GPS distance vs Cycle computer distance

[GruB]

I've put the Etrex Cx onto the Etape for the journey to work today.
After 13 miles I noticed that the distance was different between the two.
By the time I got to work it was approximately 1 mile greater distance on the GPS compared to the Polar CS400 cycle computer.

My question is - which is right?  Is GPS that accurate with measurement compared to a magnet on a wheel?

[PaulF]

I'd say neither. Assuming that your computer is 100% accurate and you have it set correctly I'd expect it to be the more accurate since the GPS distance is based on taking your position every few seconds so will 'smooth' a 'wiggly' road but should also make the distance less that the computer distance. Still a 7.5% (approx) difference is quite significant!

Is the Polar wireless? If so interference could cause it to 'miss' the occasional wheel turn?

[Jord]

Also if it is a very hilly course a GPS will measure the distance in a straight line rather than up and down (if that makes sense) so could be less the the computer - which is the opposite to what you have.

I have found on most rides both the GPS and the magnet driven computer are more or less identical.

[chris]

GPS has a 'built in error' and the error changes with time. I spent 10 years working on vehicle tracking systems and had a GPS antenna firmly bolted to the outside wall of my office. Sometimes I would plot on a map the the GPS reported location of the antenna during the day and I could see the reported position change frequently but only by a small amount. Even though the position of the antenna had not changed the accumulated distance between successive locations could become quite large over a period of time. The error is not as much now as it used to be as the americans have made the deliberate error less. This could lead to the GPS over calculating the ODO, but not by that much over a short period of time.

I also used to occasioanlly see spurious fixes (probablyone in many thousand fixes) from the GPS chip where the location could be out by a large amount, sometimes many miles. I not sure what caused these, but we had to filter them out. I'm sure that mass produced GPS units would do this too.

[perpetual dan]

Is the Polar wireless? If so interference could cause it to 'miss' the occasional wheel turn?

^^^ This - I assume the computer is calibrated properly. I ditched my wireless computer as it was too flaky to be helpful.
Also, does your route go through tunnels, dense trees etc, which might cause the GPS to get a bit confused?

If you put the route into bikeroutetoaster or similar what 3rd distance figure do you get?

[Paul D]

My Etrex is used on the commute and gives identical distances on most days to within about 0.05 mile on every trip. If it was a mile out for every 13 miles it would be easily noticable over some of the audaxing distances I've used it for.

Although there's no doubt about slight GPS errors, unless you do a mile of tunnel I would check your Polar comp. Actually, just read the OP and you've gone further on the GPS so it's not a tunnel.

After my GPS epiphany I just binned all my computers and bought Garmin QR mounts for all my bikes.

[GruB]

Der - I made a boo boo.

I have just popped out and checked my settings.
As I have a 'smart' wireless HR Polar computer - and as I have two bikes with separate sensors on both - the computer is smart enough to sense which bike one is using.

When and only when you enable it to do so.   

I have now turned on Bike 2 which in this case is the Etape. 
The Etape is running 700 x 25c wheras the TCR is running 700 x 23c

I also checked the readings again.  Etrex shows 25.1 miles whereas cycle computer has 24 miles.

So now I have set it up correctly on the way home I will see if the two match - like for like.




I'll just get my coat 

[nuttycyclist]

....
My question is - which is right?  Is GPS that accurate with measurement compared to a magnet on a wheel?

People saying GPS is wonderful and accurate really get on my nerves.  It's not.

Yesterday I was out for a walk and noted that some footpaths were missing from OpenStreetMap so tracked them.  From the road I walked down a path, walked for about 45 minutes loop through woodlands, then returned up the same path.  On my return the GPS neatly tracked me walking 130 feet or so parallel to the track I'd marked on the way out.  On its screen it proudly told me it was "accurate to 12 feet"

[Greenbank]

I'd say neither.

Yup, that's the easiest.

