Quality of color coding in maps for color deficient observers
For a color deficient observer, the quality of a map or other information design may be defined as the ability to extract features. As color is such important conveyor of information, the colors need to appear correct and be perceived in the desired and intended way. As color appearance is affected by the size of the stimuli, the task of discriminate colors may be even more difficult for a color vision deficient observer. In order to investigate the discriminability of the color coding in an official Norwegian map product, we conducted an experiment involving both color deficient and color normal observers. Also, we investigate to what extent the ability to discriminate colors is influenced by size of the visual field. The experiment revealed that the color vision deficient observers made significant more errors than the normal observers, especially when the visual angle was reduced. Introduction It is established that color is undoubted guide to visual attention [1] Color vision deficiency (CVD) is a condition that affects the ability to distinguish and discriminate colors, and it is believed that about 8% of all males and 0,5 females is affected by such condition [2, 3] Studies reviewed in the work of Cole [4] reveal that up to 60% of the users with abnormal color vision reported problems in observational task as reading color coded charts, slides and prints. Further, they are slower and make more errors in search when color is an attribute of the target object or is used to organize the visual display. It is well known that the color appearance is affected by size of the stimuli, which is often referred to as the color size effect. Carter and Silverstein [5] state that a common experience is difficulty discriminating the multiple small subtense colors on weather, financial, and other maps and charts viewed on print media and conventional size information displays. Color is a cartographic element that is used both aesthetically and as a conveyor of information and is well documented in literature [6 9] Color is the cartographic element that is most frequently misused and is often considered as a difficult cartographic element to use, as it easily draws attention away from the data and goals for the map when it is used poorly [8] Based on the assumption that quality of a map can be based on the ability for a user to extract features, the main goal in this study is to investigate the following research questions: Are the colors in an official Norwegian map product distinguishable for a color deficient observer? Is the ability to distinguish or recognize colors more influenced by variation of visual angle for color deficient observers compared to observers with normal color vision? The paper is organized as follows: First, an overview of basic concepts and related work necessary to understand the research topic is given. Then the experimental setup is described, followed by results, discussion and conclusion. Background and related work This section gives an overview of some of the basic concepts and related work which are essential for the understanding of this research topic. An overview on cartography and color coding, color vision deficiencies, color size effect and visual field is provided in the following pages. Maps and cartography In reference maps, hue is used to symbolize different kinds of features like the blue and green for water and land. Color may be used to represent qualities or quantities and often in a combination, like a topographic map will use hue in order to present qualitative data (like water) or classify data (like class of roads) and value to classify and represent quantitative data (like deepness of the water) Bertin [9] states that "Above all, color exercises an undeniable psychological attraction. In contrast to black and white it is richer in cerebral stimulation, and in numerous cases where it can appear as a luxury, this luxury nonetheless "pays off" It captures and holds attention, multiplies the number of readers, assures better retention of the information, and, in short, increases the scope of the message. Color is particularly applicable to graphic messages of a pedagogic nature" The use of color in maps is most based on conventions and traditions. Some of the conventions are to choose colors that have a similarity to real life objects, like green and blue to represent land and water. Other conventions are to use strong colors to emphasize important objects, like the use of red to represent highways or cities. Standards or specifications exists for thematic maps in different industries like geology [10, 11] hydrographic services and nautical charts [12] and sports like orienteering [13, 14] There is also a standard from the International Standardization Organization, ISO 19117:2012 Geographic informations Portrayal [15] This standard handles presentation of geographic data, but the focus is more on symbolization than use of colors. Then there are the national standards or specifications (often adopting on the ISO 19117) like the standard of the Norwegian Mapping Authority [16] and the specification for cartography on digital displays [17] Color vision and color deficiency Trichromacy is the ability to perceive colors through the three types of cones (L, M and S) that respond to different wavelengths of the visible spectrum. People with full color vision perceive color with all three cones, while people with different color vision anomalies are affected by either a lack of or dysfunction of one or more of the cones. Color vision deficiencies are most often a congenital condition, but may also be caused by medical conditions. Color vision deficiencies are well described in literature, for example in the book by Hansen [2] As for the classification of the color vision it is categorized according to available cones (monochromatism, dichromatism and anomalous trichromatism) and further categorized by the cones that are missing or dysfunctional: Protanopia or Protanomaly (missing or dysfunctional L cone (red) Deuteranopia or Deuteranomaly (missing or dysfunctional Mcone (green) Tritanopia or Tritanomaly (missing or dysfunctional S cone (blue) A missing or dysfunctional cone does not make the observer "color blind" but the color perception is limited and the observer may have problems perceiving certain colors accurately and may confuse color ranges. Image enhancement methods for color vision deficient observers A typical method for enhancing images to improve their quality for CVD observers [18] is to change the color (re coloring methods) Specializations of such methods are the daltonization methods, aiming to compensate poor color discrimination by targeting a specific type of CVD. An overview of existing methods and evaluation of these by measuring accuracy and response time in sample to match and visual search methods involving CVD are well described in the work of Simon Liedtke et al. [19] Other examples of re coloring methods is the palette method proposed by Green [20] and a version of the palette method applied to maps proposed by Kvitle et al. [21] Simulations of different color deficiencies and daltonization methods have been applied in order to create better maps for the CVD observer. The ColorBrewer [7, 22] and Colororacle [23] both based on algorithms like those described in Brettel et al. [24] are examples of applications dedicated to maps. One major problem with simulations is that they operate more like a guideline for a designer with normal color vision. The ColorBrewer is evaluated by CVD observers in experiments involving multiple choice map reading questions as described in the work of Gardner [25] but in a limited group of subjects. The color palettes described in ColorBrewer are well suited for thematic mapping such as choropleth maps, but are not directly applicable to the common reference map (which is the aim of our study) because of the limited set of colors. The Ordnance Survey [26] has developed new reference maps for CVD observers, where the maps are actually developed in an iterative way in cooperation with CVD observers. The group of ten CVD observers was recruited among employees, and these users contributed in several phases of the development. These maps have been embraced by both CVD and color normal observers, even though some of the color coding are unconventional. Color size effect and visual angle The visual angle [27] or the object's angular size is the relationship between the viewing distance and the size of the object. Years of research states that color appearance changes according to different sizes of the same color stimulus. An overview of history of this research and a structural model is given in the work of Carter and Silverstein [5] An attempt to take this effect into account in the CIECAM02 color appearance model is described by Luo and Li [28] and experimental work on variation of size on display are described in Xiao et al. [29] Exploring the color size effect for CVD observers, like the work of Pokorny and Smith [30] Nagy et al. [31] and Paramai et al. [32] indicate a correlation between stimulus size and performance in color naming tasks as many dichromats show trichromacy in color matching and name the colors in fair agreement with the normal observers in larger fields of view =3deg) Also, the experiments [31] revealed large individual difference between the observers and that for some observers the ability to discriminate varied considerably as a function of the stimulus parameters, such as field size, luminance level and method of presentation. As summarized in Pokorny and Smith [30] in large fields dichromats show weak residual trichromatic vision and at higher luminances deuteranopes and protanopes show residual deuteranomaly and protanomaly. Experimental work Based on the goals to test if the colors of a known official map are discriminable for CVD observers, and if color size
Color Imaging: Displaying, Processing, Hardcopy, and Applications