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Posts: **78** Solid Baller

So I've been putting some effort into finding a means to track skier path. I first tried latching an accelerometer to myself while skiing, which worked great for acceleration data but not so well for calculating velocity and position data without ridiculous error in the numbers. Next I played around with acoustic location (which I still think has promise), but it required an annoying emitter to be worn by the skier. I read quite a bit about GPS location but, at least for now, I don't think it's quite able to provide clean enough data for what I'm looking to do. Then, primarily from reading about the work Bud Davis was doing, it occurred to me that motion capture was the answer.

I took what Bud was doing and moved it a bit further into the digital world. I mapped out some geometrical reference points on a boat, and grabbed a video that was shot from the pylon. Using pixel location I was able to track the skier's center of mass, handle, and front foot. The graphs below are from an open rated mens skier skiing 32 off at 36 mph. The process is fairly manual for now, but seems to provide reasonable results.

Pay attention to the labels and units. The first graph is the angular position (in degrees) vs. time. The second graph is angular velocity (in degrees per second) vs. time. The third graph is a bit of a stretch, but I think it is rather informative. It is a graph of the net force on the skier (assuming a 190 lb. skier) in the direction of the center of rotation (the pylon). It is reasonably equivalent to the line load, at least for certain portions of the path. It should be reasonably accurate from center line to apex. From apex to center line I would expect the actual line load to be slightly higher than what's calculated. I could translate the polar coordinates (reference frame of the boat) to Cartesian coordinates (reference frame of the course), but frankly I don't see the point. As far as I'm concerned the angular data is way more interesting (although I reserve the right to change my mind).

I took what Bud was doing and moved it a bit further into the digital world. I mapped out some geometrical reference points on a boat, and grabbed a video that was shot from the pylon. Using pixel location I was able to track the skier's center of mass, handle, and front foot. The graphs below are from an open rated mens skier skiing 32 off at 36 mph. The process is fairly manual for now, but seems to provide reasonable results.

Pay attention to the labels and units. The first graph is the angular position (in degrees) vs. time. The second graph is angular velocity (in degrees per second) vs. time. The third graph is a bit of a stretch, but I think it is rather informative. It is a graph of the net force on the skier (assuming a 190 lb. skier) in the direction of the center of rotation (the pylon). It is reasonably equivalent to the line load, at least for certain portions of the path. It should be reasonably accurate from center line to apex. From apex to center line I would expect the actual line load to be slightly higher than what's calculated. I could translate the polar coordinates (reference frame of the boat) to Cartesian coordinates (reference frame of the course), but frankly I don't see the point. As far as I'm concerned the angular data is way more interesting (although I reserve the right to change my mind).

6

## Comments

1,396Historical Ballermaybe half of what it likely is.

752Open or 55K Rated SkierFrom what I understand the load you've calculated is centripetal force, which makes sense from an angular perspective but doesn't take into account drag on the ski. With the pylon as the reference point this won't show up in line load because the direction of travel is tangential, but with respect to the course there is motion in the direction of the rope, correct?

6,479Mega BallerI'm very intrigued by how close the angle is to a triangle wave. The corresponding path will not look like that of course, but that shape is interesting and (to me) slightly unexpected.

But what IS expected is that this food for thought...

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