Garmin v Strava climbing accuracy

knownothingbozo

I have a Garmin 200 cycle computer, which I like to keep a track of my distances etc on, but also use Strava, just to see how I am doing against others in the area. I have noticed that on the ride my Garmin shows a certain amount of ascent, but when I load it onto garmin connect, it generally loses a few a hundred feet, I know this will be because it uses some sort of science trickery out on the road and assume it then checks against a map when uploaded.
Problem is, that Strava records a entirely different amount of climb - not even the number that is shown from the Garmin before upload.
Consequently I used bike route toaster to map the ride, set it to feet, and to use Ordnance Survey data, and compared the three.
Garmin - uploaded figure: 967ft, Strava: 1337 ft, Bike Route Toaster: 1350 ft. To make matters weirder, the number initially shown by Strava, then gets changed over night, and now shows 1046ft.
Anyone know what's going on?

StillGoing

The search engine is your friend. It's all to do with how each device reads the terrain.

vorsprung

There's two factors which can lead to exactly the same map data giving different climbing figures

First, imagine you are riding a rolling road that goes up 10 metres and down 10 metres repeatedly. After a while a GPS shows you've done 100 metres of ascent. Now imagine the same but with 5 metre rises. Now 1 metre rises. There is a point at which the size of the ascent will not register on a GPS. Or else a very bumpy road would show some ascent

The second way is position and how it relates to altitude. The GPS position is a guess accurate to a few metres. So the altitude associated with that position is also a guess. Drawing a map in with bikeroutetoaster is also not exact. It could be off by the width of the road and still follow the route for example.
Garmin, Strava immediately, bikeroutetoaster and Garmin overnight all use slightly different assumptions about how to "improve" these guesses on position and therefore on the altitude.

In summary I wouldn't worry about it. Just use one of the figures to get a consistent set of records if you are going to later analyse it

meursault

Yeah, it's all estimated.

I have been using my Nokia Lumia and Strava to track rides, and when out with a friend who has Garmin, the distances are spot on. But the altitudes are off by 300m on a 30k ride.

GPS is still awesome though!

karlth

The more accurate the reading gets, the higher the figure will be and the more it approximates the real amount of ascent. This is nice, because it means you can feel good about yourself and take the highest figure, because it's the most accurate.

Of course if you got to the point you were adding up the imperfections in the tarmac that would cease to be the case, but for the tools we have which don't resolve much better than a metre it's true.

wongataa

Remember that GPS altitude measurements are a lot less accurate than GPS horizontal position measurements. This will cause a good few differences.

avoidingmyphd

karlth wrote:

The more accurate the reading gets, the higher the figure will be and the more it approximates the real amount of ascent. This is nice, because it means you can feel good about yourself and take the highest figure, because it's the most accurate.

Of course if you got to the point you were adding up the imperfections in the tarmac that would cease to be the case, but for the tools we have which don't resolve much better than a metre it's true.

Not quite. Accurate elevation data is just the first step. The next step is what you do to it to arrive at a "total ascent/descent" figure. At this step the outcome depends on what you count as climbing/descending. Different websites use a different techniques and give different answers even from the same initial elevation data. So e.g. bikeroutetoaster and mapmyride both use the same (google's) elevation dataset but brt always returns much higher climbing/descending totals than mmr because whatever it's doing counts stuff as "climbing" that mmr doesn't.
adding up imperfections in the tarmac can only happen if you measure them to start with. but having measured them, you aren't obliged to count them in your total ascent figure.

the best thing to do is to choose one provider and stick with it. there's not really such a thing as absolute right answers.

meursault

Another anomaly is this cat 4 climb

http://app.strava.com/segments/2187586

In reality, it is barely an ascent, mostly flat. Have no idea why it comes up like this.

karlth

meursault wrote:

Another anomaly is this cat 4 climb

http://app.strava.com/segments/2187586

In reality, it is barely an ascent, mostly flat. Have no idea why it comes up like this.

Faulty information in Strava. The start and finish elevations are wrong; looking at the OS map it rises from 140m to, erm, 140m.

skyblueamateur

I always correct my elevation reading on Strava and it always gives a lower reading then my Garmin or Mapmyride. Which would be the most accurate?

meursault

SkyBlueAmateur wrote:

I always correct my elevation reading on Strava and it always gives a lower reading then my Garmin or Mapmyride. Which would be the most accurate?

Neither, pick one and stick to it.

siamon

The Garmin Connect account should have an "elavation correction" facility as well? The lowest figure will be nearest.

springtide9

siamon wrote:

The Garmin Connect account should have an "elavation correction" facility as well? The lowest figure will be nearest.

Strava also allows you to correct....
These use "Elevation DB's" for height info, but are not always that granular or accurate. There are also multiple elevation DBs out there which have similar but not exact data.

This is why people state it's just an estimate... stick to one and don't worry it's out.

As for GPS accuracy for height, read:
http://gpsinformation.net/main/altitude.htm

magoo289

Normally, I would use garmin and upload this to Strava. I always noticed the elevation was different, but only recently noticed the times in Strava were also different from Garmin! So today I used the Strava App as well as Garmin. For a 40km ride the elevation was out by 47m and then I uploaded the ride from the Garmin device the time was out by 49 secs and didn't agreed with either the actual or moving Garmin time. I'll stick with Garmin as my recording device.

