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 Post subject: Re: App vs computer
PostPosted: Tue Aug 06, 2013 10:42 am 
Retro Guru
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Joined: Thu May 08, 2008 4:18 pm
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Some of Garmin's hiking-oriented GPS units have actual compasses in. I don't think any of the bike ones do, though.


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 Post subject: Re: App vs computer
PostPosted: Tue Aug 06, 2013 11:41 am 
Old School Grand Master

Joined: Tue Oct 09, 2007 1:55 pm
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Location: New Forest, UK
Bike computers include error from the zigzagging of the front wheel.
My suspicion is that smartphone GPS units use fewer channels to save power - my marine one tracks 24 satellites and picks the best for positional computation. Phone ones are hard to understand as the specs aren't published, but the iPhone 4s chip can track 12...whether it does is another matter. All get degraded by buildings and trees.
My marine one can accurately tell which side of the road you are on (not on a map, just using the track log).

In summary there are a shedload of errors. If yours is consistent then I would not worry. Remember that a 3% error on a bike computer is 2mm in diameter - so well within the range of tyre size and simple variation in inflation pressures.


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 Post subject: Re: App vs computer
PostPosted: Tue Aug 06, 2013 11:46 am 
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Location: The Cock Inn, Tillett, Herts
A Speedo measures your velocity along a surface, gps measures your velocity through 3d space, so even the most accurate examples of each device will disagree.


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 Post subject: Re: App vs computer
PostPosted: Tue Aug 06, 2013 1:33 pm 
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hamster wrote:
In summary there are a shedload of errors. If yours is consistent then I would not worry. Remember that a 3% error on a bike computer is 2mm in diameter - so well within the range of tyre size and simple variation in inflation pressures.


Diameter for a 26" wheeled bike will be around will be around 670mm so 2mm is about 0.3%. I agree though that tyre variation and deflection will make a difference so I always measure the circumference of my wheel with some weight on the bars.

Calibrate your computer well and you're not far off having one of those 'wheel on a stick' things that are used for measuring distance :wink:


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 Post subject: Re: App vs computer
PostPosted: Wed Aug 07, 2013 7:44 am 
Old School Grand Master

Joined: Tue Oct 09, 2007 1:55 pm
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Location: New Forest, UK
Oops, my mental Maths app is off. :oops:


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 Post subject: Re: App vs computer
PostPosted: Wed Aug 07, 2013 8:49 am 
retrobike rider
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MikeD wrote:
Some of Garmin's hiking-oriented GPS units have actual compasses in. I don't think any of the bike ones do, though.

Most, if not all smart phones have a built in compass.


Most apps can use the phone masts, Wi-Fi to narrow down the location. Possibly grab a satellite maps similar to TomTom satnavs so they can locate quickly with the GPS. One done though it's pure GPS. Shifted over to the OS's readout and just logged. Correction is done online afterwards normally. So distance height will be smoothed out. At least it is currently with Strava and the others where similar when I looked a year or so ago.


Viewranger/maverick on android may use a few more. Compass for instance, barometer if you have it.
I know Maverick which I use for OS and other map work give accuracy, no of SATs mapping, altitude (that's just more trig just in the extra plane) etc.


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