Strava reliability

by Spatulala   March 30, 2013  

Started using Strava and wondered if anyone knew if the stats it collects via my iPhone are reliable? Went out for a 60k ride up 7 East Devon Hills yesterday, and the app said 1200m and then corrected itself to 1015m of climbing. Then I saw a ride 2 mates did together this morning - 1 said 400m and the other said 650m!

If the elevation is off, does that mean the top speeds are as well? I was quite proud of my 50mph segment from the top of Observatory Hill down to Sidbury!

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Mobile phone tracking with any app is never 100% reliable.

I have found since getting my Garmin 500 though that Strava seem to adjust things. I put a ride into Garmin connect and also into Strava, both straight from the 500 without doing anything, both said different things.

SO I am very wary of trusting Strava with anything now.

I am also not the first to have this problem, I don't know whats up with Strava, but it changes people's data from various GPS units they use Thinking

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posted by Gkam84 [7942 posts]
30th March 2013 - 15:20

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nothing's up with strava, it can only work with the data it's given which can be pretty patchy. All GPS devices i've used tend to overestimate distance and altitude gain, sometimes by 10% or more

i'd never trust a GPS for accurate speed measurement, not since i clocked myself at 96km/h going down a hill in Bath Wink

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posted by Dave Atkinson [6974 posts]
30th March 2013 - 16:25

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A mobile phone app will not be as accurate as a purpose built unit such as a Garmin. There is a lot of information on line about the accuracy or inaccuracy of strava

Markcw

posted by Markcw [10 posts]
30th March 2013 - 16:28

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The quality of GPS data will vary from one device to another, and data is also disturbed by tree cover, tall buildings, interference from other gadgets and it doesn't work in tunnels. When it comes to hills and height data then there is even more room for error, not all devices have any means to correct or estimate height above sea level, and it's also possible for the resolution of GPS data to be so poor that it fails to even provide data for that taxing hill climb and subsequent speed fest to the bottom again, for all it knows you may have travelled through a tunnel rather that over the hill.

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posted by the_mikey [146 posts]
30th March 2013 - 16:30

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Check as well that any cycle app is not publishing to the public where you live. Good speed usually means bike/enthusiast so don't want bike theives targetting you by working out where your route started.

posted by HappySnidge [13 posts]
30th March 2013 - 17:59

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HappySnidge wrote:
Check as well that any cycle app is not publishing to the public where you live. Good speed usually means bike/enthusiast so don't want bike theives targetting you by working out where your route started.

I admit to searching on Strava while going through race start lists to check up on who's been training, but can't imagine anything duller than trawling through local rides trying to work out who's fast enough to have a bike worth stealing.

Sq

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posted by Squiggle [410 posts]
31st March 2013 - 1:39

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http://www.uttoxeter-news.co.uk/News/Uttoxeter-cycle-thefts-lead-to-smar...

“Our investigations have shown that some of the victims had been using websites and mobile phone apps to log their routes – these sites allow users to view each others routes and track their rides.

“Some of the GPS data recorded and shared on these sites is so accurate you can pin point the house where the journey’s have begun and ended.

“We suspect some thieves have been using these sites to identify potential victims and high-value bikes.

posted by HappySnidge [13 posts]
1st April 2013 - 18:07

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Like others have said, I wouldn't trust GPS alone for accurate speed or distance data. If you want proper accuracy, combine a Garmin with a speed / cadence sensor that uses the size of your wheel to know EXACTLY how far and fast you have travelled.

As for elevation, that's far more troublesome. Elevation calculated by GPS is very haphazard for a host of very complicated reasons. If you can afford a Garmin with a barometric pressure sensor, you can find out the correct height above sea level for places you start your rides from, such as home, office etc and pre-define them in your Garmin. The elevation readings it gives based on changes in atmospheric pressure as you climb or descend are then pretty accurate - certainly more accurate than GPS.

If you consult Strava's help pages you'll discover that they use data submitted from Edge 800 users over smartphones to correct their data as it is much more accurate.

To summarise, smartphone GPS not great, Garmin GPS better, Garmin GPS with speed / cadence as good as you can get, but none of this tech will be 100% accurate for elevation.

