). (c) with emphasized rail- roads (ER). (d) with emphasized time-circles (EC). The concentric circles indicate travelling-times in increments of 10 minutes from the city of Enschede to all other stations in the province of Overijssel. recent years, various methods have been developed for recording to compare four alternative designs of CTCs (i.e. constructing time cartograms [e.g. 1–3, 5, 17, 18, 28–36], with railroads, without railroads, with emphasized rail- however, the practical value of such cartograms has not roads, and with emphasized time-circles – see Figure 1) yet been established. This motivated us to conduct a de- to test whether railroads add to the readability of CTCs (or tailed and systematic usability evaluation of centered time provide a false impression of time) and to examine the im- cartograms (hereafter abbreviated as CTC) to investigate pact of visual hierarchy on the readability of CTCs. These whether they are indeed useful. designs are abbreviated WR, NR, ER, and EC, respectively. Several CTCs were created to answer spatio-temporal Four groups of participants were asked to perform spatio- questions related to the Dutch railway network. To cre- temporal tasks using these designs and report which one ate these cartograms, we used the method proposed by of these provided an effective and efficient visual repre- Ullah and Kraak [2]. Two user studies were conducted: sentation and best satisfied their requirements. Forty re- a laboratory test and an online survey. In the laboratory sponses were collected and analyzed. In the online survey, test, we used eye-tracking, thinking aloud, and video- CTCs were evaluated against geographic and schematic Usability evaluation of centered time cartograms Ë 339 maps by asking spatio-temporal questions to determine same for all TPs. To avoid potential bias, the tasks and cor- whether they can answer some questions better than the responding maps were presented in random order. Each other two solutions. Eighty-eight persons participated in test was then followed by a questionnaire where the TPs the survey. ranked each design on a Likert scale from ‘strongly agree’ The remainder of this paper is organized as follows: to ‘strongly disagree’, considering three usability aspects: Section 2 discusses the design, implementation, results, (a) pleasant to see, (b) easy to understand, and (c) use and conclusions of the laboratory test. Section 3 reports without confusion. The questionnaire aimed to capture the on the design, implementation, results, and conclusions feelings of the TPs about each design which could then be of the online survey. Section 4 draws final conclusions and interpreted in combination with their suggestions during looks at possibilities for future work. the process of thinking aloud. Potential TPs were invited to participate through an email. They were asked to complete an online survey about 2 The laboratory test their profile and whether they were willing to participate in the actual user test. Profile information was used to al- locate participants to particular test groups (see Figure 3 2.1 Objective, design, and implementation below). Afterwards, a separate invitation was sent to all willing participants to participate in the actual user test. In this research, a laboratory test was conducted applying Test completion time for each TP was on average 40 min- eye-tracking, thinking aloud, and video-recording to com- utes (10 minutes for introduction + 5 minutes for warm-up pare four different CTC designs (see Figure 1). The test ob- tasks + 25 minutes for the actual test). jectives comprised: In total, 48 persons indicated their willingness to par- • To find out which alternative CTC design (or com- ticipate in the user test by completing the profile survey, bination) performs better when answering spatio- however, among those 48 respondents 8 failed to arrive temporal tasks for testing. Among those final 40 test participants, 11 listed • To discover which usability issues are involved when Bachelor, 23 Master and 6 PhD as their highest educational test participants are working with alternative CTC degree. The participants, from 25 different nationalities, designs were postgraduate students and staff members of the Fac- • To investigate whether railroads add to the readabil- ulty of Geo-Information Science and Earth Observation at ity of CTCs (or provide a false impression of time) the University of Twente in the Netherlands and were thus • To determine whether the distortion in CTCs indeed knowledgeable about maps. Participants had varied aca- affects their readability demic backgrounds, with the majority from GIS and Re- mote Sensing fields and most were occasional users of For the laboratory evaluation, CTCs were constructed Dutch trains. Approximately half of the participants knew for the railway network in the Dutch province of Overijs- about cartograms in general, but had limited or no knowl- sel.