Suitability of colour hue, value, and transparency for geographic relevance encoding in mobile maps

Abstract

Mobile maps have become a commodity in the last decade. Despite the widespread use of mobile map apps, effectively presenting relevant geographically referenced information on small screens is still a challenge. Surprisingly, not much research has been dedicated to mobile map design issues. With presenting results from an empirical study on the suitability of the visual variables colour hue, value, and transparency for visually communicating the geographic relevance of points of interest (POI) on mobile maps, we aim at contributing to close this research gap. The premise for our study is that mobile map viewing behaviour is influenced by visual attention allocation. This, in turn is determined by perceptual, bottom-up processes dependent on the visual saliency of stimuli, and cognitive, top-down processes which are dependent on task, goal, prior knowledge, and training. With the study we address following research questions: 1) which of the three visual variables (colour hue, value, and transparency) optimally directs visual attention on small screen maps to the correct encoding of the geographic relevance? 2) do users understand the encoding of the three visual variables into three levels of high, medium and low geographic relevance of POI? and 3) do established cartographic conventions, such as value ('the darker, the more') and the traffic light metaphor (green hue as a positive/higher value, yellow as a medium value and red as a negative/lower value) work on small displays? These research questions address the saliency of POI symbols, their semantic understanding, and cartographic conventions. We expect to get insights into both perceptual (bottom-up component of attention) and cognitive (top-down component of attention) understanding of geographic relevance encodings in mobile map displays. Methods: To evaluate the aptitude and effectivity of the three visual variables we conducted a test using the eye-tracking method. Participants: The participants were 27 subjects (13 female, 14 male) between 18 and 43 years old. They represent all strata of the population and had no previous knowledge of cartography (which was tested for in a pre-experimental questionnaire). Material: The stimuli were a total of 18 mobile maps, six for each visual variable, and were designed to fit an Apple iPhone 4 display. The base map, taken from OpenStreetMap, shows a road map of the Italian municipality of Trentola-Ducenta unknown to the study participants (see Figure 1). For colour hue, the variation is green, yellow, and red; for value the variation is between light, medium, and dark blue; for transparency the variation is between no, medium and high transparency of a violet colour hue. The base map and the three depicted POIs were the same for all maps, only the position of the POIs on the maps were modified (see Figure 1). Figure 1. Mobile map stimuli for colour hue, value, and transparency condition. Study design: The study follows a within-subject design, i.e. each participant sees all three conditions, i.e. colour hue, value, and transparency. This independent variable (encoding of geographic relevance of the POIs) is then structured into three sub-levels. The dependent variables are time to first fixation (TFF), the correctness of the order of the fixated symbols (COFS), the correctness of the relevance ranking (CRR), and response time (RT). To address the bottom-up, stimulus driven aspect of geographic relevance processing, in the first part of the experiment participants have no prior knowledge. The 18 maps are presented to them on the screen scaled to the iPhone 4 size in randomised order to compensate for potential learning effects. In between, blank white screens with a black cross in the centre are shown to calibrate participants' gaze recorded by the eye-tracker. In the second part of the experiment participants had additional information in the form of a scenario. They had to imagine being in a foreign city and looking for a restaurant. This framing should set a goal, invoke expectations, and address the top-down processing. After a training phase, participants were asked to order the POI symbols displayed in the 18 maps from highly relevant to less relevant. This should provide insights, if they also had understood the semantic coding.