Calculating colour distance on choropleth maps with sequential colours – a case study with ColorBrewer 2.0

In diesem Beitrag wird eine Vorgehensweise zur Berechnung der Farbmetrik ΔE00 vorgestellt, die aus benachbarter Fachliteratur der Blick- und Wahrnehmungsforschung abgeleitet ist. Bereits vorausgehende empirische Studien deuten darauf hin, dass hinter dieser Metrik ein zuverlässiges Verfahren steht, das kartographische Gestaltungsüberlegungen unterstützen kann (Brychtová und Çöltekin 2015; 2016). In diesen vorausgehenden Studien wurde gezeigt, dass die Erhöhung von ΔE00-Werten das Unterscheiden von zwei Farben in sequenziellen und qualitativen Farbschemen verbessert. Dabei wurde auch beobachtet, dass Farbdistanz (ΔE00 = 10) hinsichtlich gleicher und unterschiedlicher Bewertung zweier Farben funktioniert, auch wenn die verglichenen Farben (räumlich) weit auseinander liegen. Auf der Grundlage dieser Erkenntnisse wird in dieser Studie eine Auswahl an Farben untersucht, die den weit verbreiteten Farbempfehlungen des Online-Tools ColorBrewer 2.0 entnommen sind. Diese Untersuchung berücksichtigt den Schwellwert ΔE00 = 10 als minimal sicher wahrnehmbare Farbdistanz. Die Resultate zeigen, dass die meisten untersuchten Farben dem Farbdistanzschwellwert entsprechen oder über ihm liegen. Es gibt jedoch auch Farbdistanzen, die beachtlich unter dem Schwellwert liegen. Daraus ist abzuleiten, dass manche weit verbreiteten Farbschemen zu optimieren sind, was weitere empirische Studien erfordert. In this paper, we first present a procedure derived from related vision and perception literature to calculate the colour metric ΔE00, as our previous empirical research let us believe that this is a reliable metric that can be useful in cartographic design decisions (Brychtová and Çöltekin, 2015; 2016). In earlier work, we demonstrated that increasing ΔE00 values consistently improves the human judgement of whether two colours are the same or different both with sequential and qualitative schemes. Furthermore, we observed that colour distance ΔE00=10 ‘works’ in terms of same/different judgements for two colours, even if the compared colours are (spatially) far apart. Using this knowledge from previous work of others as well as our previous own work, we evaluate a subset of colours used in the well-known online colour recommender ColorBrewer 2.0 against ΔE00 = 10 threshold as a minimum perceptually safe colour distance. The results of the evaluation showed that overall, majority of the evaluated colours are equal to or larger than the perceptually safe ΔE00 = 10, however, there are also colour distances that are considerably lower. These findings suggest that some widely adopted colour schemes might not be ideal under some circumstances, and call for more research

The effect of spatial distance on the discriminability of colors in maps

The spatial distance (gap) between map symbols can have a great impact on their discriminability, however, there is little empirical evidence to establish spatial and attribute thresholds. In this paper, we examine the effect of the spatial gap in discriminability of color hue and value, that is, we conducted an online study to obtain performance metrics; then an eye- tracking study to understand participants’ strategies and cognitive processes. Participants completed two experimental tasks (compare two areas and decide if their color is the same; and compare three areas and rank them from the lightest to the darkest). The color distances and the spatial distances were strictly controlled for the compared areas. Our analyses confirmed that, overall, increasing the gap between colors has a consistent negative impact on the ability to differentiate them with both sequential and qualitative schemes. Furthermore, we observed that sequential schemes require larger color distances than qualitative schemes for discriminability. Finally, our results suggested that for qualitative colors, the largest tested color distance ΔE00 = 10 yields considerably higher levels of accuracy in color discrimination (even when the spatial gap between the two colors is large), thus we recommend ΔE00 = 10 to practicing cartographers and other information visualization designers

An empirical user study for measuring the influence of colour distance and font size in map reading using eye tracking

The primary goal of this study is to empirically analyse the influence of colour distance and font size on map readability. We utilized eye-tracking to complement the classical usability metrics; thus, we studied performance metrics such as effectiveness (i.e. success, accuracy), efficiency (i.e. time to answer, task completion time), and selected eye tracking metrics fixation frequency, fixation duration and scanpath speed as well as conducting an area-of-interest (AOI) analysis to understand the performance and strategy issues that may be influenced by colour distance and font size during map reading. The user experiment was carried out in a controlled laboratory where participants were asked to conduct a visual search task and mark the correct answer with a mouse click on a static map on a computer screen. Collected data was analysed through descriptive and inferential statistics. Task completion times for the five tested colour distances show that as the colour distances grow larger, the relative differences in task completion times become statistically significant; empirically confirming our intuition that larger colour distances are better for map readability. The comparison of the scanpath speeds for the tested font sizes suggests that the medium font size leads to a more efficient search

Towards assessing generalization quality with visual complexity measures

This paper presents preliminary results on a visual complexity study with maps where we attempt quantifying generalization quality using algorithmic visual complexity measures

Discriminating classes of sequential and qualitative colour schemes

We present an analysis of the influence of colour distance on the user experience with choropleth and chorochromatic maps. We systematically evaluated five sequential and five qualitative colour schemes in a two-stage user experiment. At first, we conducted an online study to obtain performance metrics accuracy and response time from a large, 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. In this process, along with accuracy and response time, eye-tracking metrics fixation frequency, fixation duration and scanpath speed as well as a gaze transition analysis were utilized. In both experiments, participants were asked to compare two areas with controlled colour distances between them, and decide whether these areas were of the same colour. Results suggest that we are able to discriminate narrower colour distances than commonly used in practice, however, not as narrow as suggested in previous literature

Gaze and feet as additional input modalities for interacting with geospatial interfaces

Geographic Information Systems (GIS) are complex software environments and we often work with multiple tasks and multiple displays when we work with GIS. However, user input is still limited to mouse and keyboard in most workplace settings. In this project, we demonstrate how the use of gaze and feet as additional input modalities can overcome time-consuming and annoying mode switches between frequently performed tasks. In an iterative design process, we developed gaze- and foot-based methods for zooming and panning of map visualizations. We first collected appropriate gestures in a preliminary user study with a small group of experts, and designed two interaction concepts based on their input. After the implementation, we evaluated the two concepts comparatively in another user study to identify strengths and shortcomings in both. We found that continuous foot input combined with implicit gaze input is promising for supportive tasks