There's a good blog from @StravaTips comparing smartphones to dedicated GPS linked below - I've never regretted investing in an Edge 800!

http://www.scarletfire.co.uk/2012/07/iphone-gps-fail/

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posted by StuayEd [62 posts]
1st April 2013 - 20:03

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HappySnidge wrote:
http://www.uttoxeter-news.co.uk/News/Uttoxeter-cycle-thefts-lead-to-smartphone-apps-warning-20130309110405.htm

“Our investigations have shown that some of the victims had been using websites and mobile phone apps to log their routes – these sites allow users to view each others routes and track their rides.

“Some of the GPS data recorded and shared on these sites is so accurate you can pin point the house where the journey’s have begun and ended.

“We suspect some thieves have been using these sites to identify potential victims and high-value bikes.

There have been lots of stories like this in the press recently and it's definitely a concern - it's worthy of note that in the privacy settings on Strava, you can set up a 500m exclusion zone around your house that means followers can't see the start or end of your ride if it enters that exclusion zone. Tat way no one can possibly identify where you live. I'd recommend that everyone should set it up!

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posted by StuayEd [62 posts]
1st April 2013 - 20:05

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If I remember correctly, the iphone doesn't actually have GPS - it triangulates it's position based on cellphone towers*, aided by the location of local wifi networks**. I'm not sure if altitude is calculated based on the same triangulation (that sounds flaky) or a co-ordinates lookup of your location against an OS map with altitude data derived from there.

Contrast all that with a Garmin, for example, which is a 'true' GPS device relying on satellite positioning.

Over a 100km ride, my phone and Garmin differ by maybe 1km distance, so not a big deal. I'm not sure about difference in ascent.

Overall, I think the phone app is definitely useful. If I have the battery life, I record on both, then if I'm in a rush later, upload from the phone (easier) rather than booting the laptop and hooking up the GPS, which is a bit of a faff.

* = If a phone has 'proper' GPS, then in theory you can still navigate when in aircraft mode (i.e. no phone or data signals), but the iphone 4 will do no such thing.
** = This may have changed for the iphone 5 when maps data switched from google to Apple.

I got this phone ages ago, so I may be talking bollocks, but hopefully not!

Dodging the saccadic masking

posted by notfastenough [2559 posts]
2nd April 2013 - 11:19

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the iphone 4 uses a Broadcom BCM4750IUB8 single-chip GPS receiver. so yes, it does have proper GPS

not all chipsets (and not all chips of the same type) are equally capable. I've had two HTC One X handsets and the GPS tracking on the first was noticeably better than on the second

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posted by Dave Atkinson [6974 posts]
2nd April 2013 - 12:53

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Dave Atkinson wrote:
the iphone 4 uses a Broadcom BCM4750IUB8 single-chip GPS receiver. so yes, it does have proper GPS

not all chipsets (and not all chips of the same type) are equally capable. I've had two HTC One X handsets and the GPS tracking on the first was noticeably better than on the second

I checked - you're quite correct, although it appears to be supplemented by cellphone signal triangulation and wi-fi networks. I assume the GPS chip alone isn't as good as others, otherwise why bother with the other stuff etc.

Dodging the saccadic masking

posted by notfastenough [2559 posts]
2nd April 2013 - 19:52

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you certainly should not trust your Strava segment times obtained with a Strava phone app; somehow my Strava app tells me I'm much faster than I really am, as shown by my Garmin Edge 500 with speed/cadence sensor.

Not surprisingly, a lot of the KOMs in my locality are recorded using Strava phone apps...

posted by Metjas [238 posts]
2nd April 2013 - 20:35

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Looks like a Garmin is the way to go then, plus sensors for speed, cadence, power, altitude and heart rate. Better start saving.

posted by Spatulala [31 posts]
2nd April 2013 - 20:43

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Strava always gives a lower avg speed than the data directly from my garmin. I assumed this was due to what im going to call gps drift (i.e when stood still the garmin reads 0.5 mph or so)?
Also, does anyone know how do you go about defining a known elevation for an edge 800? (i'm a bit technolically challenged!)

posted by MattT53 [118 posts]
2nd April 2013 - 20:51

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Garmin connect has a feature to enable elevation corrections, which I think are taken from proper maps. The feature is on the bottom right of each individual ride. Not that you should obsess too much, it's only a guide and your legs will certainly tell you when you've climbed enough!

