Tuesday, February 27, 2018

Travel Time Fireworks

The usual way to visualize travel times on a map is by using an isochrone layer. An isochrone layer uses lines to connect locations on a map that can be reached in the same travel time.

Isochrone maps are an established method of visualizing travel times on a map and have been used at least since the nineteenth century. That doesn't mean that cartographers shouldn't experiment with other methods of visualizing travel times on a map. For example 'accessibility fireworks' and 'time cartograms' are two other methods that can be used if you want to display travel time on a map.

Topi Tjukanov has created a really interesting travel time map which shows journey times from Kallio in Helsinki to everywhere else in the Helsinki region using different modes of transport. In Topi's animated map Accessibility Fireworks a number of colored dots are all released on Kallio on the map at the same time. They are then animated as they travel through the city to their destination when they then disappear from the map. Each second of the animation represents one minute of travel time.

This animated isochrone technique works really way in some areas. For example the slowest yellow dots, representing public transit, which linger on the map in the north-east of Helsinki really emphasize how ill-served this part of the city is by public transport. The uniform distribution of the green dots. representing travel by bike, reveals how most parts of the city can be reached in a fairly uniform time by bicycle (although again the far north-east of the Helsinki region takes the longest to reach).

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 area which has been mapped.

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 so much from the usual problems of illegibility common to many time cartograms. This is partly due to the fact that the map's users are already familiar with the concept that transit maps distort geography and are not strictly geographically accurate.
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