Thursday, April 10, 2008

the imprecision of measuring devices

Question: you have multiple measuring devices, which do you trust?
(Answer is none.)

Here's the story. Today was a beautiful day. 70F and sunshine.

I just ran the Charles River loop between the Museum of Science and River St bridges fully instrumented.

Full image here

This is the route recorded by my Garmin Etrex Vista Cx in my back pocket. I also had my Apple iPod Nano with the Nike+ kit.

The Nano computes speed/distance by measuring foot strike time. It overestimates my distance by about 5% usually. But I don't bother to recalibrate it because it would depend on the running surface: treadmill, grass, hard mud, gravel, tarmac, concrete all affect the foot strike time differently.

Clearly, there are some issues with the GPS data points recorded. I'm pretty sure I can't run at nearly 30 mph.

Zooming in at the Museum of Science reveals some oddities:

According to the tracklog, I've been doing some impossible zipping around. Of course, we need to delete those outlier points. Google Earth then reports:

The Nike+ kit doesn't record every footfall. It saves a reading every 2 seconds and performs lots of smoothing:

Finally, there is a well-known reference guide to this course (probably obtained by someone with a measuring wheel):

Summarizing, we have:
Source Distance
Garmin tracklog 9.66
Google Earth 8.9
iPod Nano Nike+ 8.13
My best estimate 7.6
Reference table 7.0

I didn't run the exact reference course, which is a good lower bound.

Additional mileage included running from the MIT Stata Center out to Memorial Drive. (Plus I had to double back at one point since a gate was locked.)

So I can't rely solely on the reference table.

Lesson? The GPS isn't too reliable in this urban environment. As it turns out, the lower tech device (Nike+ kit @ $30 + iPod Nano) is closer to the ground truth.

The possibility of bad track points (e.g. from interference) means that on a short run you have to inspect the GPS tracklog that's recorded. Wish there was filtering software built into the Garmin.


  1. we have both experienced some aberrant readings using the GPS. The tall surroundings seem to confuse the
    GPS in its acquisition of signals.

    What results as the GPS regains its position is a sudden increase in speed.

    On a bicycle recently, I found that my max speed was 70mph. Not likely.

  2. This comment has been removed by a blog administrator.