Brychtová, A: Visual distance of map symbols: evaluation of map readability with eye-tracking

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  • 1. 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
  • 2. 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
  • 3. 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
  • 4. 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
  • 5. 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
  • 6. 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
  • 7. 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
  • 8. 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
  • 9.  20% 40% 60% 80% 98%8 pt11 pt14 pt First InDOG Doctoral Conference, 29th October - 1st November 2012, Olomouc
  • 10. 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
  • 11. 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
  • 12. 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
  • 13. 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
  • 14. 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
  • 15.  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
  • 16. Results  post-hoc Kruskal-Wallis tests  differences of perception between pairs of map of concrete color distancefixation count scanpath length time to answercolor observed critical differen color observed critical differen color observed critical differendistance difference difference ce distance difference difference ce distance difference difference ce20%-40% 39.125000 68.99673 FALSE 20%-40% 48.36111 68.99673 FALSE 20%-40% 51.24306 68.99673 FALSE20%-60% 31.665398 68.87767 FALSE 20%-60% 20.10352 68.87767 FALSE 20%-60% 31.85699 68.87767 FALSE20%-80% 82.903329 68.87767 TRUE 20%-80% 79.71386 68.87767 TRUE 20%-80% 95.19837 68.87767 TRUE20%-98% 63.527778 68.99673 FALSE 20%-98% 69.28472 68.99673 TRUE 20%-98% 72.83681 68.99673 TRUE40%-60% 7.459602 68.87767 FALSE 40%-60% 28.25759 68.87767 FALSE 40%-60% 19.38606 68.87767 FALSE40%-80% 43.778329 68.87767 FALSE 40%-80% 31.35275 68.87767 FALSE 40%-80% 43.95532 68.87767 FALSE40%-98% 24.402778 68.99673 FALSE 40%-98% 20.92361 68.99673 FALSE 40%-98% 21.59375 68.99673 FALSE60%-80% 51.237931 68.75840 FALSE 60%-80% 59.61034 68.75840 FALSE 60%-80% 63.34138 68.75840 FALSE60%-98% 31.862380 68.87767 FALSE 60%-98% 49.18120 68.87767 FALSE 60%-98% 40.97981 68.87767 FALSE80%-98% 19.375551 68.87767 FALSE 80%-98% 10.42914 68.87767 FALSE 80%-98% 22.36157 68.87767 FALSE First InDOG Doctoral Conference, 29th October - 1st November 2012, Olomouc
  • 17. First InDOG Doctoral Conference, 29th October - 1st November 2012, Olomouc
  • 18. 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