Which is more accurate? GPS or Wheel magnet

[Davidc]

I've compared my Garmin Foretrex GPS with the VDO C2 and there's not much difference, provided there's a GPS signal.

There's a bit more lag in speed measurement on the GPS, the distances agree quite closely, as do the speeds. The GPS can produce some stupid numbers though.

 

[Bollo]

My guess is that they'll be some form of signal processing going on inside the Garmin that combines the position measurement from the GPS with the speed measurement of the wheel magnet. Combining them both will smooth out the jitter on the GPS position and calibrate the wheel measurements.

Kalman filters and that if you want to see the maths.

(Spent a few years doing this kind of thing with aircraft GPS + inertial nav systems)

 

[killiekosmos]

If the cycle computer - with magnet- is set up properly then the possible errors would be:
* poor measurement of circumference of wheel (should be very small)
* wheel slip (no boy racer starts!)
* badly programmed device - unlikely
So, all in all you should be able to get a very accurate measure.

For GPS, the device works by measuring the distance between two locations and the time between them. It will always use straight line measures, so depending how many measures it takes per minute, it could have small errors due to you not always cycling in straight lines.

 

[zacklaws]

Here is some comparisons between my Garmin Edge 305 and VDO C4 of some recent rides, all in hilly terrain, and difference is hardly nothing to worry about, the last two rides, I have pumped the tyres up to 125lbs max instead of the usual 120lbs and that may have made the difference of the VDO having a slightly higher mileage.:-

Miles

Garmin 32.18
VDO 32.10

Garmin 48.33
VDO 48.19

Garmin 80.15
VDO 79.84

Garmin 31.77
VDO 31.84

Garmin 52.63
VDO 52.91

As for speeds, they both read the same, but I have spent a lot of time setting up the VDO to be as accurate as possible, not just measuring one revolution but as many as I can up the footpath, usually about 5, and working in millimetres rather than tenths of an inch, and marking my tyre with a tiny bit of white paint so it leaves a tiny mark on the footpath to measure instead of the usual valve position, and then remeasuring for a double check. Using the white square markers that are painted on the roads are useful to check your computer if you come across any, as they do not seem so common now, providing that you have tested them previously in your car to determine their distance, and providing your car odometer is correct. Usually they are a mile apart but some can be an odd measurement.

 

[buddha]

Does GPS take elevation into account?
If not then on a 10degree climb (or descent), for example, the GPS reading would be 1.5% short.

 

[HLaB]

Does GPS take elevation into account?
If not then on a 10degree climb (or descent), for example, the GPS reading would be 1.5% short.

I believe so, gps works on xyz coordinates.

 

[Coco]

But vertical measurement on GPS is even less accurate than horizontal.

 

[BentMikey]

AFAIK wheel magnet bike computers are most accurate, enough that they are far better than car odometers and often used by homebuilders for kit cars.

 

[ufkacbln]

Debated in the past....

If you set up the wheel properly then magnets are accurate, but how accurate do you need?

Tyre pressure can alter the absolute circumference of a wheel by a few mm which on a long ride can result in metres of difference.

I am not bothered to the stage where this type of inaccuracy is going to affect my cycling

 

[GrasB]

AFAIK wheel magnet bike computers are most accurate, enough that they are far better than car odometers and often used by homebuilders for kit cars.

But only because they can be setup far more accurately quickly & easily. A pickup from a normal gearbox once adjusted to the same level of accuracy will give the same result. It's just typically much harder to setup on most dash boards.

 

[BigSteev]

AFAIK wheel magnet bike computers are most accurate, enough that they are far better than car odometers and often used by homebuilders for kit cars.

Always used to amuse me when PeterC would remove his Striker's speedo/odometer when he got out of the car.

 

[Maz]

I think you'd be faster without either (less weight).

 

[garrilla]

Back to the OP, irrespective of which is better, as long as your set up with reasonable precision the error will not be anywhere close to 1% (as someone else wrote 1m in 1km) even at 1% you'd still be reading 99% of 15mph, 14.8ish... so the question you should ask your doubting collegues is how much slower do they think you're going than the 15mph of your reading, then calculate how far out your magnet position and wheel/tyre diameter would have to be to be that slow.

 

[MacB]

I noted variations on my commute distance and it took a while for the penny to drop. It was most noticeable when I'd just pumped up the tires. Having ridden with others a fair bit I've never noticed any big variations in the readings from our various machines. Depends what you need the data for. If it's just for tracking distances and fitness progress then I'd say we're already more accurate than most of us have a use for.

 

[Bollo]

Please gentleman (and ladies?), if I weren't preoccupied with end of financial year stuff at work I'd go very very jimbolee over your collective asses. This is one of the few (possibly only) times that I can speak with some authority.

To summarise as non mathematically as possible - yer Garmin will maintain estimates of your position and velocity and the cumulative errors on those figures. It probably also stores an estimate of the correction to your wheel diameter estimate. Collectively these are called the state, and the error on these figures is the covariance. This is used to derive the little circle that you get around the position to show error. When you get a new measurement in, be it a velocity (possibly with a systematic error due to your estimate of wheel diameter being wrong) or a position (with an error due to GPS accuracy), the Garmin will compare the measurement it was expecting, derived from your state, with the one it actually got from whatever sensor. If there's a difference, it could be due to either the random variations of the signal (for example the GPS error) or because you've accelerated, decelerated or changed direction. There's then a lot of matrix algebra that decides how much of that difference is real (i.e. not due to error) and updates the state and its error accordingly. So, for example, if you stay in one place, the error on your position estimate gradually heads towards zero. At the other end, if you accelerate, decelerate or change direction more rapidly than the equations expect, then you get a temporary increase in the error on your position and speed estimates as the maths gets used to your new 'state'. Simples.

If it's just for tracking distances and fitness progress then I'd say we're already more accurate than most of us have a use for.

Exactly! Now where's that power meter leaflet?