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PostPosted: Sun Apr 20, 2014 5:21 pm 
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Joined: Thu Mar 26, 2009 11:06 pm
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I have noticed recently that when uploading .fit files to Garmin Connect and Strava that the interpretation of the .fit files differs between the 2 sites. Eg my ride yesterday - pre-plotted to be 101.83 miles - was reported at 102.3 by Garmin Connect (and on the device itself) but is quoted as being 105.0 by Strava. Similarly during a ride a few days ago my Garmin 800 reported a max speed of 50.8 whilst Strava says I did 52.8.

I had assumed that Strava read the same .fit file as Garmin Connect but obviously not. Can anyone explain how the data is recorded, stored and read such that the different web-sites give varying results. I assume that the Garmin data is more accurate?

If Strava data is so inaccurate it makes the whole principle of Segments etc even more of a mockery than ever!


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Posted: Sun Apr 20, 2014 5:21 pm 


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PostPosted: Sun Apr 20, 2014 5:49 pm 
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Location: Haines, AK - Temporarily
Do you have a speed sensor?


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PostPosted: Sun Apr 20, 2014 5:56 pm 
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Joined: Fri Jan 03, 2014 7:37 am
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afaik all gps systems have a margin of error, i used to have a gps for hiking etc and it always measured elevation to + or - 15ft

and using strava on my phone I've noticed whenever i go over a bridge it reads elevation to water lever, not the bridge deck


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PostPosted: Sun Apr 20, 2014 10:54 pm 
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Location: France
Presume that Strava apply their own proprietary sampling and smoothing to the raw GPS data. When you think about it this will be necessary if the data is coming from a variety of hardware units with a variety of sampling rates. Over time the delta in the algorithms will add up. For the purposes of Strava it is fine.

Dogg, GPS systems used to have a large error that was inbuilt into the system known as selective availability. This was turned off in 2000 and GPS is generally accurate to +/- 15m now.

GPS altitude is generally not very accurate and dependent on satellite position. Garmin units have barometric altimeters which are always more accurate than a phone which relies purely on GPS altitude. However a barometric altimeter will only report the correct height if it is calibrated to a known altitude/pressure. Some route recording apps do not sense altitude directly but rather do a look up of known datum altitude for a particular geographic co-ordinate, data often recorded by satellite radar topography.


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PostPosted: Mon Apr 21, 2014 3:37 pm 
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Location: Haines, AK - Temporarily
maquisard wrote:
Garmin units have barometric altimeters
That's not universally true, one example is the Garmin 200 - it's GPS based.


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PostPosted: Mon Apr 21, 2014 3:49 pm 
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Funnily enough was about to try and answer the same question. Been out this morning and my power readings between garmin and strava are miles apart. Will do some research later.....


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PostPosted: Tue Apr 22, 2014 10:56 am 
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Location: Urbana, Illinois
They do vary. I've also seen 2 tenths of a mile difference in my average speed at times.

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PostPosted: Tue Apr 22, 2014 12:16 pm 
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Hello Guys,

I make training software and read in data from most devices on the market.
I have experiences with the .FIT file format and Garmins. We recently had similar issues and the cause was a misinterpretation of auto-pause, start/stop, power up/down events.
You can email me the FIT file with the results in Garmin (a screenshot perhaps) and I will take a look - my email is softsell@telenet.be

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Posted: Tue Apr 22, 2014 12:16 pm 


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