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Colour in map displays: issues for task-specific display design
Interacting
with Computers
vol 7 no 2 (1995) 151-165
Colour in map displays: issues for task-specific display design Walter Smith, John Dunn*, Kim Kirsner and Mark Randell
Colour is generally regarded as a desirable property of computer displays chiefly because it supports users’ preattentive visual processes, such as texture segregation, which rapidly organize and structure
screen information. This paper examines the use of colour in computerized map displays of the sort used by geographic information systems. In particular, it focuses on the perception of patterns formed by subclasses of map symbols, defined by colour or shape. Three experiments are reported which confirm the utility of colour, but which also identify two potential problems: interference of taskirrelevant colour and superficial processing of spatial configurations of colour-defined symbols. These findings support a general argument that colour should not be preferred automatically, but rather its utility depends on the cognitive demands of the task for which the display is designed. Keywords:
geographic
information
systems, map displays, colour
Well-designed computer displays for geographic information systems (GIS) must effectively exploit users’ perceptual and cognitive architecture for visual inspection. Particularly relevant to the design of map displays are the preattentive visual processes which underlie perceptual phenomena such as texture segregation, rapid search for unique objects and the grouping of similar objects into perceptual wholes (e.g. Nothdurft, 1992). Preattentive processing is characterised as relatively rapid, unlimited in capacity and resource-free, and occurs before the slower, deliberate and consciously directed attentive visual processes (e.g. Julesz, 1981; Neisser, 1967). Colour is generally regarded as a desirable property of displays (e.g. Davidoff, 1987; Murch, 1987) chiefly because it supports these efficient preattentive processes. Experimental studies have shown, for example, that observers are quicker at picking out symbols defined by colour (e.g. red as opposed to blue symbols) than symbols defined by shape
Department of Psychology, * Department of Psychiatry WA 6907, Australia 0953-5438/95/02/0151-15
University of Western Australia, Nedlands, WA 6907, Australia and Behavioural Science, University of Western Australia, Nedlands,
@ 1995 Elsevier Science Ltd
151
(e.g. square as opposed to circle symbols) from multi-element displays like maps (e.g. Davidoff, 1987; Treisman and Gelade, 1980). Furthermore, colour searches are found to be processed in parallel; that is, their speed is unaffected by the density of irrelevant elements in the display. The research reported here was conducted as part of a wider project concerned with the design of displays for work in complex management domains such as military command, control, communication and intelligence, and emergency management. A distinction can be made between two approaches to the design of interactive worksystems for these domains. The first approach is to provide the user with a single general-purpose map display and general database support. The second approach is to decompose the domain into its component tasks and then design special-purpose displays for each task identified at an appropriate level (Smith et al., 1994; Weiland et al., 1992). Separate tasks for emergency management, for example, might be: mobilization of personnel resources; allocation and reallocation of resources; evacuation decisions and clean-up operations. The current research addresses the use of colour in map displays from the perspective of developing such special-purpose task-specific displays. The argument presented here is that designers should not assume that colour is always desirable, but rather its full perceptual and cognitive implications need to be understood and considered in relation to the demands of each task and therefore each map display. The general argument extends to other coding dimensions, such as flashing and luminance (Van Orden et al., 1993). For investigative purposes, this research focused on a basic map-reading activity which is here described as symbol grouping. Symbol grouping refers to an observer’s ability to perceive spatial configurations formed by particular subclasses of symbols within a map display. This ability is likely to be important for management tasks in the interpretation and assessment of depicted domain situations. Taking a military example, it is often necessary for the user to selectively attend to the configuration of all the ‘friendly’ units on the display, to consider such things as what area they cover or what mutual suport they offer. The ease with which a user is able to focus, maintain and switch attention across different symbol classes depends largely on the relationship between the map’s physical coding system (the use of colour, shape, shading, etc) and the nature of preattentive vision. Figure 1 shows a schematic example of a map display which depicts a geographical area and the location of a number of units represented by symbols. Using stimuli like this, an experimental task was devised to investigate observers’ ability to perform symbol grouping: that is, for example, to perceive the configuration formed by the diamond-shaped symbols in Figure 1 as opposed to the circle symbols. The roles of three symbol coding dimensions are investigated: colour, shape and wholeness.
GIS and cartography David Rhind (1988) produced a ‘GIS research agenda’ in which he identified four key areas of required development: enabling technology (faster networks, greater storage, etc); intelligent knowledge-base system (IKBS) approaches (e.g. 152
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Figure 1. Schematic
GlS map display
of the sort used in experiments
for object extraction from spatial data); non-IKBS (e.g. better statistical algorithms); and finally, non-technical issues. Within the so-called ‘non-technical’ issues, Rhind identified four subareas: visualization; organizational aspects; legal aspects; and, cost/benefit analysis. This paper is intended to contribute to the visualization area of GIS research. Rhind and Mounsey (1987), as described in Rhind (1988), have argued that the non-technical aspects generally are the most important factors affecting the uptake of GIS. Many issues in visualization and the design of effective maps for GIS are simply those of cartography, the tradition of map-making in general. It is widely noted that designers of computerised maps rarely have specialized cartographic skills and that consquently the presentation of spatial information in GIS is poor relative to their paper-based ancestors and cousins. As Weibel and Buttenfield (1992) note: “Poorly designed maps may obliterate the patterns in displayed information”. An important difference between map design for GIS and traditional cartography, however, is that computers enable the use of a task-specific display approach; that is, the design of multiple maps each tailored for specific task demands. Paper maps, in contrast, are usually general purpose in nature, designed to serve a multitude of tasks. The GIS designer has an opportunity to avoid some of the problems resulting from multiple and conflicting user requirements. Place-name labels may be desirable for one task while constituting irrelevant clutter in another. To take an example more pertinent to the present work, colour coding might be turned on for one map and turned off for another to suit the changing cognitive demands of different situations. With dramatic reductions in the cost of GE software, and the SmithEt ul.
153
introduction of GIS-like functions into office applications, computerised map design is likely to become an increasingly prevalent design problem faced by human-computer interaction practitioners. Buttenfield and Mark (1991) provide an analysis of the complete cartographic process, breaking it down into three major stages: generalization (the extraction of spatial data); symbolization; and production. The present work is relevant to the process of symbolization; that is, the process of creating a set of symbols which have some logical relation to the real-world entities being described (e.g. Dent, 1990). The most widely discussed theorist in the area of effective symbology is Bertin (1981; 1983) who attempts to provide a complete framework of the cognitive and visual consequences of using different physical coding dimensions: size, value, texture, colour, orientation, shape. Colour, it is implied by Bertin’s framework, can support a set of symbols to be perceived as similar and support them to be perceived as a group. Shape, in contrast, can support only the former of these. Neither colour nor shape is effective in promoting a set of symbols to be perceived as an ordered dimension or to be perceived in proportion to each other. The experiments reported in this paper are, in part, an empirical examination of that aspect of Bertin’s framework which addresses the properties of colour and shape coding with respect to the perceptual grouping of map symbols.