Cycle computers won't be perfectly accurate as they rely on the assumption that a single revolution of the wheel moves you an exact (to a mm) distance down the road. If that wheel circumference measurement is out by even 1mm then it won't be correct. It also relies upon the wheel circumference measurement to remain constant throughout the ride under all loads (leaning back on a descent with the weight over the rear wheel; or leaning forwards grimping up a steep hill), including as the pressure in the tyre may change. Even turning will slightly change the effective circumference of a tyre.

A 700c x 25mm tyre is approximately 2105mm circumference, but some cycle computers only allow you to set the circumference in cm, and a 25mm tyre is listed as 211cm.

Let's assume a cycle computer set to 211cm reports exactly 300km for a ride. If we'd set it to 210mm it would have reported 298.58km (2dp), and set to 212cm it would have reported 301.42km (2dp).

A 23mm tyre is usually 2097mm circumference, a 25mm tyre is 2105mm. 25.1*2097/2105 = 25.0 miles, so it won't account for a 1.1mile discrepancy.

So now I have set it up correctly on the way home I will see if the two match - like for like.

Don't count on it. I've done my commute hundreds of times with cycle computer and GPS on. The two rarely match up, plus there are reasonable variations (in both) even though I've taken the same route. Even taking just the cyclecomputer readings, my commute in can be anywhere between 11.3km and 11.8km. Same bike. Same route. No correlation with tyre pressure. I don't have to weave about at all on my commute in as the traffic is clear enough all of the way. I'd have to be taking along at ~16^o off straight to add 500meters to a 11.3km commute!

As for GPS, it isn't accurate for distance measurement because the GPS signal is designed for positioning with a stated error. You are within 4 meters of *right here*. Distance measurement is an algorithm applied on top, it's an interpretation of the position/error data.

Take this image, although it's a simplification it should illustrate the point:-



The red splodges represent location fixes and relative errors. The black lines represent 3 different paths through the same splodges. The 3 lines are obviously of different lengths, therefore there's no way for the GPS to know exactly what path was taken and so it cannot be relied upon for a truly accurate figure. The smoothing algorithm applied destroys the ability to guarantee accuracy.

But the algorithm (in some GPSes at least) is smarter than some people think, it does take into account elevation gain, and will (based on certain bounds) assume a straight line has been taken between points where the signal has been lost. Again, this interpretation will reduces the accuracy.

Pick one measuring device and use that, but don't bother comparing two (or more) readings from different devices for an individual ride as there's little point. The more devices you consult the more disparate readings you'll end up seeing.

[frankly frankie]

Good illustration!  I like that a lot.

Plus if its an HCx and moving slowly (in traffic, or stopped at lights), a reflected signal (eg off a big building) could cause a temporary much larger offset for one or more of those points.  HCx are known to be prone to 'wander' at very low speeds.  The older Cx and C are a bit better here, though not so good in other ways.

[GruB]

Okay, home now.
On the way home I ensured my cycling computer recognised the correct bike was being used. 

Greenbank,
I have read on here and elsewhere that cycle computer wheel measurement can often be a bit hit and miss.  So I was intrigued when the Polar had the option to input the ETRO value. 
The TCR has 23-622 - that equates to 2074 mm
The Etape has 25.622 - that equates to 2086 mm.

My journey home today measured 24.1 miles.
The Etrex Vista Cx GPS measured 24.6 miles.
Compared to this morning that is a lot closer. 

[PloddinPedro]

...............Pick one measuring device and use that, but don't bother comparing two (or more) readings from different devices for an individual ride as there's little point. The more devices you consult the more disparate readings you'll end up seeing............

I'd say Greenbank has it spot on. When I first got a GPS I drove myself nuts trying to get my Cateye computer "fine-tuned" to the exact mm setting so as to generate a reading precisely equal to the GPS result of 15miles ride to our weekly club meeting point. I couldn't do it. I used the "ride the bike for one wheel revolution and measure the distance travelled between chalk marks" method and found a difference of several mm. I concluded that it affected the Cateye measurably if I rode the whole way seated versus honking up the hills (the front tyre compressed - smaller circumference - with more body weight on the 'bars).

Then I got another GPS and that gave a whole different set of readings for the same ride; then they updated the software in the new GPS and it changed again - it's a fruitless quest!