Strava reliability

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

No problem!

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!

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!)

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...

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

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!

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/

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.

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.

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

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.

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

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