Experimental goals The user’s ability to attend to the configuration created by subsets of symbols in maps like Figure 1 is likely to be affected by two preattentive processes: visual search, to locate the target symbols, and perceptual grouping, to perceive the emergent configuration. Based on the research described earlier concerning the relationship between colour and preattentive processing, colour is likely to play a significant facilitating role in symbol grouping. This is investigated in Experiment 1. However, two possible problems with colour are also addressed in the paper. The first (examined in Experiment 2) is whether task-related variation in colour can inhibit the perception of configurations of map symbols. More specifically, the experimental question tested was whether grouping of a non-colour symbol class is inhibited when those symbols are randomly and irrelevantly coloured. The issue here is the extent to which colour-driven symbol grouping is governed by top-down factors. The visual search phenomenon known as ‘pop-out’ (where a single unique element, say the only red one, is immediately detected in a multi-element display) is thought to be influenced by top-down factors by some theorists (e.g. Treisman, 1988; Treisman and Gelade, 1980; Treisman and Sato, 1990) who argue that performance is facilitated by knowing in advance on which stimulus dimension the unique element will differ from its surroundings. Against this, some evidence suggests that the task-relevance of a stimulus dimension does not affect pop-out (Theeuwes, 1992). Turning to perceptual grouping, the conventionally reported phenomenon occurs where similar elements in a display are automatically grouped to produce an emergent percept of their combined form. However, evidence exists to suggests that perceptual grouping is not resource-free and 154
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Figure 2. Symbol parentheses)
0.
whole diamond (red)
A
half diamond (red)
0.
whole circle (blue)
0
half circle (blue)
set used in experiments
(symbols
appeared
in colours
indicated
in
questions whether it is preattentive at all (Ben-Av et al., 1992). Opinion is thus divided on the extent to which visual search and perceptual grouping are affected by top-down factors. Therefore no predictions were made concerning whether task-irrelevant colour would interfere in the symbol grouping task investigated here. The second possible problem with colour (examined in Experiment 3) concerns memory for symbol configurations. Following a depth-of-processing approach (Craik and Lockhart, 1972) it was predicted that users’ memory for colour-defined configurations would be worse than for those defined by other dimensions because the application of rapid preattentive processes would lead to a more shallow level of processing. As users of GIS must often move between many different map displays and other types of information, memory for configurations can be an important aspect of some tasks.
Experimental
methodology
The experiments reported here used the naval map symbols shown in Figure 2. These symbols vary on three coding dimensions: colour (red vs blue), shape (diamond vs circle), and wholeness (half vs whole). Colour is redundantly combined with shape so that symbols are either red diamonds (indicating hostile units) or blue circles (indicating friendly units). Variation of this colour+shape dimension with the wholeness dimension produced the four types of symbol shown in Figure 2. Whole symbols depict surface units such as ships and armies, while half symbols depict air-based units. In Experiments 2 Smithet al.
155
and 3, the symbols were sometimes seen with colour turned on, and sometimes with colour turned off. In colour-off conditions, the colour+shape dimension is reduced to a pure shape dimension. The symbol grouping task devised for the current investigation required observers to perceive the configuration formed by a subset of symbols in a map display. On each trial, the subject saw three different windows which replaced each other in succession. First, a cue window appeared, indicating the target class of symbols to be attended to, e.g. blue circles or whole symbols. Second, a map window appeared, like Figure 1, and the subject attended to the configuration of target symbols. When satisfied that a clear ‘mental image’ of the target configuration had been formed, the subject pressed a key initiating a test window containing a configuration of dots. The subject decided whether or not the dots matched the target configuration. Test configurations were either identical to the target configuration (requiring a ‘same’ judgment) or were identical except for one dot being slightly displaced (requiring a ‘different’ judgment). The dependent variables were: map inspection time (i.e., the time spent viewing the map screen), and error rate for the same-different judgment. Both these measures reflect the ease of forming a mental representation of the configuration of target symbols. The symbol grouping task was intended to be more ‘ecologically valid’ than other tasks reported in the experimental search literature which, for example, require subjects to count target symbols or to select the quadrant of the screen in which they appear most numerous (Van Orden et al., 1993). In the present task subjects had to construct a mental representation of the configuration formed by the target symbols which more closely reflects a basic map-reading process. The subjects used in the present investigation were naive volunteers, rather than domain experts. No mention was made to them about the semantics of the symbol codes; that is, the naval meaning of hostile, friendly, surface-based and air-based units. The use of novice subjects reflected the broad aim to learn about the basic nature of preattentive processing of colour-coded and formcoded map symbols in the context of symbol grouping. It also avoided problems associated with expert subjects’ imposed meaning on the stimuli. The experiments are necessarily limited in the number of variables addressed. The experimental search literature points to the importance of a number of other variables (for a recent review, see Wolfe, 1994) which might influence mapreading performance. The present research illustrates the application of the concepts from this literature to GIS design.
Experiment
1
Experiment 1 investigated the effect on performance in the symbol grouping task of two dimensions defining target symbols: first, colour combined redundantly with shape, and second, wholeness.
Method The independent follows. 156
variables
under
investigation,
Interacting
and
their
levels,
with Computers
were
as
vol 7 no 2 (1995)
l
Target Type o colour+shape o colour+shape o wholeness o wholeness -
l l
l
the type of target symbols
searched
for:
- red diamonds (versus blue circles) - blue circles (versus red diamonds) whole symbols (zlersus half symbols) half symbols (versus whole symbols).
Size - the number of target symbols present in the map window: 3 or 4. Density - the density of the map window: 5 distractor symbols with no land contour; 15 distractor symbols with no land contour; 5 distractor symbols with land contour; 15 distractor symbols with land contour. Match - whether or not the test configuration matched the target configuration: same or different.
Nine students and staff at the University of Western Australia served as subjects. The stimuli were generated, sequenced and presented by an Acorn Archimedes microcomputer. Following practice, 16 blocks of trials were presented across two sessions, each block being dedicated to one of the four target Types and comprising a random order of every combination of Size, Density and Match. Symbols were positioned randomly in the map window but under the following constraints. Symbols were prevented from overlapping because no two symbols could occupy the same box of an 8 x 8 invisible grid covering the window. Target configurations were prevented from being too spread out or too dense, by not allowing target symbols in the outer rows or columns of the invisible grid and by not allowing two target symbols to occupy adjacent boxes of the grid. The map window subtended a visual angle of approximately 16”, while individual symbols subtended about 0.5”. For each condition, there were always a random mixture of the two kinds of target symbol and the two kinds of distractor symbol.
Results Instructions to avoid errors were successful and the range of subject mean error rates was 0.39% to 4.83%, except for one subject who was dropped with an error rate of 24.93%. Error rates were not high enough for an analysis by condition. Figure 3 shows the mean inspection times for each target Type against map window Density. A repeated measures ANOVA was carried out on these data with five repeated factors: Block-Series, Type, Size, Density and Match. A main effect of target Type was found (F = 54.66; df = 3,21; p < 0.0001) which was mostly due to searches for colour+shape targets (red diamonds and blue circles) being faster than searches for wholeness targets (whole and half symbols). Target Type also interacted with map window Density (F = 26.22; df = 9,63; p < 0.0001). As seen in Figure 3, this interaction was caused by searches for wholeness targets being greatly affected by the number of distractor symbols presented in the map window, while colour+shape searches were unaffected by the number of distracters. None of the target Types was affected by the presence of the land contour. Perhaps the most striking aspect of Figure 3 is the magnitude of the Smithet al.
