iPhone running app accuracy – is GPS enough?

Four wheels better than two?

I’m not always looking for speed accuracy.

I fondly remember dodging speeding tickets in my post-college Jeep CJ-7. The speedometer read low (honest, officer!) because of the oversized tires. But true mileage is far more important when I run.

Accuracy counts in a running app. Nobody likes missing out on their hard-fought mileage. Worse is grinning over a speedy workout and discovering you got played by a buggy app (this happened to me!).

Speed (and distance) can be measured through many different systems. I won’t trouble you with pitot tubes, but I’m about to get my geek on (I used to be a mechanical engineer). If the gory details bore you, skip to the bottom to find out what’s in version 1.05.

Rotations – how your car works

Rotations measure distance well when calibrated correctly (most people don’t rely on fat tires to flout the law). One app listens for clicks between your spokes and a straw stuck to your front fork (goofy, but it works even on the iPod touch). Too bad nothing rotates during runs or walks.

Accelerometer – how Nike plus works

An accelerometer (tiny chip that senses movement) feels the shock from your steps and multiplies by stride length. Unfortunately, stride length changes during a run. And those freakish cyclists have no stride length at all!

Nike uses a sophisticated formula that measures contact time. This improves accuracy, but is often wrong by 10% and doesn’t map your workout.

GPS satellite orbits

GPS – how Roadbud, Garmin, and Runkeeper work

GPS checks time on atomic clocks orbiting around the earth. It converts this time into distance and then calculates position in three dimensions. That’s the easy part – the Defense Department sweated the details years ago.

Applying GPS to running is a bit more difficult.

• Accuracy depends on listening to a number of satellites in good position. Since the satellites are always moving, sometimes they are too bunched up or just out of sight. The signal can also get blocked or bounced by buildings.
• High accuracy positions are mixed with wide guesses.
• The iPhone tries to “help” GPS by reporting location based on the wifi routers or cell phone towers it hears. A good idea in theory that doesn’t work well in real life. Skyhook Wireless provides this service to Apple and claims higher accuracy than reality – which leads to a lot of ugliness.
• GPS elevations can be wildly inaccurate because the satellites are closer to each other vertically.

Rainy day, precise position

Roadbud (and any other GPS device) has three big jobs:

1. Improve the raw GPS data
2. Determine which positions are legit
3. Combine with other data sources

Correcting GPS is tricky – if you include all the inaccurate positions, the trace jumps all over the map. If you go to the other extreme and drop all the fuzzy positions, you cut out corners and reduce mileage. The magic is in finding the right positions to include.

We’ve created a number of sophisticated methods to improve accuracy – and we’re still adding to the craft.

Here’s what we don’t do:

• Artificially smooth paths (shows a different, incorrect trace)
• Average current speed (takes forever to show correct speed)
• Ask users to turn off wifi (annoying and occasionally cuts out useful data)
• Blame cloud cover (or witches)

Roadbud isn’t perfect, but it works well, even when there aren’t many satellites around. If you ever meet up with an inaccurate run, email your screenshot to support@roadbud.com. We’ll do our best to track down the problem and keep it from happening again.

No related posts.