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Is That My Real Speed?

Last Updated: Nov 05, 2012 12:25PM EST


We spend a lot of time analyzing and massaging data to make sure it’s accurate. Accuracy is important for two reasons 1.) If we claim to have the best service for giving skiers and snowboarders stats, it better be accurate 2.) once you introduce the concept of a leaderboard, inaccurate stats leads to users loosing motivation to perform better for the leaderboard (not to mention faith in their own stats being accurate).
The most common question and concern is with speed. There are numerous devices out there--often also using GPS for speed measurements--and sometimes their readings are higher than those on AlpineReplay. This is intentional. Take the following graph of speed in kph for a single ski run:



Notice that peak right around the middle. The reading is 79.9kph. The reading right before it is 50.1kph. Now for some physics. Since these readings are a second apart, we can figure out the average acceleration that happened over this time:

acceleration = change in velocity / change in time
acceleration = (79.9 - 50.1) / 3.6 / 1 = 8.2 m/s2

That’s some serious acceleration! Had you been falling through the air, your acceleration would be 9.8m/s2 and chances are you aren’t falling through the air for a full second on a standard resort run.

So what we do is look at the next value and see if it makes more sense over a two second period. It almost always does. That’s the point we take as your maximum speed.

What does a more typical run look like? The slope at the peak is much more shallow as with this graph.

This snowboarder reached her peak speed only once, but the path to this speed is much more gradual.


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