157
inspection time (ms) 4 target symbols
3 target symbols half
5ooo half
whole
4ooo whole
3ooo
2ooo I ,_
1000
I ,_
blue
blue
red
red
0I_ 5
15
15
5
number of distractor
symbols
Figure 3. Experiment 1: mean map inspection times for symbol grouping task shown for number of target symbols, target symbol type and number of distracters present differences in inspection time. The most appropriate measurement of map inspection time is for 3 target symbol conditions which were slower overall than for the four target symbol conditions (F = 19.94; df = 1, 7; p < 0.025). Maps containing three target symbols required an exhaustive search which more closely reflects the real situation where map users do not know in advance how many target symbols are present. Detection of a fourth symbol told the subject that no further targets would be found. Based on the three target symbols conditions then, the time to perceive configurations defined by colour+shape took approximately 1.3 seconds, regardless of how many distractor symbols were present. For target symbols defined by wholeness, mean inspection time rose to 2.8 seconds with five distractor symbols present and rose to 4.8 seconds when 15 distracters were present. Experiment
2
Experiment 2 addressed two questions First, were the slow wholeness searches 158
raised by the results of Experiment 1. inhibited by the presence of irrelevant Interacting
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colour variation? In general, does task-irrelevant colour variation impair performance in the symbol grouping task? Secondly, was the superiority of the colour+shape searches purely a result of rapid parallel processing of colour, or did the redundant addition of shape contribute to the advantage? The technique used was to repeat part of Experiment 1 and to compare ‘colour-on’ with ‘colour-off’ conditions.
Method Twelve new subjects were run through a similar design and procedure to that of Experiment 1. The key modification were to the variables Target Type and Density, as follows. l
Target Type: two conditions
with colour turned on -
o colour+shape - red diamonds (versus blue circles) o wholeness(+colour) - whole symbols (versus half) two conditions with colour turned off o shape - diamonds (versus circles) o wholeness - whole symbols (versus half symbols) l
Density:
0,4,8
or 16 distractor
symbols,
with a land contour always present.
The description wholeness( +colour) indicates a discrimination along the wholeness dimension with the presence of task-irrelevant variation in colour. Under colour-off conditions, all symbols appeared in white.
Results Figure 4 shows the mean map inspection times for different target Types against the number of distractor symbols. The first issue, whether taskirrelevant colour inhibits performance, was tested by comparing wholeness and wholeness(+colour) conditions. As seen in Figure 4, there was no significant difference in map inspection times for these two conditions. However, a comparison of errors did reveal a difference. Taking just those trials where there were eight or more distractor symbols present, a significantly higher number of incorrect same-different judgments were made for wholeness( +colour) than wholeness (x ’ = 4 .57; df = 1; ~7 < 0.05). Thus there is some indication that performance was impaired through the addition of task-irrelevant colour. Figure 4 indicates that for shape, wholeness and wholeness( +colour) searches, increases in the number of distractor symbols produced a monotonic rise in the mean map inspection time, with a significant main effect of Density (F = 102.19, df = 3,33; p < 0.0001). However the curve for colour+shape searches is flat indicating independence from map density. This yields a significant Type X Density interaction (F = 37.86, df = 9,99; p < 0.0001). These data show that shape alone (circles vs diamonds) produces a steeper search slope than wholeness (half vs whole symbols) and is therefore a poorer search Smith et al.
159
inspection time (ms)
shape
colour OFF
6000 wholeness
wholeness (+colour)
4000
colour ON l shape +colour
2000
0 4
0
16
8
number of distractor symbols
Figure 4. Experiment 2: mean map inspection times for symbol grouping task shown for tat-get symbol type, whether colour was on or off and number of distractors present dimension.
It seems
likely
that
shape
contributed
little
or nothing
to the
in Experiments 1 and 2. For this particular set of symbols, the dimensions investigated might be placed in order of descending effectiveness for search as: colour, wholeness and then shape.
superiority
of performance
with
colour+shape
found
Experiment 3 Experiment 3 addressed a general question concerning the utility of the rapid map inspection times observed for colour searches in Experiments 1 and 2. Is the additional time spent inspecting maps for shape searches wasted effort, or does it lead to superior memory for symbol layout? This question was investigated by giving subjects an incidental recognition memory test for symbol configurations on completion of the experimental task. That is, after subjects had completed a series of trials of the symbol grouping task, as in Experiments 1 and 2, they were given a surprise test in which they were presented with a mixture of new and previously seen symbol configurations and asked to decide whether they had seen them before or not. The prediction, based on depth-of-processing theories of memory (Craik and Lockhart, 1972), was that subjects would show better memory for configurations which they had seen previously under shape search conditions (i.e., with colour turned off) than under colour+shape conditions (i.e., with colour turned on). 160
Interacting with Computers vol 7 no 2 (1995)
Method Nine new subjects completed eight blocks of a modified version of the map grouping task as used in Experiments 1 and 2. The target Type variable was restricted on two levels both involving shape but one with colour turned on: colour+shape
-
red diamonds
(versus blue circles)
the other with colour turned off: shape -
diamonds
(versus circles)
So that memory of symbol layout was possible, the maps were not generated purely at random for every trial but instead were variants of a small set of predetermined patterns. For trials with colour turned on, the layout of target symbols were derived from one of two patterns and each layout of distracters were also derived from one of two patterns. Similarly, for trials with colour turned off, the layout of targets and distracters were each derived from either of two patterns. This made four patterns of targets and four patterns of distracters to be incidentally learned by the subjects throughout the symbol grouping task phase of the experiment. The actual patterns used were reversed between two groups of subjects for the colour-on and colour-off conditions. On each trial the exact positions of symbols were slightly and randomly perturbed from those specified in the assigned pattern. The number of distractor symbols was fixed at eight throughout. Green square symbols were added to the set of distractor symbols; like all symbols, they appeared white under the colour-off conditions. Following the symbol grouping task, subjects were given an incidental recognition memory test. Subjects were shown target configurations alone and distractor configurations alone, and for each they decided if they had seen them before or not. Half of the test items were generated according to the previously seen patterns while half were randomly generated new patterns. Thus no stimulus in the recognition text exactly resembled the stimuli seen earlier in the symbol grouping task. The independent variables manipulated in the memory test and their levels were as follows: l
l l
Colour: configuration pattern seen previously with colour on; configuration pattern seen previously with colour off. Configuration type: target configuration, distractor configuration. Pattern type: configuration based on pattern seen previously, new random configuration.
Results Figure 5 shows the probabilities that items in the recognition task were judged to have been seen before, i.e., were given a positive identification. These probabilities were calculated for each subject across pattern instances and repetitions in the recognition test. A repeated measures ANOVA was conducted on these data with factors of Colour, Configuration type and Pattern type. As expected, recognition for target patterns was superior to recognition Smithet al.