The Human Cyclist A blog. Try it, you might like it...

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posted by sm [329 posts]
2nd April 2013 - 21:15

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notfastenough wrote:
Dave Atkinson wrote:
the iphone 4 uses a Broadcom BCM4750IUB8 single-chip GPS receiver. so yes, it does have proper GPS

not all chipsets (and not all chips of the same type) are equally capable. I've had two HTC One X handsets and the GPS tracking on the first was noticeably better than on the second

I checked - you're quite correct, although it appears to be supplemented by cellphone signal triangulation and wi-fi networks. I assume the GPS chip alone isn't as good as others, otherwise why bother with the other stuff etc.

most phones use other networks to augment GPS data, because it can extend battery life. there's a new system in development which takes very small snippets of GPS data (fractions of a second rather than the tens of seconds it can sometimes take to get a full fix) and computes location from that, other network data and the phone's past locations via the cloud. the developers reckon this could hugely extend battery life when GPS is on

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posted by Dave Atkinson [6974 posts]
2nd April 2013 - 21:28

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MattT53 wrote:
Strava always gives a lower avg speed than the data directly from my garmin. I assumed this was due to what im going to call gps drift (i.e when stood still the garmin reads 0.5 mph or so)?
Also, does anyone know how do you go about defining a known elevation for an edge 800? (i'm a bit technolically challenged!)

Matt,

Goto this website and navigate to the location that you want to predefined the elevation for. It will tell you (using your choice of NASA or OS data) exactly how high above sea level a given location is. Best to use a proper computer, rather than a phone or tablet for this website! http://www.osola.org.uk/elevations/

Once you know the elevation of your given location:
1) Goto the map screen on your Garmin and press the icon with the four arrows, to enter full screen mode.
2) Pan around and zoom out / in to find your chosen location? Once you've found it, zoom in to make your saved location as accurate as possible.
2) Tap at the precise place you want to define the elevation and a pin will drop in place on the display. If you mess up, just tap again and it'll move.
3) at the top of the display you'll see a little information box with the coordinates and elevation displayed (carry on even if the elevation is correct as you want to override the barometric sensor on the Garmin and ensure it's always correct at the start of a ride - this only happens when you predefine).
4) Tap on the information box and you'll get a new screen with 3 buttons along the bottom. The middle one is a little flag on an "x" a as in "x marks the spot". Press it!
5) your Garmin will say something like "location saved as 001" a click ok.
6) A new screen will appear like before except the middle button is now a pencil - press this to enter edit mode to edit the location.
7) now you can change the name, the symbol it is displayed as on the map (think office block for work or little house for home) and crucially the correct elevation.
8) Scroll down to "change elevation" and select it.
9) Use the delete key top right to get rid of all the zeros and type in the correct elevation. You must fill all 5 spaces, so if your location is 45 metres above sea level, you should out 00045. Then press the green tick.

That's it! Now whenever you start the Edge's timer within 50 metres of that location it will automatically set the elevation to whatever you put in and then give much more accurate readings based on the barometric pressure. It's like automatically calibrating it every time you start a ride.

You'll also find that you can navigate to your saved locations easily by going to the "where to" menu and selecting "locations". Great if you're half way round a route and decide for some reason that you just need the quickest route home possible etc and want to use your edge as a sat nav!

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posted by StuayEd [62 posts]
3rd April 2013 - 0:53

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Thanks, a lot. That's a great help, much appreciated!

posted by MattT53 [118 posts]
3rd April 2013 - 9:10

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No problem!

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posted by StuayEd [62 posts]
3rd April 2013 - 11:53

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Taking my last Strava GPX file and putting it into Google Earth shows that the Strava height gain is underestimating by 14%. My Suunto T6 with barometric sensor gave more like 5% error on the Google Earth figure.

The total altitude gain Strava estimates is a calculation based on map database of heights for a given Lat/Lon as recorded by the phone's GPS. Some devices with barometric sensors use these in prefeence - like the Garmon 500 et al. https://strava.zendesk.com/entries/20965883-Elevation-for-Your-Activity

I was told there would be Cake. Luckily there's http://TestValleyCC.org.uk

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posted by KiwiMike [323 posts]
16th April 2013 - 13:56

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