GPS, Measurements, and Why RunKeeper Is Lying To You

(image courtesy

I’ve reviewed quite a few run-tracking apps for Gear Diary, and just about everyone of them has had some margin of error. Some of them have been minor, and others have just been mind-blowingly egregious. My personal favorite is when RunKeeper for Android told me a 1.5-mile run had been a 7 mile run. Apparently I broke several world records in my first month of running! All joking aside, it’s a common issue for people to comment that a race is short, or long, based on their Garmin or iPhone returning a different distance than the race was supposed to be.

Apparently the folks at the Hampton Half-Marathon were really, really sick of getting yelled at by angry runners brandishing Garmin data, and they’ve laid out a fascinating explanation of both why GPS is a flawed measurement tool AND how races determine certified courses. It’s really fascinating stuff even if you don’t run road races!

For example, I finally know why my GPS is often off on exit markers, telling me a turn is in a 1/2 mile when the exit marker says 1/4 mile or vice versa:

What affects the accuracy of a GPS?

Aa comprehensive comparision of all commercially available GPS devices is beyond the scope of this article. However, many runners use a leading brand, Garmin and the stated current specification using the latest WAAS is 3-5 meters. However that is only for 95% of the time. the other 5% of the time,your GPS may be off as much as 10 meters or more.

10 Meters off, that doesn’t seem like a lot.

(Check again – Thats as much as 30 FEET PER READING)

There is a general misunderstanding of what a GPS device is.  People have a picture in their mind that it follows your path like a traditional wheel.  The image being of a consistent and continuous line being drawn along the path that you run.

FICTION: Your GPS measures the same path you are running.

Not so. Instead your GPS records a series of reading that can be plotted on a chart. So instead of a constant line it is a reading every 1 to 20 seconds. These dots are to the left or right or you or front or back. See chart below. According to the MANUFACTURERS Spec… 95% of the time that dot is within 10 feet or 3 meters. (not that close really when you think about it.)

To better illustrate, let’s just say your GPS is very special Border Collie that will run with you anytime, anywhere. Like any well trained dog they run right next to you, or within 10 feet 95% of the time, but 5% of the time, they see a squirrel and chase it just for a short time. Or they run through a large puddle up to 30 feet away. But like a good dog they start and finish right with you.

Well, that certainly explains the times I’ve received seriously wacky data while running…the GPS satellite took a detour! But what’s really crazy is what races have to go through to receive USA Track and Field certification (basically, approval that the race distance is exact and finishing times can be used for qualifying purposes):

Race directors pay to have race course certified according to USATF standards.  Using a calibrated bicycle, the official course certifier rides the exact course multiple times and carefully takes each corner and rides each tangent. Measurement requires at least 2 rides over the entire length of the course, often more.

To measure a road race course, you cannot use a standard bike or car odometer. Instead, a highly accurate device called a Jones-Oerth device is required. This is a mechanical counter that mounts on the front wheel of a bicycle and shows a series of digits in a row, just like a car odometer reading. Each digit it registers represents only a fraction of a bicycle wheel revolution, so we call this a  “high resolution” measurement: it is very sensitive. One bike wheel revolution may increment the Jones-Oerth counter 3-4 counts per foot. Since the number of counts that the device registers varies according to the wheel diameter, the certifier will see how many counts it takes to ride a known accurate course (calibration course).

There’s far more in the article, but it’s mind-blowing that with the amount of technology we have available to us today that races need to rely on a guy riding a bike MULTIPLE TIMES to accurately calibrate and measure a distance. Later in the article, they explain the calibrator most ride a certified track at the start and end of the day, that weather impacts the exact calibration on a daily basis, and that the rider has to ride through traffic and other everyday obstacles to complete their readings. So not only is this person risking life and limb riding anywhere from 3.1 to 26.2 and beyond miles on a bike, through city streets and parks and trails, they then have to do geometry and algebra to determine how right or wrong they are! All because GPS is simply not accurate enough to be trusted.

Have you ever been fooled by GPS? Do you use a Garmin or Runkeeper and always wondered why it comes up long or short sometimes? Let us know in the comments!

Smuttynose Hampton Half Marathon via Runners World

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4 replies

  1. I learned a bit about that when I geocached!

  2. Runkeeper just stores the GPS data obtained from the phone. If you’re getting inaccurate distances from Runkeeper, blame the GPS accuracy of your phone, not the app.


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