161
p (positive identification)
distractor contigurations
target config&ations
1.0
0.5 colour +shape shape 0.0
colour +shape
seen previously
new random
new random
seen previously
pattern type Figure 5. Experiment 3: mean probabilities of positive identifications of test configurations in incidental memory task, shown for target distractor configurations, for target type (colour on versus colour off) and pattern type (seen previously versus new random)
for distractor patterns. For targets, the mean probability of a hit (positively identifying a target configuration that had been seen previously) was 0.82 while the mean probability of a false alarm (positively identifying a target configuration that was in fact new) was only 0.06. In contrast, the probabilities of hits and false alarms for distractor patterns were 0.61 and 0.45 respectively, producing a significant interaction between Pattern type (old vs new) x Configuration type (target vs distractor) (F = 52.92; df = 1, 8; p < 0.001). The results support the main prediction that memory for symbol configurations is superior under shape search (colour off) conditions compared to colour+shape (colour-on) conditions. This is seen in the significant interaction between Pattern type (old vs new) and Target type (colour-on vs colour-off) (F = 16.00; df = 1,8; p < 0.01). The magnitude of this interaction was slightly larger for distractor symbols, 0.223, than for target symbols, 0.195, but the three-way interaction was not significant.
Summary
and discussion
The research reported here confirms the advantage of colour coding over shape and wholeness 162
for the basic map-reading
process of perceiving Znteracting
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formed by classes of symbols in map displays. However, it also indicates two potential difficulties with the use of colour: interference of task-irrelevant and colour and superficial processing of colour symbol configurations. Experiments 1 and 2 showed that the perception of configurations of map symbols defined by colour can be carried out by rapid parallel visual search, while shape-defined and wholeness-defined configurations must be ‘picked out’ using slower, deliberate and exhaustive searching. As such, the results are consistent with previous findings in the literature concerning the superiority of colour over form in visual displays (e.g. Davidoff, 1987; Van Orden et al., 1993). For the task used in Experiment 1, map inspection times for configurations defined by colour+shape were approximately 1.2 seconds, regardless of how many distractor symbols were present. For target symbols defined by wholeness, inspection times were generally greater and were strongly affected by the number of distracters present. Experiment 2 found some evidence that task-irrelevant variation in symbol colour could inhibit performance on the symbol grouping task. Subjects made less errors in the perception of configurations defined by symbol wholeness when task-irrelevant colour was turned off; although it should be noted that there was no change in map inspection time. In Experiment 3 subjects were given an incidental memory test separately for configurations of target symbols and configurations of distractor symbols. For both targets and distracters, memory for configurations was better for patterns which were incidentally learned under colour-off conditions. This suggests that the extra time spent searching for configurations of non-colour targets is not wasted but leads to deeper and longer lasting instantiation in memory of the layout of both target and distractor symbols. The design implications of these findings are best interpreted within the approach of designing displays to fit specific tasks (Smith et al., 1994). In principle, the designer’s aim is to achieve the optimal mapping between the display’s physical codes and the task semantics. In practice, this will mean balancing the advantages and disadvantages of various coding dimensions including colour. For tasks which make up complex management domains, colour may be an advantage for some purposes but a disadvantage for others. As predicted from the experimental literature, colour coding is an advantage for tasks like monitoring, where the user must survey a complex map display and switch visual attention smoothly and rapidly between different classes of symbol which define task-relevant categories, for example, hostile units or friendly units in the military domain. In these situations the inspection time advantage observed in Experiments 1 and 2 will be multiplied many times over. However, as indicated by Experiment 2, task-irrelevant colour may also inhibit perceptual searches made on other dimensions, such as symbol wholeness. Here the designer must decide whether the advantages of colour outweigh its interfering effect. For tasks requiring firm attention to form-coded dimensions, the best solution might be a task-specific display with colour turned off. Similarly, for tasks where users must learn or become familiar with symbol layouts, a colour-off display might be most appropriate, as indicated by the results of Experiment 3. This is sometimes the case where the user studies the Smith Pf al.
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map for a certain period, but then works with other displays, such as message interpreters or databases, or works away from computerised displays completely.
Acknowledgments This work was supported by a grant from the Australian Research Council (no. AM9280018). Thanks are also due to our collaborator Australian Defence Industries Limited, Perth, and to Yean Wei Ong for helpful assistance and comments.
References Ben-Av, M.B., Sagi, D. and Braun., J. (1992) ‘Visual attention and perceptual grouping’ Perception and Psychophysics 52, 3, 277-294 Bertin, J. (1981) Graphics Gruyter
and graphic
Bertin, J. (1983) Semiology of Graphics Semiologie graphique: les diagrammes,
information-processing University les reseaux,
(English
translation)
of Wisconsin Press (Translation le cartes Mouton, 1967)
de of
Buttenfield, B.P. and Mark, D.M. (1991) ‘Expert systems in cartographic design’ in Taylor, D.R.F. (ed) Geographic Information Systems: The Microcomputer and Modern Cartography Pergamon Press Craik, F.I.M. and Lockhart, R.S. (1972) ‘Levels of processing: a framework for memory reseach’ 1. Verbal Learning and Verbal Behaviour 11, 671-684 Davidoff, J. (1987) ‘The role of color in visual displays’ in Osborne, D. (ed) Int. Reviews of Ergonomics (VI) Taylor Francis Dent, B.D. (1990) Cartography: Julesz, B. (1981) ‘Textons, Nature 290, 91-97
Thematic
Map Design William Brown Publishers
the elements of texture perception
Murch, G.M. (1987) ‘Colour graphics - blessing Buxton, W. (eds) Readings in Human-Computer Appronch Morgan Kaufman Neisser, U. (1987) Cognitive
and their interactions’
or ballyhoo? in Baecker, R. and Interaction - A Multidisciplinary
Psychology Appleton-Century-Crofts
Nothdurft, H (1992) ‘Feature analysis and the role of similarity in preattentive Perception and Psychophysics 52, 4, 355-375 Rhind, D.W.
vision’
(1988) ‘A GIS research agenda’ Int. 1. Geog. Info. Syst. 2, 1, 23-28
Smith, W., Randell, M., Kirsner, K. and Dunn, J. (1994) ‘Trade-offs in task-specific display design: the balance between specialization and control’ in Proc. 1st Int. Workshop on Cognitive Modelling and User Interface Development (Vienna, Austria) Theeuwes, J. (1992) ‘Perceptual selectivity for color and form’ Perception sics 51, 6, 59%606
and Psychophy-
Triesman, A. (1988) ‘Features and objects’ (14th Bartlett Memorial Lecture) Quarterly Experimental Psychol. 40A, 201-237 Treisman, A. and Gelade, G. (1980) ‘A feature integration Psychol. 12, 97-136
theory of attention’ Cognitive
Triesman, A. and Sato, S. (1990) ‘Conjunction search revisited’ J. Experimental Human Perception and Performance 16, 3, 459478 164
Interacting
J.
