13/11/2017

Strava Global Heatmap: Explore the world’s athlete playgrounds

 

What happens when you combine data from 1 billion activities, 3 trillion latitude/longitude points, and a total recorded activity duration of 200,000 years? This new heatmap from Strava of course.

Image: Bike riding in Europe.

The heatmap leverages data from Strava, a community platform that lets athletes track and analyse their activities. Whether its data from marathon runners in Boston, or a cycling adventure in the Mekong, so long as the user conducted the activity publicly, its reflected on the map. For example, check out this comparison of mountain biking versus skiing in Whistler.

Image: It's clear that skiiers and bikers use the mountain in different ways.


As with all user-based data projects, privacy is an inherent concern and, due to the nature of the Strava dataset, the team faced a unique problem.

"Data from non-moving activities can have the undesirable effect of highlighting homes or businesses," they explained, but "a new algorithm does a much better job of classifying stopped points. If the magnitude of the time averaged velocity of an activity stream gets too low at any point, subsequent points from that activity are filtered until the activity breaches a specific radius in distance from the initial stopped point."

Image: Zooming out from a single tile in London to the rest of the world.


Once filtered, the team translated each GPS point into Web Mercator Tile coordinates and used Bresenham’s line algorithm to connect sequential GPS points to create a journey. Then, the team applied a normalisation function to give the map its heat look.

"A slick normalization technique is to use the CDF (cumulative distribution function) of the raw values. That is, the normalized value of a given pixel is the percentage of pixels with a lower heat value. This method yields maximal contrast by ensuring that there are an equal number of pixels of each color. In photo processing, this technique is known as Histogram equalization. We use this technique with a slight modification to prevent quantization artifacts in areas of very low raw heat."

You can read more about how the heatmap was created here or explore it here.
 

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