Perception accuracy and user acceptance of legend designs for opacity data mapping in GIS

Abstract

In a GIS system, the need to encode geospatial data without standardized cartographic representations, such as population data, weather information, etc., on top of a map is common. This can be done with a layer on top of a base map, with the effect that the base map is partially or fully hidden. Reducing the opacity (or its equivalent - increasing the transparency) of this overlay data layer is a frequently seen solution to show the geographic context. With the colours of the base map being combined with the overlay layer’s colours, the resulting visualization can become difficult to interpret for the end-user.

To help the user decode the values encoded in maps, legends are a common tool for non- interactive maps and data visualizations. This user study investigates the decoding accuracy using a map without a legend, as well as 4 different legend designs for opacity-mapped data overlayed on a static base map. A secondary objective is to measure how helpful the users considered the different legend designs were to decode values.

Baseline categories for comparison were (i) no legend - only having the range of the data values being expressed in text and (ii) a legend design imitating the ArcGIS legend for opacity data mapping.

Three different legend designs were produced to introduce more contextualisation from the map background to the background of the legend and reducing the distance from the legend to the data. This was done using (iii) a sample of the map as background for the legend (iv) having the most common colours of the map base layer as legend background (v) attaching a legend directly to the edge of the overlay data area.

Using a web interface, the users were requested to visually estimate the value at the location of a marker within the overlay data area. In statistical analysis of the results, there was clear statistical effect in reduced errors when having a legend compared to when no legend was included. There was, however, no statistically significant difference in estimation/perception errors between the legend designs tested.

The acceptance of respondents - defined as how useful they considered the legend types were to help estimate the value - did have statistically higher estimates when sampling the map background (marked as iii when introduced in text above) and when attaching the legend to the data area (v) compared to the default ArcGIS design (i).

Popular Abstract

Everyday people all around the world use maps on their phones or on webpages to find information on businesses, traffic, and other data. Putting data that is connected to an area on the map that doesn’t have a common cartographic representation (such as boxes outlining the outer walls of a building or block of houses) usually results in the data-layer hiding the map behind it. Data that can be needed to be shown on top of a map could be for example population density, weather phenomena and COVID-19 rates. A common method is to make the data semi-transparent, so the user can see part of the map background as well. This study investigates if different legend designs (a guide between numbers and visual representation) result in more accurate interpretations of data displayed as a see-through layer.
Examples of data mappings where only the legend types were changed were shown in a web browser to a group of participants. These participants were chosen to already have some experience with maps.

In the examples there was a marker, similar to the type used in Google maps at a location in the data, and the participants were asked to estimate the value at that location by using the legends. All participants were shown 4 different legend designs (b to e in figure below), as well as a version without a legend (a in figure) for comparison. After completing the estimation task, the users were asked how useful they considered the different legend types were for helping them estimating the value of the data at the position of the marker.

The results shown that the presence of a legend helped the users make significantly better estimates. This indicates that including legends is helpful for users when having semi-transparent data over a base map. However, within the sample of responses with legends, none of legend design variations created for the study was measurably better than the others.
The respondents did prefer 2 of the legends designs (c and e in figure below) when asked about how which designs they subjectively thought were easier to use. These legend types made use of including the background of the map in the legend, and moving the legend closer to the data. The improvement in reported user experience that can result from considering factors such as context for the legend and their placement in designing maps ought to be considered when designing maps that displays data without common map representation conventions.

• author: Tillman, Jonas
• supervisor: Micael Runnström
• organization: Dept of Physical Geography and Ecosystem Science
• course: GISM01 20212
• year: 2022
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: GIS, Data Visualisation, Legends, Opacity, Transparency, Visual Data Encoding, Perception
• publication/series: Master Thesis in Geographical Information Science
• report number: 139
• language: English
• id: 9073341
• date added to LUP: 2022-01-25 10:55:51
• date last changed: 2022-01-25 10:55:51