Psychol.
with Computersvol 7 no 2 (1995)
Van Orden, K.F., Divita, J. and Shim, M.J. (1993) ‘Redundant use of luminance and flashing with shape and color as highlighting codes in symbolic displays’ Human Factors 35, 2, 195-204 Weiland, M.Z., Cooke, B. and Peterson, B. (1992) ‘Designing and implementing decision aids for a complex environment using goal hierarchies’ in Proc. Human Factors Sot. 36th Ann. Meeting Wiebel, R. and Buttenfield, B.P. (1992) ‘Improvement of GIS graphics for analysis and decision-making’ Inf. J. Geog. Info. Syst. 6, 3, 22%245 Wolfe, J.M. (1994) ‘Guided Search 2.0: a revisited model of visual search’ Psychonomic Bull. and Rev. 1, 2, 203-238
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Colour in map displays: issues for task-specific display design Walter Smith, John Dunn*, Kim Kirsner and Mark Randell
Colour is generally regarded as a desirable property of computer displays chiefly because it supports users’ preattentive visual processes, such as texture segregation, which rapidly organize and structure
screen information. This paper examines the use of colour in computerized map displays of the sort used by geographic information systems. In particular, it focuses on the perception of patterns formed by subclasses of map symbols, defined by colour or shape. Three experiments are reported which confirm the utility of colour, but which also identify two potential problems: interference of taskirrelevant colour and superficial processing of spatial configurations of colour-defined symbols. These findings support a general argument that colour should not be preferred automatically, but rather its utility depends on the cognitive demands of the task for which the display is designed. Keywords:
geographic
information
systems, map displays, colour
Well-designed computer displays for geographic information systems (GIS) must effectively exploit users’ perceptual and cognitive architecture for visual inspection. Particularly relevant to the design of map displays are the preattentive visual processes which underlie perceptual phenomena such as texture segregation, rapid search for unique objects and the grouping of similar objects into perceptual wholes (e.g. Nothdurft, 1992). Preattentive processing is characterised as relatively rapid, unlimited in capacity and resource-free, and occurs before the slower, deliberate and consciously directed attentive visual processes (e.g. Julesz, 1981; Neisser, 1967). Colour is generally regarded as a desirable property of displays (e.g. Davidoff, 1987; Murch, 1987) chiefly because it supports these efficient preattentive processes. Experimental studies have shown, for example, that observers are quicker at picking out symbols defined by colour (e.g. red as opposed to blue symbols) than symbols defined by shape
Department of Psychology, * Department of Psychiatry WA 6907, Australia 0953-5438/95/02/0151-15
University of Western Australia, Nedlands, WA 6907, Australia and Behavioural Science, University of Western Australia, Nedlands,
@ 1995 Elsevier Science Ltd
151
(e.g. square as opposed to circle symbols) from multi-element displays like maps (e.g. Davidoff, 1987; Treisman and Gelade, 1980). Furthermore, colour searches are found to be processed in parallel; that is, their speed is unaffected by the density of irrelevant elements in the display. The research reported here was conducted as part of a wider project concerned with the design of displays for work in complex management domains such as military command, control, communication and intelligence, and emergency management. A distinction can be made between two approaches to the design of interactive worksystems for these domains. The first approach is to provide the user with a single general-purpose map display and general database support. The second approach is to decompose the domain into its component tasks and then design special-purpose displays for each task identified at an appropriate level (Smith et al., 1994; Weiland et al., 1992). Separate tasks for emergency management, for example, might be: mobilization of personnel resources; allocation and reallocation of resources; evacuation decisions and clean-up operations. The current research addresses the use of colour in map displays from the perspective of developing such special-purpose task-specific displays. The argument presented here is that designers should not assume that colour is always desirable, but rather its full perceptual and cognitive implications need to be understood and considered in relation to the demands of each task and therefore each map display. The general argument extends to other coding dimensions, such as flashing and luminance (Van Orden et al., 1993). For investigative purposes, this research focused on a basic map-reading activity which is here described as symbol grouping. Symbol grouping refers to an observer’s ability to perceive spatial configurations formed by particular subclasses of symbols within a map display. This ability is likely to be important for management tasks in the interpretation and assessment of depicted domain situations. Taking a military example, it is often necessary for the user to selectively attend to the configuration of all the ‘friendly’ units on the display, to consider such things as what area they cover or what mutual suport they offer. The ease with which a user is able to focus, maintain and switch attention across different symbol classes depends largely on the relationship between the map’s physical coding system (the use of colour, shape, shading, etc) and the nature of preattentive vision. Figure 1 shows a schematic example of a map display which depicts a geographical area and the location of a number of units represented by symbols. Using stimuli like this, an experimental task was devised to investigate observers’ ability to perform symbol grouping: that is, for example, to perceive the configuration formed by the diamond-shaped symbols in Figure 1 as opposed to the circle symbols. The roles of three symbol coding dimensions are investigated: colour, shape and wholeness.
GIS and cartography David Rhind (1988) produced a ‘GIS research agenda’ in which he identified four key areas of required development: enabling technology (faster networks, greater storage, etc); intelligent knowledge-base system (IKBS) approaches (e.g. 152
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Figure 1. Schematic
GlS map display
of the sort used in experiments
for object extraction from spatial data); non-IKBS (e.g. better statistical algorithms); and finally, non-technical issues. Within the so-called ‘non-technical’ issues, Rhind identified four subareas: visualization; organizational aspects; legal aspects; and, cost/benefit analysis. This paper is intended to contribute to the visualization area of GIS research. Rhind and Mounsey (1987), as described in Rhind (1988), have argued that the non-technical aspects generally are the most important factors affecting the uptake of GIS. Many issues in visualization and the design of effective maps for GIS are simply those of cartography, the tradition of map-making in general. It is widely noted that designers of computerised maps rarely have specialized cartographic skills and that consquently the presentation of spatial information in GIS is poor relative to their paper-based ancestors and cousins. As Weibel and Buttenfield (1992) note: “Poorly designed maps may obliterate the patterns in displayed information”. An important difference between map design for GIS and traditional cartography, however, is that computers enable the use of a task-specific display approach; that is, the design of multiple maps each tailored for specific task demands. Paper maps, in contrast, are usually general purpose in nature, designed to serve a multitude of tasks. The GIS designer has an opportunity to avoid some of the problems resulting from multiple and conflicting user requirements. Place-name labels may be desirable for one task while constituting irrelevant clutter in another. To take an example more pertinent to the present work, colour coding might be turned on for one map and turned off for another to suit the changing cognitive demands of different situations. With dramatic reductions in the cost of GE software, and the SmithEt ul.
153
introduction of GIS-like functions into office applications, computerised map design is likely to become an increasingly prevalent design problem faced by human-computer interaction practitioners. Buttenfield and Mark (1991) provide an analysis of the complete cartographic process, breaking it down into three major stages: generalization (the extraction of spatial data); symbolization; and production. The present work is relevant to the process of symbolization; that is, the process of creating a set of symbols which have some logical relation to the real-world entities being described (e.g. Dent, 1990). The most widely discussed theorist in the area of effective symbology is Bertin (1981; 1983) who attempts to provide a complete framework of the cognitive and visual consequences of using different physical coding dimensions: size, value, texture, colour, orientation, shape. Colour, it is implied by Bertin’s framework, can support a set of symbols to be perceived as similar and support them to be perceived as a group. Shape, in contrast, can support only the former of these. Neither colour nor shape is effective in promoting a set of symbols to be perceived as an ordered dimension or to be perceived in proportion to each other. The experiments reported in this paper are, in part, an empirical examination of that aspect of Bertin’s framework which addresses the properties of colour and shape coding with respect to the perceptual grouping of map symbols.
