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 visualition techniques. 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.

Legend Design

Due to a lack of generalized, objective guidelines regarding digital legends, the designers mostly rely rather on their subjective judgment and personal experience, than on objective principles. Guidance for the design of digital legends in literature is often surprisingly conservative, hence [Dykes et al. 2010] developed and proposed broad guidelines for design considerations, based on extensive research on different kind of resources. Their guidelines are oriented on the following categories:

Selections : To uphold the cognitive focus and avoid dividing the attention of the user between map and legend, show only relevant information. Such is information essential to understand the map, unknown symbols or symbols, which are not self-explanatory.

Layout : As legend uses space, it is of important matter 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. Also they should 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 be depended on the task at hand. For example, while it can be of use to support the user with a legend even before the map is visible in some cases (e.g. bivariate legends, unfamiliar symbolism), it can be of lesser need when it's expected that the legend is used less frequently.

Dynamism: Dynamism comes in handy when dealing with change and alternative possibilities regarding design decisions. New options can be added, when used in combination with the categories above.

Design and Process: During the design process the legend should be handled like map or 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 be combined with each other.

"The Map is Legend" : This concept fuses the roles of map and legend together, this concept can be divided into three subthemes. The "no legend" approach - hiding symbols when precise identification is not needed or self-explanatory ones (also under the aspect of showing symbols only on demand in a dynamic environment); the "map-selection" focus - under which the legend becomes a natural legend, taking into account a two-dimensional spatial mapping; and furthermore the "map becomes legend" theme - which is implemented 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 and furthermore 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. Mapping those arrangements would be the problem to be solved by such a "legend of legends".

"The Relevant Legend" : Relevance of selection is the key motivation of this approach. Only relevant information gets conveyed through the legend (e.g. things which are not on the map are not shown in the legend) . There are several variants derivable from the possibilities offered by this concept. The "pop legend" influenced by community usage; "My Legend, my Map" influenced by group or individual requirements according to tasks at hand; "The Paint-Box Legend" utilizing the possibility to drag symbols from the legend onto the map, for example to identity regions with certain features; "The shopping Basket Legend", which enables the user to drop symbols from the map into legend when needed.

"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 local community customized imagery .

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