Tuesday, December 12, 2017

How Time Can Bend Space

Showing how long it takes to travel between two different points on a map can be difficult. The most common approach is probably to use an isochrone layer, which uses color to visualize journey times geographically.

In the example above, from Mapbox, driving times from a selected point on the map are shown using different colors. In this example there is a continual gradation between the different colors. However in lots of isochrone maps lines are drawn on the map connecting points which can be reached in the same travel time. For example lines might be used to show how far you can travel in 10, 20 & 30 minute increments.

Another approach to visualizing travel time on a map is to use a time cartogram. In a time cartogram geographic distance on the map is replaced by a time attribute such as travel time (Eric Fischer has posted a few time cartograms of San Francisco to Flickr). However the problem with time cartograms (as with all cartograms) is legibility. When you distort a map by some other variable apart from distance the map can quickly become illegible, as users struggle to recognize the geography.

Nate Parrott has created an interactive time cartogram to show NYC Subway Travel Time. If you click on a subway station on Nate's map then the subway map automatically redraws itself so that the distance to all stations is based on the journey time from your selected station. This interactive time cartogram works really well as a visualization of journey times and it doesn't suffer from the usual problems of illegibility common to many time cartograms.

There are a number of reasons why the NYC Subway Travel Time Map works so well. To start with users are already familiar with the concept that transit maps distort geography and are not strictly geographically accurate. Users are also familiar with the use of colored lines to show the transit system's different lines. If you are already familiar with a line and its stations on the New York subway map then you will still be able to pick it out on a distorted time cartogram based on the line's color. Even if the NYC Subway Travel Time map confuses you then you can still mouse-over a station on the map and quickly reorient yourself with the New York subway.

If you want to make your own isochrone travel time maps then you might like this How to Make a Travel Time Map post.

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