Experimental goals The user’s ability to attend to the configuration created by subsets of symbols in maps like Figure 1 is likely to be affected by two preattentive processes: visual search, to locate the target symbols, and perceptual grouping, to perceive the emergent configuration. Based on the research described earlier concerning the relationship between colour and preattentive processing, colour is likely to play a significant facilitating role in symbol grouping. This is investigated in Experiment 1. However, two possible problems with colour are also addressed in the paper. The first (examined in Experiment 2) is whether task-related variation in colour can inhibit the perception of configurations of map symbols. More specifically, the experimental question tested was whether grouping of a non-colour symbol class is inhibited when those symbols are randomly and irrelevantly coloured. The issue here is the extent to which colour-driven symbol grouping is governed by top-down factors. The visual search phenomenon known as ‘pop-out’ (where a single unique element, say the only red one, is immediately detected in a multi-element display) is thought to be influenced by top-down factors by some theorists (e.g. Treisman, 1988; Treisman and Gelade, 1980; Treisman and Sato, 1990) who argue that performance is facilitated by knowing in advance on which stimulus dimension the unique element will differ from its surroundings. Against this, some evidence suggests that the task-relevance of a stimulus dimension does not affect pop-out (Theeuwes, 1992). Turning to perceptual grouping, the conventionally reported phenomenon occurs where similar elements in a display are automatically grouped to produce an emergent percept of their combined form. However, evidence exists to suggests that perceptual grouping is not resource-free and 154
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Figure 2. Symbol parentheses)
0.
whole diamond (red)
A
half diamond (red)
0.
whole circle (blue)
0
half circle (blue)
set used in experiments
(symbols
appeared
in colours
indicated
in
questions whether it is preattentive at all (Ben-Av et al., 1992). Opinion is thus divided on the extent to which visual search and perceptual grouping are affected by top-down factors. Therefore no predictions were made concerning whether task-irrelevant colour would interfere in the symbol grouping task investigated here. The second possible problem with colour (examined in Experiment 3) concerns memory for symbol configurations. Following a depth-of-processing approach (Craik and Lockhart, 1972) it was predicted that users’ memory for colour-defined configurations would be worse than for those defined by other dimensions because the application of rapid preattentive processes would lead to a more shallow level of processing. As users of GIS must often move between many different map displays and other types of information, memory for configurations can be an important aspect of some tasks.
Experimental
methodology
The experiments reported here used the naval map symbols shown in Figure 2. These symbols vary on three coding dimensions: colour (red vs blue), shape (diamond vs circle), and wholeness (half vs whole). Colour is redundantly combined with shape so that symbols are either red diamonds (indicating hostile units) or blue circles (indicating friendly units). Variation of this colour+shape dimension with the wholeness dimension produced the four types of symbol shown in Figure 2. Whole symbols depict surface units such as ships and armies, while half symbols depict air-based units. In Experiments 2 Smithet al.
155
and 3, the symbols were sometimes seen with colour turned on, and sometimes with colour turned off. In colour-off conditions, the colour+shape dimension is reduced to a pure shape dimension. The symbol grouping task devised for the current investigation required observers to perceive the configuration formed by a subset of symbols in a map display. On each trial, the subject saw three different windows which replaced each other in succession. First, a cue window appeared, indicating the target class of symbols to be attended to, e.g. blue circles or whole symbols. Second, a map window appeared, like Figure 1, and the subject attended to the configuration of target symbols. When satisfied that a clear ‘mental image’ of the target configuration had been formed, the subject pressed a key initiating a test window containing a configuration of dots. The subject decided whether or not the dots matched the target configuration. Test configurations were either identical to the target configuration (requiring a ‘same’ judgment) or were identical except for one dot being slightly displaced (requiring a ‘different’ judgment). The dependent variables were: map inspection time (i.e., the time spent viewing the map screen), and error rate for the same-different judgment. Both these measures reflect the ease of forming a mental representation of the configuration of target symbols. The symbol grouping task was intended to be more ‘ecologically valid’ than other tasks reported in the experimental search literature which, for example, require subjects to count target symbols or to select the quadrant of the screen in which they appear most numerous (Van Orden et al., 1993). In the present task subjects had to construct a mental representation of the configuration formed by the target symbols which more closely reflects a basic map-reading process. The subjects used in the present investigation were naive volunteers, rather than domain experts. No mention was made to them about the semantics of the symbol codes; that is, the naval meaning of hostile, friendly, surface-based and air-based units. The use of novice subjects reflected the broad aim to learn about the basic nature of preattentive processing of colour-coded and formcoded map symbols in the context of symbol grouping. It also avoided problems associated with expert subjects’ imposed meaning on the stimuli. The experiments are necessarily limited in the number of variables addressed. The experimental search literature points to the importance of a number of other variables (for a recent review, see Wolfe, 1994) which might influence mapreading performance. The present research illustrates the application of the concepts from this literature to GIS design.
Experiment
1
Experiment 1 investigated the effect on performance in the symbol grouping task of two dimensions defining target symbols: first, colour combined redundantly with shape, and second, wholeness.
Method The independent follows. 156
variables
under
investigation,
Interacting
and
their
levels,
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were
as
vol 7 no 2 (1995)
l
Target Type o colour+shape o colour+shape o wholeness o wholeness -
l l
l
the type of target symbols
searched
for:
- red diamonds (versus blue circles) - blue circles (versus red diamonds) whole symbols (zlersus half symbols) half symbols (versus whole symbols).
Size - the number of target symbols present in the map window: 3 or 4. Density - the density of the map window: 5 distractor symbols with no land contour; 15 distractor symbols with no land contour; 5 distractor symbols with land contour; 15 distractor symbols with land contour. Match - whether or not the test configuration matched the target configuration: same or different.
Nine students and staff at the University of Western Australia served as subjects. The stimuli were generated, sequenced and presented by an Acorn Archimedes microcomputer. Following practice, 16 blocks of trials were presented across two sessions, each block being dedicated to one of the four target Types and comprising a random order of every combination of Size, Density and Match. Symbols were positioned randomly in the map window but under the following constraints. Symbols were prevented from overlapping because no two symbols could occupy the same box of an 8 x 8 invisible grid covering the window. Target configurations were prevented from being too spread out or too dense, by not allowing target symbols in the outer rows or columns of the invisible grid and by not allowing two target symbols to occupy adjacent boxes of the grid. The map window subtended a visual angle of approximately 16”, while individual symbols subtended about 0.5”. For each condition, there were always a random mixture of the two kinds of target symbol and the two kinds of distractor symbol.
Results Instructions to avoid errors were successful and the range of subject mean error rates was 0.39% to 4.83%, except for one subject who was dropped with an error rate of 24.93%. Error rates were not high enough for an analysis by condition. Figure 3 shows the mean inspection times for each target Type against map window Density. A repeated measures ANOVA was carried out on these data with five repeated factors: Block-Series, Type, Size, Density and Match. A main effect of target Type was found (F = 54.66; df = 3,21; p < 0.0001) which was mostly due to searches for colour+shape targets (red diamonds and blue circles) being faster than searches for wholeness targets (whole and half symbols). Target Type also interacted with map window Density (F = 26.22; df = 9,63; p < 0.0001). As seen in Figure 3, this interaction was caused by searches for wholeness targets being greatly affected by the number of distractor symbols presented in the map window, while colour+shape searches were unaffected by the number of distracters. None of the target Types was affected by the presence of the land contour. Perhaps the most striking aspect of Figure 3 is the magnitude of the Smithet al.
