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 Post subject: App vs computer
PostPosted: Mon Aug 05, 2013 4:50 pm 
Retro Guru
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Joined: Tue Feb 20, 2007 3:39 pm
Posts: 1846
Which is more accurate?

I run a Cateye computer on my road bike, which is 6 months old and set-up as per instructions, and also use the mapmyride app at the same time.

Funny thing is that the app always gives me a better average speed (around 0.5 kph).
Does anyone else have this happening? And which should I trust in - magnets or satellites? :wink:


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 Post subject: Re: App vs computer
PostPosted: Mon Aug 05, 2013 5:27 pm 
Old School Grand Master

Joined: Tue Oct 09, 2007 1:55 pm
Posts: 8222
Location: New Forest, UK
I get much larger discrepancies than that, usually Endomondo gives 5% or so lower distance covered. There is huge mapping and positional inaccuracy in apps, for example in exact road spot heights. Furthermore there is no compansation when accuracy is degraded due to poor GPS satellite constellation positioning (my boat's GPS warns).


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 Post subject: Re: App vs computer
PostPosted: Mon Aug 05, 2013 7:28 pm 
Retro Guru

Joined: Sun Dec 28, 2008 12:12 am
Posts: 680
Location: Anglesey
Surely GPS wouldn't allow for gradients anyway?


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 Post subject: Re: App vs computer
PostPosted: Mon Aug 05, 2013 7:38 pm 
Old School Grand Master

Joined: Tue Oct 09, 2007 1:55 pm
Posts: 8222
Location: New Forest, UK
GPS can measure altitude...badly.


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 Post subject: Re: App vs computer
PostPosted: Mon Aug 05, 2013 8:20 pm 
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Joined: Sat Oct 03, 2009 2:37 pm
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Location: UK Southwest
Assuming you have your wheel size set up accurately then the computer will always be more precise than the GPS.

I have a Garmin for running and one for my bike. When I look at the data on Strava after a race the trace over the map only roughly follows the path I actually took. Also when I look at other peoples data from the same race the total distances differ from mine and again the traces don't follow the path that accurately. Lastly the data from people using phone apps is significantly less accurate than that from the Garmin users. The bike computer just counts wheel revolutions over time, so as I said, assuming the wheel circumfrence is set correctly it can't be far off can it.


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 Post subject: Re: App vs computer
PostPosted: Mon Aug 05, 2013 8:49 pm 
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Joined: Tue Feb 20, 2007 3:39 pm
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Thanks for the answers, they pretty much confirm my suspicions. I've been using the computer as my main source of figures and the app as back-up.
Just got back from an evening ride and the app was only 0.3 kph higher today, which I guess isn't bad after reading the above.

As you say hamster they measure altitude badly, looking at the speed/altitude graph, I go faster uphill! :lol:


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 Post subject: Re: App vs computer
PostPosted: Tue Aug 06, 2013 12:33 am 
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Joined: Tue Jun 19, 2007 3:34 pm
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Location: Launceston, Australia
well assuming that the computer and the app give the same distance and the same time the average should be exactly the same... but getting the distances to be exactly the same is not likely.


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 Post subject: Re: App vs computer
PostPosted: Tue Aug 06, 2013 8:45 am 
retrobike rider
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Joined: Tue Jul 14, 2009 7:52 pm
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Location: Trancecentral
To be dead on balls accurate the GPS/Location signal would need to be quicker.

My GPS, a Garmin, will take a reading every second, or on Smart reading about every 4 seconds. It then smooths the data between the GPS points. Take a big corner fast and the GPS will 'cut' the corner. Leave the GPS unit stationary and it'll spaz out all over the place too, small distances but it's there. Then you have GPS signal, or mobile mast signal, trees, buildings, weather, error correction, changing satellites....

The Garmin Edge has a magnet too, for speed readings I think. Accurate enough for me to play with. I'm no pro.

I've used Sporttracks and Google Earth to import the GPX from the Garmin, they show the actual GPS points on the route, when zoomed in you can see the jumps between them.

