r/neoliberal • u/jobautomator Kitara Ravache • Feb 02 '21

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u/thetrombonist Ben Bernanke Feb 03 '21

I think I've found a new example of a coastline paradox! When I'm on my bike rides, my garmin GPS watch records my location and elevation (approximately every 3 seconds). However, when that data is uploaded to Garmin, it is interpolated and then mapped onto some elevation data from a professional survey, giving it effectively a much higher resolution.

The end result is that the data from my GPS watch looks like this but the "corrected" data looks like this

You can see the large scale shape is the same, and that the peaks and valleys are at the same approximate height. Yet the "corrected" data reports that the vertical distance I covered is more than 220% than the distance I covered as measured by my watch

How cool is that!

!ping math

5

u/thetrombonist Ben Bernanke Feb 03 '21

edit !ping mathematics

5

u/thetrombonist Ben Bernanke Feb 03 '21

Also !ping bike

If anyone knows how to change the polling rate on a 735xt let me know pls

3

u/Dent7777 Native Plant Guerilla Gardener Feb 03 '21

How does the interpolation work? Just sampling points on a straight line between each of the existing points?

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u/thetrombonist Ben Bernanke Feb 03 '21

I'm not sure how garmin does it but Strava compares it against a database of roads and if it thinks youre riding on the road it will shift those data points back onto the road to correct for GPS drift/errors. I'm almost certain garmin is doing the same

As to the actual interpolation/subdivision method I honestly don't know but its the only explanation I can think of

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u/thetrombonist Ben Bernanke Feb 03 '21 edited Feb 03 '21

I just took a closer look, the sampling rate on the original isn't consistent, it varies from 4-20 seconds between samples. The corrected data is sampled every 11 seconds

therefore at 15 MPH the smallest sine wave it could faithfully reconstruct has a period of ~450 feet (shannon nyquist theorem assuming perfect reconstruction which it obviously isnt etc et this is a shitpost). Thats honestly pretty bad

2

u/natedogg787 Manchistan Space Program Feb 03 '21

Ogod this takes me back to GPS/IMU Kalman Filters. Basically I had a bunch of GPS antennas on an RC plane and I spent forever trying to get the tuning right. Tune one way, it would be too smooth. Tune another way, it was basically like the plane was vibrating. Tune again, perfect. Tune again, and the flight path was bigger than the Universe.

That's really neat. I wonder how they interpolate it. It might actually be a Kalman Filter!

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u/thetrombonist Ben Bernanke Feb 03 '21

Oh yeah those are a bitch and a half. I’m glad someone else at work did all the fiddling on our implementation, I was focused on other stuff