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inspection time (ms) 4 target symbols
3 target symbols half
5ooo half
whole
4ooo whole
3ooo
2ooo I ,_
1000
I ,_
blue
blue
red
red
0I_ 5
15
15
5
number of distractor
symbols
Figure 3. Experiment 1: mean map inspection times for symbol grouping task shown for number of target symbols, target symbol type and number of distracters present differences in inspection time. The most appropriate measurement of map inspection time is for 3 target symbol conditions which were slower overall than for the four target symbol conditions (F = 19.94; df = 1, 7; p < 0.025). Maps containing three target symbols required an exhaustive search which more closely reflects the real situation where map users do not know in advance how many target symbols are present. Detection of a fourth symbol told the subject that no further targets would be found. Based on the three target symbols conditions then, the time to perceive configurations defined by colour+shape took approximately 1.3 seconds, regardless of how many distractor symbols were present. For target symbols defined by wholeness, mean inspection time rose to 2.8 seconds with five distractor symbols present and rose to 4.8 seconds when 15 distracters were present. Experiment
2
Experiment 2 addressed two questions First, were the slow wholeness searches 158
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colour variation? In general, does task-irrelevant colour variation impair performance in the symbol grouping task? Secondly, was the superiority of the colour+shape searches purely a result of rapid parallel processing of colour, or did the redundant addition of shape contribute to the advantage? The technique used was to repeat part of Experiment 1 and to compare ‘colour-on’ with ‘colour-off’ conditions.
Method Twelve new subjects were run through a similar design and procedure to that of Experiment 1. The key modification were to the variables Target Type and Density, as follows. l
Target Type: two conditions
with colour turned on -
o colour+shape - red diamonds (versus blue circles) o wholeness(+colour) - whole symbols (versus half) two conditions with colour turned off o shape - diamonds (versus circles) o wholeness - whole symbols (versus half symbols) l
Density:
0,4,8
or 16 distractor
symbols,
with a land contour always present.
The description wholeness( +colour) indicates a discrimination along the wholeness dimension with the presence of task-irrelevant variation in colour. Under colour-off conditions, all symbols appeared in white.
Results Figure 4 shows the mean map inspection times for different target Types against the number of distractor symbols. The first issue, whether taskirrelevant colour inhibits performance, was tested by comparing wholeness and wholeness(+colour) conditions. As seen in Figure 4, there was no significant difference in map inspection times for these two conditions. However, a comparison of errors did reveal a difference. Taking just those trials where there were eight or more distractor symbols present, a significantly higher number of incorrect same-different judgments were made for wholeness( +colour) than wholeness (x ’ = 4 .57; df = 1; ~7 < 0.05). Thus there is some indication that performance was impaired through the addition of task-irrelevant colour. Figure 4 indicates that for shape, wholeness and wholeness( +colour) searches, increases in the number of distractor symbols produced a monotonic rise in the mean map inspection time, with a significant main effect of Density (F = 102.19, df = 3,33; p < 0.0001). However the curve for colour+shape searches is flat indicating independence from map density. This yields a significant Type X Density interaction (F = 37.86, df = 9,99; p < 0.0001). These data show that shape alone (circles vs diamonds) produces a steeper search slope than wholeness (half vs whole symbols) and is therefore a poorer search Smith et al.
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inspection time (ms)
shape
colour OFF
6000 wholeness
wholeness (+colour)
4000
colour ON l shape +colour
2000
0 4
0
16
8
number of distractor symbols
Figure 4. Experiment 2: mean map inspection times for symbol grouping task shown for tat-get symbol type, whether colour was on or off and number of distractors present dimension.
It seems
likely
that
shape
contributed
little
or nothing
to the
in Experiments 1 and 2. For this particular set of symbols, the dimensions investigated might be placed in order of descending effectiveness for search as: colour, wholeness and then shape.
superiority
of performance
with
colour+shape
found
Experiment 3 Experiment 3 addressed a general question concerning the utility of the rapid map inspection times observed for colour searches in Experiments 1 and 2. Is the additional time spent inspecting maps for shape searches wasted effort, or does it lead to superior memory for symbol layout? This question was investigated by giving subjects an incidental recognition memory test for symbol configurations on completion of the experimental task. That is, after subjects had completed a series of trials of the symbol grouping task, as in Experiments 1 and 2, they were given a surprise test in which they were presented with a mixture of new and previously seen symbol configurations and asked to decide whether they had seen them before or not. The prediction, based on depth-of-processing theories of memory (Craik and Lockhart, 1972), was that subjects would show better memory for configurations which they had seen previously under shape search conditions (i.e., with colour turned off) than under colour+shape conditions (i.e., with colour turned on). 160
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Method Nine new subjects completed eight blocks of a modified version of the map grouping task as used in Experiments 1 and 2. The target Type variable was restricted on two levels both involving shape but one with colour turned on: colour+shape
-
red diamonds
(versus blue circles)
the other with colour turned off: shape -
diamonds
(versus circles)
So that memory of symbol layout was possible, the maps were not generated purely at random for every trial but instead were variants of a small set of predetermined patterns. For trials with colour turned on, the layout of target symbols were derived from one of two patterns and each layout of distracters were also derived from one of two patterns. Similarly, for trials with colour turned off, the layout of targets and distracters were each derived from either of two patterns. This made four patterns of targets and four patterns of distracters to be incidentally learned by the subjects throughout the symbol grouping task phase of the experiment. The actual patterns used were reversed between two groups of subjects for the colour-on and colour-off conditions. On each trial the exact positions of symbols were slightly and randomly perturbed from those specified in the assigned pattern. The number of distractor symbols was fixed at eight throughout. Green square symbols were added to the set of distractor symbols; like all symbols, they appeared white under the colour-off conditions. Following the symbol grouping task, subjects were given an incidental recognition memory test. Subjects were shown target configurations alone and distractor configurations alone, and for each they decided if they had seen them before or not. Half of the test items were generated according to the previously seen patterns while half were randomly generated new patterns. Thus no stimulus in the recognition text exactly resembled the stimuli seen earlier in the symbol grouping task. The independent variables manipulated in the memory test and their levels were as follows: l
l l
Colour: configuration pattern seen previously with colour on; configuration pattern seen previously with colour off. Configuration type: target configuration, distractor configuration. Pattern type: configuration based on pattern seen previously, new random configuration.
Results Figure 5 shows the probabilities that items in the recognition task were judged to have been seen before, i.e., were given a positive identification. These probabilities were calculated for each subject across pattern instances and repetitions in the recognition test. A repeated measures ANOVA was conducted on these data with factors of Colour, Configuration type and Pattern type. As expected, recognition for target patterns was superior to recognition Smithet al.
