GPS Distance vs Computer Distance

zipboingarialwhoop

I bought a Cateye computer and went for a ride over cannock and tracked the ride using Viewranger on my iPhone.

I had set the computer up pretty accurately measuring the wheel circumference by rolling the wheel and checking it using maths!! The speed seemed about right but the distance travelled shown on the computer was about 2.5 miles more than that shown by the phone.

I thought the may be a slight difference as I assume the GPS doesn't take into account of the ups and downs.

My phone shows 11.6miles for the follow the dog and monkey trail combined (with some bits diverted due to logging).
The computer showed 14.1 miles.

Which is right?? Should I remeasure my wheel or chuck the GPS - in fairness the track shown on the gps was accurate on a 1:50,000 map

Cheers

Zip

paul.skibum

I dont know how accurate GPS on an iphone is I know the gps on my old nokia when I last tried it was a pretty broad you are somewhere in this mile diameter circle.

The best way to check it would be to ride between two road signs showing distances ie Sign A says Stockport 50 miles, Sign B says Stockport 40 miles (though I would be inclined to ride in the other direction away from Stockport if it was me - not even sure why I chose Stockport as my example) and check what phone and computer say for distances. Cateye computers if set up right are usually pretty accurate.

There is always the "who gives a sh*t how far you've ridden as long as you enjoyed it" arguement too.

cooldad

paul.skibum wrote:

There is always the "who gives a sh*t how far you've ridden as long as you enjoyed it" arguement too.

Very much this.

YeehaaMcgee

cooldad wrote:

paul.skibum wrote:

There is always the "who gives a sh*t how far you've ridden as long as you enjoyed it" arguement too.

Very much this.

+Potato.
There's an error to both (ssssh, some people are bound to jump in explaining how their GPS gives them precisely the same distance, down to umpteen decimal places, each time they ride to work... These people are what we politely call "crazy").
Ultimately, they're both accurate enough.

cooldad

One of our guys who logs all this sh1t made a couple of trips to Algeria last time we pootled around Swinley Forest.
I'd say there are possible inaccuracies.

allthegearnoidea

my endomondo lady went Mental today when out, tracked a good 5 miles extra on the route and also zigzagged all over the place!! i'd expect it in heavy tree cover, but it missed out the first few sections with no cover at all,
i used to have a cateye micro wireless, was great, until i had it on the back with the girls bike on it and not noticed the brake lever rubbing it and knocking it off somewhere between my pops and inlaws! haha! (opposite coasts)
phone gps's are no way near as good as a proper dedicated one, but then, you'd expect that!!
have noticed now though, without the speedo on the bars i ride a lot better as i'm not constantly looking to see how slow i'm going!!! :-D
agree hugely with above, doesnt matter how far youve been, aslong as youve enjoyed it thats all that matters!!

zipboingarialwhoop

I totally agree with the "just ride" mentality.

But, I am a techno geek and a 20% error on a device grates on my OCD:-)

I love driving as well but if the odometer on my car was 20% out I'd have something To say to the manufacturer.

Cheers
Zip

Rigga

My strava is always 2 miles more than my bike computer!

allthegearnoidea

Zip, gps test the accuracy of your car! Most are in a bracket of upto 10% Overread! Some come out clearer as a mph over read, the wifes fiat for example has a 6mph over read through out, my old Bx has an 8mph,

YeehaaMcgee

zipboingarialwhoop wrote:

I totally agree with the "just ride" mentality.

But, I am a techno geek and a 20% error on a device grates on my OCD:-)

I love driving as well but if the odometer on my car was 20% out I'd have something To say to the manufacturer.

Cheers
Zip

I don't think either's going to be 20% off. Unless you have one of those strange GPS trails that is convinced you shot off to New Zeland for half a second

Anonymous

I enjoyed those trips to Algeria. Though I'm probably on some blacklist now for having done them twice in the same day. Nice place.

Anyway, bike computers I believe are reading the wheel spin and like a car odometer you're getting the actual mileage the wheels have done as they spin over a surface, up down, going round in the same spot, etc.

GPS is tracking a position and logging points at intervals then working out a distance between them. If you were to look at the GPS plot in detail you'll see it doesn't follow the trail exactly but goes in straight lines between each plot point. The trail may have had all kinds of detail that would involve more distance than just going in a straight line on the flat.

Accuracy of GPS is down to a few metres in ideal conditions, but many factors will degrade it at times or it can lose you entirely and then catch up later. Then if you're roughly in the same location it can think you haven't moved but you could be moving and the wheels are clocking up distance but GPS is not.

Finally phone GPS units are adjusted for accuracy for car navigation, which is low. Internally the chips of some of them will do small pauses to filter out spikes that would confuse car navigation (making them jump between roads), but it filters out fine detail in bike riding and walking too.

A dedicated Garmin-like device would be better tuned for the trails and I think some come with a wheel speed sensor and in theory can work out distance moved when the GPS itself is not thinking you've moved, and stop it from logging movement when GPS thinks you've moved but you haven't.

cooldad

Two trips to Algeria, and no interesting local tobaccos, not even a crappy T shirt. Some friend.

zipboingarialwhoop

Going for a ride tomorrow, I have an ideal solution - Ill judge the ride distance on No. of pubs frequented with a pint in each.

This will be accurate unto about 8 pints, then it'll probably start with the GPS / Algerian accuracy...

Zip (hic)

bluechair84

x beers is a fantstic way to measure most things, but it's usually reserved for ladies...

YeehaaMcgee

"Ladies"? Seriously?
I'm not sure the creatures we judge in terms of pints can be classed as "ladies".

bluechair84

that's true, I've pulled a few goats that made it into the ladies classification when measuring on the beer scale... those were bad times...