For the bike computer, if it's setup properly I guess pretty accurate, unless you skidded, did a wheelspin, popped a wheelie/end/stoppie/manual....

The GPX overlay doesn't accurately fit maps either, OS maps are not accurate they're always kept to the same scale, Google maps or aerial images the same, there's angles and elevation. So just plotting the route on those isn't distance accurate. That's why the GPS doesn't seem to follow the routes either, the GPS is pretty damn close to where you were on the earth, the map isn't.

I suppose to be most accurate, a GPS unit, taking readings as quickly as possible, backed up by mobile location too, combined with a bike computer setup correctly taking many readings as close to the hub as possible to get more, and both hubs in case of wheelie action.

But then how accurate do you want to be? If you're comparing your own rides it doesn't matter, if you're comparing with other then the likes of strava will try their best but it depends on the users' equipment. You could use digital EPO, or edit the GPX to cheat online stuff.


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 Post subject: Re: App vs computer
PostPosted: Tue Aug 06, 2013 9:02 am 
retrobike rider
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Location: Yorkshire, England
Problem with 'phone apps' is people chuck it in the bag, pocket etc. It really needs to be as high up as possible so you are not blocking line of site to the satellites. Many Apps don't make use of the other functions on a phone, compass, accelerometer to help out. Probably a bit to complicated to use direction to double check GPS position to position or see if you are in motion or going up or down hill and at what angle.

Garmins are a bit more evolved and cost a lot more for what they do.

No doubt as the phone power increases and Garmin/TomTom etc move over they can use the many years and extra sensors with better GPS or similar dedicated chipsets. Build them robust and waterproof too. Things might improve. Of course tag a Bluetooth magnetic sensor to it and expand away.. Rear and front wheel and cranks would be all easy to add.


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 Post subject: Re: App vs computer
PostPosted: Tue Aug 06, 2013 10:21 am 
Special Retro Guru
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FluffyChicken wrote:
Problem with 'phone apps' is people chuck it in the bag, pocket etc. It really needs to be as high up as possible so you are not blocking line of site to the satellites. Many Apps don't make use of the other functions on a phone, compass, accelerometer to help out. Probably a bit to complicated to use direction to double check GPS position to position or see if you are in motion or going up or down hill and at what angle.

Garmins are a bit more evolved and cost a lot more for what they do.

No doubt as the phone power increases and Garmin/TomTom etc move over they can use the many years and extra sensors with better GPS or similar dedicated chipsets. Build them robust and waterproof too. Things might improve. Of course tag a Bluetooth magnetic sensor to it and expand away.. Rear and front wheel and cranks would be all easy to add.


To my mind, there seems to be a couple of factors for gps reception. Smartphones tend to use other things (network towers, data access to servers to assist in locating sats, potentially WiFi locating data), and also do tend to have things like accelerometers and "compass" functionality to assist / augment in location. That said, whether all sports tracking apps make use of them (and as an additional example, I know Viewranger downloads some data to assist with altitude.

My only other experience of sat navs has been automotive ones, and whilst they used various things to help with positioning and ephemeris data, and tended to have improved chipsets, sensitivity and reception over time, smartphones do seem to have the edge in terms of filling the gap (and after all, WiFi positioning and NFC are technologies and schemes mooted to help with indoor positioning - eg large malls, say, in the US).

So I can't help but think that decent smartphones, at present at least, probably have the edge in acquiring and maintaining a decent perspective of position. That said, when it gets coarse grained, it probably looks more unreliable that dedicated GPS devices that wouldn't do such a thing.

Decent GPS / sat nav devices where supposed to use acclerometers and digital compasses for dead-reckoning when losing or suffering unreliable GPS reception. Can't say as I've ever used any, though, that did it. I'd like to think they'd thought of, and were using that sort of idea with smartphones - but I fear not - I suspect it just goes to a rather more coarse-grained "generalised" location, based on the other location techniques that can be used - such a shame, or at least that there appears no option, because they look to have all the hardware devices needed.


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