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p (positive identification)
distractor contigurations
target config&ations
1.0
0.5 colour +shape shape 0.0
colour +shape
seen previously
new random
new random
seen previously
pattern type Figure 5. Experiment 3: mean probabilities of positive identifications of test configurations in incidental memory task, shown for target distractor configurations, for target type (colour on versus colour off) and pattern type (seen previously versus new random)
for distractor patterns. For targets, the mean probability of a hit (positively identifying a target configuration that had been seen previously) was 0.82 while the mean probability of a false alarm (positively identifying a target configuration that was in fact new) was only 0.06. In contrast, the probabilities of hits and false alarms for distractor patterns were 0.61 and 0.45 respectively, producing a significant interaction between Pattern type (old vs new) x Configuration type (target vs distractor) (F = 52.92; df = 1, 8; p < 0.001). The results support the main prediction that memory for symbol configurations is superior under shape search (colour off) conditions compared to colour+shape (colour-on) conditions. This is seen in the significant interaction between Pattern type (old vs new) and Target type (colour-on vs colour-off) (F = 16.00; df = 1,8; p < 0.01). The magnitude of this interaction was slightly larger for distractor symbols, 0.223, than for target symbols, 0.195, but the three-way interaction was not significant.
Summary
and discussion
The research reported here confirms the advantage of colour coding over shape and wholeness 162
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formed by classes of symbols in map displays. However, it also indicates two potential difficulties with the use of colour: interference of task-irrelevant and colour and superficial processing of colour symbol configurations. Experiments 1 and 2 showed that the perception of configurations of map symbols defined by colour can be carried out by rapid parallel visual search, while shape-defined and wholeness-defined configurations must be ‘picked out’ using slower, deliberate and exhaustive searching. As such, the results are consistent with previous findings in the literature concerning the superiority of colour over form in visual displays (e.g. Davidoff, 1987; Van Orden et al., 1993). For the task used in Experiment 1, map inspection times for configurations defined by colour+shape were approximately 1.2 seconds, regardless of how many distractor symbols were present. For target symbols defined by wholeness, inspection times were generally greater and were strongly affected by the number of distracters present. Experiment 2 found some evidence that task-irrelevant variation in symbol colour could inhibit performance on the symbol grouping task. Subjects made less errors in the perception of configurations defined by symbol wholeness when task-irrelevant colour was turned off; although it should be noted that there was no change in map inspection time. In Experiment 3 subjects were given an incidental memory test separately for configurations of target symbols and configurations of distractor symbols. For both targets and distracters, memory for configurations was better for patterns which were incidentally learned under colour-off conditions. This suggests that the extra time spent searching for configurations of non-colour targets is not wasted but leads to deeper and longer lasting instantiation in memory of the layout of both target and distractor symbols. The design implications of these findings are best interpreted within the approach of designing displays to fit specific tasks (Smith et al., 1994). In principle, the designer’s aim is to achieve the optimal mapping between the display’s physical codes and the task semantics. In practice, this will mean balancing the advantages and disadvantages of various coding dimensions including colour. For tasks which make up complex management domains, colour may be an advantage for some purposes but a disadvantage for others. As predicted from the experimental literature, colour coding is an advantage for tasks like monitoring, where the user must survey a complex map display and switch visual attention smoothly and rapidly between different classes of symbol which define task-relevant categories, for example, hostile units or friendly units in the military domain. In these situations the inspection time advantage observed in Experiments 1 and 2 will be multiplied many times over. However, as indicated by Experiment 2, task-irrelevant colour may also inhibit perceptual searches made on other dimensions, such as symbol wholeness. Here the designer must decide whether the advantages of colour outweigh its interfering effect. For tasks requiring firm attention to form-coded dimensions, the best solution might be a task-specific display with colour turned off. Similarly, for tasks where users must learn or become familiar with symbol layouts, a colour-off display might be most appropriate, as indicated by the results of Experiment 3. This is sometimes the case where the user studies the Smith Pf al.
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map for a certain period, but then works with other displays, such as message interpreters or databases, or works away from computerised displays completely.
Acknowledgments This work was supported by a grant from the Australian Research Council (no. AM9280018). Thanks are also due to our collaborator Australian Defence Industries Limited, Perth, and to Yean Wei Ong for helpful assistance and comments.
References Ben-Av, M.B., Sagi, D. and Braun., J. (1992) ‘Visual attention and perceptual grouping’ Perception and Psychophysics 52, 3, 277-294 Bertin, J. (1981) Graphics Gruyter
and graphic
Bertin, J. (1983) Semiology of Graphics Semiologie graphique: les diagrammes,
information-processing University les reseaux,
(English
translation)
of Wisconsin Press (Translation le cartes Mouton, 1967)
de of
Buttenfield, B.P. and Mark, D.M. (1991) ‘Expert systems in cartographic design’ in Taylor, D.R.F. (ed) Geographic Information Systems: The Microcomputer and Modern Cartography Pergamon Press Craik, F.I.M. and Lockhart, R.S. (1972) ‘Levels of processing: a framework for memory reseach’ 1. Verbal Learning and Verbal Behaviour 11, 671-684 Davidoff, J. (1987) ‘The role of color in visual displays’ in Osborne, D. (ed) Int. Reviews of Ergonomics (VI) Taylor Francis Dent, B.D. (1990) Cartography: Julesz, B. (1981) ‘Textons, Nature 290, 91-97
Thematic
Map Design William Brown Publishers
the elements of texture perception
Murch, G.M. (1987) ‘Colour graphics - blessing Buxton, W. (eds) Readings in Human-Computer Appronch Morgan Kaufman Neisser, U. (1987) Cognitive
and their interactions’
or ballyhoo? in Baecker, R. and Interaction - A Multidisciplinary
Psychology Appleton-Century-Crofts
Nothdurft, H (1992) ‘Feature analysis and the role of similarity in preattentive Perception and Psychophysics 52, 4, 355-375 Rhind, D.W.
vision’
(1988) ‘A GIS research agenda’ Int. 1. Geog. Info. Syst. 2, 1, 23-28
Smith, W., Randell, M., Kirsner, K. and Dunn, J. (1994) ‘Trade-offs in task-specific display design: the balance between specialization and control’ in Proc. 1st Int. Workshop on Cognitive Modelling and User Interface Development (Vienna, Austria) Theeuwes, J. (1992) ‘Perceptual selectivity for color and form’ Perception sics 51, 6, 59%606
and Psychophy-
Triesman, A. (1988) ‘Features and objects’ (14th Bartlett Memorial Lecture) Quarterly Experimental Psychol. 40A, 201-237 Treisman, A. and Gelade, G. (1980) ‘A feature integration Psychol. 12, 97-136
theory of attention’ Cognitive
Triesman, A. and Sato, S. (1990) ‘Conjunction search revisited’ J. Experimental Human Perception and Performance 16, 3, 459478 164
Interacting
J.
Psychol.
with Computersvol 7 no 2 (1995)
Van Orden, K.F., Divita, J. and Shim, M.J. (1993) ‘Redundant use of luminance and flashing with shape and color as highlighting codes in symbolic displays’ Human Factors 35, 2, 195-204 Weiland, M.Z., Cooke, B. and Peterson, B. (1992) ‘Designing and implementing decision aids for a complex environment using goal hierarchies’ in Proc. Human Factors Sot. 36th Ann. Meeting Wiebel, R. and Buttenfield, B.P. (1992) ‘Improvement of GIS graphics for analysis and decision-making’ Inf. J. Geog. Info. Syst. 6, 3, 22%245 Wolfe, J.M. (1994) ‘Guided Search 2.0: a revisited model of visual search’ Psychonomic Bull. and Rev. 1, 2, 203-238
Smith et al.
165