Gizmodo

This is taken from the Garmin Edge forum (https://forums.garmin.com/showthread.php?t=16046)

Which is more accurate [Garmin Connect Websit or Garmin Edge 800]? That is a good question. Regardless of the device or method used it is important to realize these devices are consumer grade and can introduce a level of error that is not acceptable for survey level data. The Edge 800 is more likely to have more accurate elevation information than the Edge 705 so elevation corrections should be minor.

Are the "elevation corrections" more accurate than the Edge 800? Using the auto-calibration feature on the device, the accuracy is +/- 50-125ft (same as GPS elevation). With WAAS and good reception you can obtain an accuracy of +/- 25-50ft. When manually calibrating to a known elevation it is approximately +/- 10ft for the first 15 minutes. Accuracy can change due to natural pressure changes (changes in weather). Potentially the information on our website may be more accurate as many things can effect the altimeter of your GPS. There is a limit however to this potential accuracy, as indicated on our website, "Elevation Corrections has known limitations for certain geographic features such as bridges and cross country borders where different elevation datasets prevail."

So in summary, in certain circumstances the Edge 800 will be more accurate than Elevation Corrections (and usually more accurate than the Edge 705), but overall the Elevation Corrections may provide the additional accuracy you need. Regardless of your choice, please be aware these devices are consumer grade and only survey grade devices should be used if sub-meter accuracy is desired.

This is what the Strava web sit has to say (https://strava.zendesk.com/entries/20965883-elevation-for-your-activity)

Elevation data recorded by a device can come from one of two sources: either a barometric altimeter on the device (such as those on devices like the Garmin Edge 500 and Forerunner 910XT) or derived from GPS signals (on most other devices, such as the Garmin Edge 200, other Forerunner models, the iPhone, and Android devices).

Strava detects devices with barometric altimeters and recognizes the data from that source. Since elevation data derived from a GPS signal is fairly inaccurate, Strava automatically corrects elevation derived from a GPS source by consulting elevation databases to determine the elevation at each point in the activity. 

If you suspect that your elevation data is incorrect, you can request an elevation lookup.  Click on the "Elevation" text under the elevation statistic above the map. The dialogue pops up with a button to "Correct Elevation". This button will only appear for data collected with a Barometric Altimeter, or if a previous automatic elevation correction has failed. After a short while, the "Calculating" text will change to "Updated", and if you click on "Updated" your page will refresh with the new elevation data. 

ForumNewbie

meursault wrote:

Another anomaly is this cat 4 climb

http://app.strava.com/segments/2187586

In reality, it is barely an ascent, mostly flat. Have no idea why it comes up like this.

As you hover the curser up over the line for the first 0.05 of a mile on that segment, it shows the percentage rising to over 100% at times - so it is clearly a faultly segment.

adr82

There are loads of segments like that if you look around. What seems to have happened in most cases is that someone has ridden along a road with a device that has a barometric altimeter, and for whatever reason it has registered a totally incorrect elevation profile (wet days make the one in my 705 do very strange things!). If the supposed "climb" is big enough to be categorised then Strava usually creates a segment for it automatically ("<name of road> climb"), and then you're left with all kinds of wacky statistics.

knownothingbozo

Thanks for your answers everyone - pick the one that makes me feel good seems to be the answer! I am making more of an effort to go for hills rather than avoiding them, as I've finally realised this is the only way I am going to improve my all round cycling. This obsession is getting rather painful as well as expensive!

meursault

If you suspect that your elevation data is incorrect, you can request an elevation lookup. Click on the "Elevation" text under the elevation statistic above the map. The dialogue pops up with a button to "Correct Elevation". This button will only appear for data collected with a Barometric Altimeter, or if a previous automatic elevation correction has failed. After a short while, the "Calculating" text will change to "Updated", and if you click on "Updated" your page will refresh with the new elevation data.

Does anyone know if this is still active? I am a lot thick, but can't seem to find it on the segment, I was banging on about earlier.

adr82

It is still active, but it's a per-ride thing, not per-segment. If you go back to your Strava ride page you should find the link just above the map.

meursault

adr82 wrote:

It is still active, but it's a per-ride thing, not per-segment. If you go back to your Strava ride page you should find the link just above the map.

Hmmm still not there, is it premium only feature? I have a 'more' dialogue box (with no elevation options), but nothing when clicking above map near elevation.

adr82

Nope, it's not premium-only. Should look like this when you click on the grey "Elevation (?)" link:

If it's not there maybe they don't have data for the route of your ride or something like that?

Edit: also once you've clicked the "Correct elevation" button the link disappears, if you only see "Elevation" and it's not a clickable link that's probably why.

meursault

adr82 wrote:

Nope, it's not premium-only. Should look like this when you click on the grey "Elevation (?)" link:

If it's not there maybe they don't have data for the route of your ride or something like that?

Edit: also once you've clicked the "Correct elevation" button the link disappears, if you only see "Elevation" and it's not a clickable link that's probably why.

Thanks adr, but still no go. No link or ? on ride or segment. Deffo have data as it has recorded my attempt on the 'climb'. Maybe original segment creator has already flagged it? still incorrect though.

adr82

meursault wrote:

Thanks adr, but still no go. No link or ? on ride or segment. Deffo have data as it has recorded my attempt on the 'climb'. Maybe original segment creator has already flagged it? still incorrect though.

I don't think it should matter if a segment on your ride is flagged or other people have already corrected their elevation readings, there's no reason for that to affect your elevation data. If you can't see the link on any of your rides submit a question to Strava support, they're usually pretty helpful (although it can take them several days to respond).