During this year’s fall DH training, the DHA’s got some practice using ArcGIS, an online mapping tool that makes it relatively easy to create your own customized maps in one sitting. This post discusses some of the pros and cons, advantages and pitfalls of mapping data. (Note that by mapping I am referring strictly to the use of geospatial maps, not to the more general application of the term that includes graphing.)
Why use a map? Mapping is fun and exciting, and it’s a relatively easy way to build a data visualization that’s interactive and easily facilitates instantaneous spatial comprehension of the data. For these reasons, people are often quick to jump on the “let’s map it!” train whenever there is spatially relevant data. But it’s important to stop and ask this question first: what will a map add to this project that other data visualizations will not? Sometimes, sparsity or lack of variation in your data should disqualify the map idea.
Take this example from Stanford’s Professor Martin Evans, which maps specific locations in and around London that are referenced in works written by authors from London. There’s an abundant amount of data in this data set, and the locations are spread all over London – mapping helps us understand the data, so mapping was a good choice. If, however, you were mapping only locations in London referenced by Sylvia Plath, you might think twice about whether the <10 data points clustered in one small location is worth putting on an interactive map.
Once you’ve determined that a map is worth your time, you might next consider what kind of spatial information you want to convey. Is the data represented well by points on a map? Or is there a path or order to these points? How can you visually differentiate between different paths or groups of points (hint: colors)? Try to create a map that accurately visualizes the story you’re trying to tell with your data. In this example, students at the Georgia Institute of Technology recreated the paths taken throughout the day by characters in Mrs. Dalloway. The smooth, continuous paths tell a better story than a series of sequential points would, and the colors make each path stand out from the others. Above all else, mapping should make it easier for your audience to understand your data, so think hard about how you’re transferring your data to your map. And use colors!
Don’t forget that an important part of mapping is the base map itself, not just the points you put on it. Much of the time, simpler will be better – if the story you’re trying to tell has nothing to do with the terrain of the area, don’t clutter your visual with a terrain base map. Humanities scholars are often excited about using historical base maps, which are historical maps that can be georeferenced onto a modern, digital map of the same location by matching specific points between the two locations. One common problem with historical base maps is that many historical maps are not geographically accurate, so georeferencing them can stretch and distort them to an unusable extent. For example, this 1853 map of Maine from the David Rumsey Map Collection is quite geographically accurate, and would work well as a georeferenced historical base map, but this 1935 world map of post office and radio/telephone services from the same collection is highly geographically inaccurate and would have to be significantly distorted to be georeferenced onto a modern 2-dimensonal map of the world.
Finally, consider how you will communicate the data for each point or path on your graph. Points and paths don’t always speak for themselves, and there will often be metadata or a paragraph of information that necessarily accompanies each data point. How will your user access this information? Is there a key that goes with the map? Do you click on a point to reveal the associated text? Does each point link to more information?
There are many ways to address the above issues and questions, facilitating lots of creativity and flexibility within each project. Above all else, no matter how you approach a mapping project, your map should always give a clear and intuitive answer to the question: what story is this map trying to tell?