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This is a 2 part question:

  1. I have concerns over the general accuracy of a GPS device to calculate speed. Mainly because I will be using the device in Eastern Europe, and not sure on the accuracy of the coverage.

  2. Because of this concern I was thinking of pairing an android based gps app to wireless sensors mounted on the wheel, much like you would do with a Garmin 800.

I'm looking for the following reply:

  1. Is my concern justified, or will a phone based GPS be accurate enough to give me a realistic speed indication?

  2. If my concerns are justified, can you recommend some ways to pair the wheel sensors with apps?

  3. What general features should I look for in a phone to bring out the best of it for use as a cycle computer / gps.

Thanks in advance!

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It should be noted that wheel sensors aren't the most accurate things in the world. Even if you take the time to do a wheel roll-out when programming your computer, things like your wheels flexing, the compression of the tire itself as it meets the road, slippage, etc. cause accuracy problems for these things. On 50 mile rides, there has been as much as a half mile difference between me and the next guy. –  Billy ONeal Jun 16 '11 at 17:57
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Billy, if your computer is setup correctly, you might see 10 feet of difference on a 50 mile ride. If you are half a mile off, you're doing something wrong. It's pretty simple math, really. –  zenbike Jun 19 '11 at 6:30
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2 Answers

up vote 3 down vote accepted

The wheel sensor add on will do quite a bit to improve the accuracy of the GPS based current speed. The distance will generally be accurate, to within error values of 50 meters, or so depending on hardware. Most phones, especially with android, are on the lower spec of GPS design, using wifi databases and cellular triangulation to augment the GPS positioning sensor.

They generally use open source maps, like Google Maps, which often change, and are less than consistent about coverage. I would recommend using an ANT+ add on, if you are concerned about exact speed and distance.

Wahoo makes decent ANT+ sensors for the iPhone, but I'm not aware of a quality set up for android. There are too many devices, so it would be difficult to create a single device guaranteed to work across the board on android, even with the same OS version. I assume this is why they haven't made an Android version yet.

Hope that helps.

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Google maps is not OpenSource while OpenStreetMap are –  Chmouel Boudjnah Jun 23 '11 at 21:08
    
True, sorry for the inaccuracy. I meant openly accessible, not truly Open Source. –  zenbike Jun 25 '11 at 7:39
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I think your general GPS concern for accuracy is reasonable, but it is the same worldwide, not just outside the US.

The full GPS constellation is in orbit with spares, so there is world wide 3-D coverage of the planet.

It happens that there are moments in time in any arbitrary location, where the coverage could be better, and the US military has a group that actually forecasts these for their armed forces.

I.e. Do not try a precision bomb drop at a moment that the satellite configuration is not optimal.

GPS has other issues (trees, buildings, clouds) that get in the way. As Billy ONeal notes, wheel sensors are not perfect either. An example of GPS issues, occurred when two friends of mine with Garmin Forerunner 201's (GPS only devices) on their arms ran a known measured half marathon course side by side, and had a 4K discrepancy at the end of it! (We blame firmware for that one). So firmware can make a big difference on GPS devices.

I would be very interested in comparing results from a GPS and wheel sensor on a longer ride.

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As noted above, the computer requires proper setup. With a rollout, I would trust the computer, more than that "known distance" course. Who measured it, after all. And what type of computer did they use? The Forerunner 200 uses only GPS for distance, BTW, and they were running. No wheels. So what does that have to do with Wheel sensor accuracy? –  zenbike Jun 19 '11 at 6:32
    
@zenbike: I mixed two topics up. The Garmins have no wheel sensors, they are GPS only devices. Thus that was a comment about the GPS accuracy, based on firmware differences. –  geoffc Jun 19 '11 at 12:10
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@geoffc: Garmins for cycling use do have wheel sensors, specifically to improve the accuracy of speed and distance measurement when good sattelite coverage is not available, or for riders who prefer accuracy better than the 5 meters which is commercially available on a civilian device. –  zenbike Jun 20 '11 at 8:19
    
@zenbike: I knew they had sensor support in newer ones, but my example was to demonstrate how much the accuracy of a GPS can vary, between two almost identical devices, the difference being just a firmware upgrade. I.e. GPS is not perfect by a long shot. –  geoffc Jun 20 '11 at 10:15
    
@geoffc: No it isn't. But that isn't the question here. Whether a wheel sensor will increase accuracy of speed and distance measurement is. And that seems to be something you were either wrong or confused about. Garmin, BTW, has had speed/cadence sensors as long as they've had bike oriented GPS units. "Newer" has nothing to do with it. –  zenbike Jun 20 '11 at 12:34
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