Wildly varying elevation gains

pibrahim

I did the Evans Ride It Liphook (the 60m one - although it was about 56) this weekend. I wanted to see what the total elevation gain was. I've found the following all posted on Strava:

http://app.strava.com/rides/14124370 - 2,969 ft [55.8 mi]
http://app.strava.com/rides/14178896 - 3,016 ft [55.8 mi]
http://app.strava.com/rides/14133151 - 4,093 ft [55.7 mi]
http://app.strava.com/rides/14123609 - 3,920 ft [56.8 mi]
http://app.strava.com/rides/14136519 - 3,894 ft [55.7 mi]
http://app.strava.com/rides/14146809 - 3,530 ft [56.0 mi]
http://app.strava.com/rides/14127352 - 3,058 ft [56.5 mi]
http://app.strava.com/rides/14124777 - 2,992 ft [56.2 mi]

I could go on and on... another lists it at 2,618 ft, another at 4,295 ft. These are all for the same course and all on the exact same day (22 July 2012) i.e. it's not like a previous year's route or anything.

What the heck? Why the variation? When you know the range is only between 2,618 and 4,295 ft then it's pretty useless! Shouldn't Strava be disregarding what the device says and just calculating it based on it's inbuilt topographical data so everyone that did the same route has (broadly speaking) the same result??

danowat

Does Strava do elevation correction then?

amaferanga

Take an average of all those as a best guesstimate. That's probably as close as you'd get to a 'true' figure.

thistle_

I'm not sure how Strava works out elevation, vertical accuracy is very poor from general purpose GPS receivers because you can't pick up signals from satellites far below the horizon.

Some sites take the GPS elevations and smooth out the spikes, other websites use the GPS positions and height data from another source (e.g. radar data from the Space Shuttle).

danowat

I use Sport tracks, and there is an elevation plugin for that which uses various external data to get correct elevation data

snoopsmydogg

did a recent run over 70 miles with a few others and 2 of us used Strava. Mine was through the android app on a htc and his from a garmin. The difference was over 1000ft of climbing (in his favour) but would probably assume the garmin to be more accurate anyway as it was built for purpose other than a phone with an add on.

Might be worth checking how the routes were uploaded to get a more accurate view.

pibrahim

Good point about seeing what device was used.

Here's what I found:

Garmin Edge 800 - 4,093 ft, 3,920 ft, 3,530 ft, 4,295 ft
Garmin Edge 705 - 3,894 ft
iPhone App - 3,016 ft, 3,058 ft, 2,992 ft
Garmin Edge 605 - 2,969 ft

So it seems they can generally be clumped together - and presumably the iPhone App data (and the older Garmin data) can be taken with a greater pinch of salt...

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.