Teaching:TUW - UE InfoVis WS 2010/11 - Gruppe 06 - Aufgabe 2
Rethinking Map Legends with Visualization
The following article is a summary of [Dykes et al. 2010], where the design of map legends is combined with visualization techniques.
Introduction
Maps are cartographic projections from the 3 dimensional Earth’s surface to a 2 dimensional plane. Due to the loss of information caused by this transformation legends are used in order to make the maps easily accessible. But many of the information designs used in maps were developed for a static context whereas map legends in a dynamic environment offer new possibilities. In order to develop new guidelines for legend design current problems and successes with on-screen maps were identified. Based on these results aspirations, tasks and functionality of digimap legends were determined. All the work was done in cooperation with EDINA who provide digital mapping services for the UK.
Legend Design
Due to a lack of generalized, objective guidelines regarding to digital legends, the designers mostly rely on their subjective judgment and personal experience rather than on objective principles. Guidance for the design of digital legends in literature is often surprisingly conservative, hence broad guidelines for design considerations were developed, based on extensive research on different kind of resources. These guidelines are oriented on the following categories:
Selection : To uphold the cognitive focus and avoid dividing the attention of the user between map and legend, only relevant information should be shown, i.e. unknown or not self explanatory symbols.
Layout : As a legend uses space, it is important to arrange its elements in a meaningful way. The representation structure should follow a visual logic. The most effective legends are those, which are reflecting relationships among data.
Symbols : The way features are represented by symbols must be consistent with the ones used in the map and to each other. They should also relate to the mapped phenomenon they represent and fulfill expectations arising within a social and cultural context.
Position: When, where or how a user is encountering a legend should depend on the task at hand. On the one hand it can be useful to support the user with a legend even before the map is visible (e.g. bivariate legends, unfamiliar symbolism) but on the other hand a legend is less important when it is used only sporadically.
Dynamism: Dynamism supports the interactive change of the above described categories.
Design and Process: During the design process the legend should be handled like a map or a data graphic. The legend should be seen as spatial information visualization, to which aesthetic and relational qualities apply.
Legend Themes
To provide support in the process of software development and to frame ideas, legend-themes were created. Those themes describe perspectives on the legend utilizing the guidelines, in accordance with the defined problem domain. The themes can be used alone or can be combined with each other.
"The Map is Legend" : This concept fuses the roles of map and legend together and can be divided into three subthemes. The "no legend" approach hides symbols when precise identification is not needed or self-explanatory(also under the aspect of showing symbols only on demand in a dynamic environment). In the "map-section" the legend becomes a natural legend, taking into account a two-dimensional spatial mapping (Fig. 1). And finally the "map becomes legend" theme, which is designed as a dynamic map, allowing transformation between legend elements and the according spatial map allocation.
"The Legend as Statistical Graphic" : In this case the legend acts as an statistical overview of the current selection, reflecting a summary of map content. Furthermore it provides the basis for exploring the map in a query-like manner.
"A Legend of Legends" : The possibility of having different styles of symbol schemes raises the question how to relate and navigate between those alternative symbol sets. To solve this problem alternative styles are displayed simultaneously and related using layout.
"The Relevant Legend" : Only the relevant information gets covered by the legend. As relevance changes over time and depends also on the data and the users, several variants can be derived from this concept. For example the legend may be changed according to community usage, task, knowledge, location or symbolism.
"The Source independent Legend" : The characteristics of how the items are being grouped are not related to the sources the described data comes from. Yet the symbols should give information of their origins to make the sources of related data distinguishable.
"The Ground Truth Legend" : In this case, the symbolism used depends on local community based decisions. Relevance between map and legend gets augmented by community customized imagery .
Digital Prototypes
In order to demonstrate the possibilities along with the prepared guidelines four prototypes were developed. The objective of this task was to communicate the ideas behind the guidelines and to get feedback for further developments. After the development of the first four prototypes a second round of discussion with EDINA followed. After some consideration time, EDINA issued priorities for the next round of prototypes. The focus was directed at a specific data set (Strategi), and the general enhancement of the features of the prototypes was requested. Briefly, the following six prototpyes were developed:
"The Map is the Legend" : In this prototype the 1D legend can be transformed into a 2D map, which can furthermore be transformed into a 2D legend.
"The Legend as Statistical Graphic – Bar Chart" : The legend in this prototype consists of bar charts, which represent the summery of the features (e.g. Building Area, Road Or Track Area, …) of the current map view. The program also supports bidirectional interactions for selection and highlighting between the map and the legend.
"The Legend as Statistical Graphic – Matrix" : This prototype supports the exploration of co-occurring features. For this purpose two feature classes can be assigned to the two axes of this 2D legend. The resulting matrix describes the degree of co-occurrence of these features in the current part of the map.
"The Legend as Statistical Graphic – Hierarchy" : Prototype 4 explores the idea of using a hierarchical legend for data that is hierarchical in nature. In the case of this paper a 2D spatial treemap [Wood and Dykes, 2008] is used for the depiction of the national bedrock geology classification (Fig. 2) . The prototype supports bi-directional interactions between the map and the legend.
"The Map is the Legend – Strategi" : Here “The Map is the Legend” theme is applied to Strategi. Because of the significant amount of features, there were some new design issues. The screen space didn’t suffice to harbor the whole legend, in the case of a large font and a number of features selected.
"The Legend as Statistical Graphic – Strategi" : In this prototype the legend reflects the hierarchical structure of the strategi dataset (point, line, area). The total occurrences of the statistical data values are encoded in the size of the symbol (national) and in the color (local).
Conclusion
The impact of this study was in general very positive. Many of the identified problems with on-screen map legends could be addressed. The impact evaluation made also clear that visual legends are much more advantageous than static legends. Moreover EDINA, the initiator of the study, regarded the work as “inspirational” and has secured funding for an additional project.
References
[Dykes et al., 2010] Jason Dykes, Jo Wood, Aidan Slingsby. Rethinking Map Legends with Visualization.IEEE Transactions on Visualization and Computer Graphics, 16(6): 890 - 899 , November/December 2010
[Wood and Dykes, 2008] J. Wood and J. Dykes. Spatially ordered treemaps. IEEE Trans. Vis. Comp. Graphics, 14(6):1348–1355, 2008
External Links
- EDINA - http://edina.ac.uk/digimap/
- Map symbols - http://www.ordnancesurvey.co.uk/oswebsite/education/pdf/50KLegend-Eng.pdf
- Strategi - http://data.gov.uk/dataset/os-strategi