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Visual distance of map symbols evaluation of map readability with eye-tracking Alžběta BrychtováThis presentation is co-financed by theEuropean Social Fund and the statebudget of the Czech Republic
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Visual distance Jan T. Bjørke, Norway (1996): „It is necessary to maintain sufficient visual distance between map symbols to make them distinguishable.“ visual distance 1. Euclidean distance between symbols influenced by the real spatial location of mapped objects, topology, generalization and map purpose 2. Rate of difference between symbols appearance experiences and ability of map makers to design easily distinguishable map symbols First InDOG Doctoral Conference, 29th October - 1st November 2012, Olomouc
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Visual distance definition “Visual distance of map symbols is exactly determined numerical value describing the degree of variation of visual variables of compared map symbols.” variation of visual variables = change of information transmitted by a map easily distinguishable change of visual variable = easy to read the information First InDOG Doctoral Conference, 29th October - 1st November 2012, Olomouc
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Visual distance necessity to emphasize sufficient difference of visual variables Jacques Bertins visual variables (7) size position shape orientation color hue color value texture First InDOG Doctoral Conference, 29th October - 1st November 2012, Olomouc
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Research task detect influence of color distance between two map elements on the readability of the map assumption: increasing color distance will have positive impact on map readability experimental stimuli were designed to reflect changes in color value (color hue is currently in progress) First InDOG Doctoral Conference, 29th October - 1st November 2012, Olomouc
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Color distance Visual distance definition: “Visual distance of map symbols is exactly determined numerical value describing the degree of variation of visual variables of compared map symbols.” The International Commission on Illumination (CIE) defines the color distance as Euclidean distance of two colors in the CIELuv color space In this case study the distance were computed as a dot product of two RGB vectors in the RGB color space: First InDOG Doctoral Conference, 29th October - 1st November 2012, Olomouc
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Experimental design Eye-tracking experiment was performed statistical analyses of eye-tracking metrics Lab setup: SMI RED 250 eye-tracker 120 Hz sampling rate 0.4°accuracy and 0.03°spatial resolution gaze data classification by dispersion threshold algorithm (ID-T) dispersion threshold = 50 px, duration threshold = 80 ms SMI BeGaze R Project First InDOG Doctoral Conference, 29th October - 1st November 2012, Olomouc
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Experimental design - stimuli 15 simple map stimuli varying in color distance of map labeling and background 20%, 40%, 60%, 80% and 98% color distance 8, 11 ad 14 pt size of labels reduction of the number of independent variables to a minimum participants were asked to find a concrete administrative unit by its name avoid the effect of geographical knowledge First InDOG Doctoral Conference, 29th October - 1st November 2012, Olomouc
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20% 40% 60% 80% 98%8 pt11 pt14 pt First InDOG Doctoral Conference, 29th October - 1st November 2012, Olomouc
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Experimental design - process within subject design – all participant tested under the same condition randomization of trials – prevention of the learning effect 15 First InDOG Doctoral Conference, 29th October - 1st November 2012, Olomouc
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Experimental design - respondents 53 volunteers – students of Palacký University data from 3 respondents with the tracking ratio less than 90% wasn’t taken into account 50 respondents 20-25 years 30 cartographers + 20 non-cartographers 30 men + 20 women data were collected within bachelor thesis of Veronika Obadálková First InDOG Doctoral Conference, 29th October - 1st November 2012, Olomouc
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Monitored metrics fixation count more overall fixations indicates less efficient searching average duration of fixation longer fixation duration indicates difficulty in extracting information, or the object is more engaging in some way scanpath longer scanpath (the length of gaze trajectory over the stimulus) indicates less efficient searching time to answer reflects the success during searching the information First InDOG Doctoral Conference, 29th October - 1st November 2012, Olomouc
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Results Shapiro-Wilk test of normality average fixation fixation count scanpath length time to answer duration p-value 2.2e-16 3.129e-16 2.2e-16 2.2e-16 on the significance level α = 0.05 no one measured eye-tracking metric comes from normal distribution Mann-Whitney test for median comparison different perception between groups of cartographers and non-cartographers average fixation fixation count scanpath length time to answer duration p-value 0,09238 0,988 0,7801 0,2094 On the significance level α = 0.05 no differences between two groups of respondents in measured metrics were proven First InDOG Doctoral Conference, 29th October - 1st November 2012, Olomouc
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Results different perception between groups men and women Mann-Whitney test for median comparison average fixation fixation count scanpath lenght time to answer duration p-value 0.008283 3.875e-09 0.02236 0.6384 On the significance level α = 0.05 the significant result was proven for fixation count, average fixation duration and scanpath length fixation count F<M average fixation duration F>M scanpath length F<M First InDOG Doctoral Conference, 29th October - 1st November 2012, Olomouc
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Kruskal-Wallis ANOVA for mean rank comparison different perception of maps with varying color distance no categorization average fixation fixation count scanpath lenght time to answer duration p-value 0,009817 0,9073 0,005274 0,0012 On the significance level α = 0.05 the significant result was proven for fixation count (H= 13.3192, DF = 4, N=50, P= 0,009817), scanpath length (H= 14.7391, DF = 4, N=50, P= 0,005274) and time to answer metric (H= 17.9129, DF = 4, N=50, P= 0,009817) the mean ranks of these metrics are significantly different among maps with different colour- distance between map labeling and background. First InDOG Doctoral Conference, 29th October - 1st November 2012, Olomouc
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First InDOG Doctoral Conference, 29th October - 1st November 2012, Olomouc
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Conclusions highest values of all analyzed metrics were observed on the map with the minimal color distance (20%), which means that respondents had difficulties in extracting information from these maps of low color distance; increasing color distance leads to decreasing count of fixations, which can mean the higher color distance the more successful information mining; similar statement can be done for scanpath length and time to answer, except the local maximum of measured metrics for maps with % color distance; color distance has evident influence on map readability, but its improvement can be observed only between stimuli with high differences of the color distance. First InDOG Doctoral Conference, 29th October - 1st November 2012, Olomouc
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