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Elevation correction is incorrect

Former Member over 5 years ago

Hello all,

I had a few runs and hikes where I suspected the elevation correction (go to the activity and below the device the drop down menu) is incorrect. So I started to record my morning walks from my house to the train station, every day the exact same way, starting and finishing about at the exact same spot (start from a traffic sign, finish at the top of the stairs).
I was expecting that the elevation won't wary because I walked 12 times the same way. However, the elevation after the correction is between 35 and 42 meters, the difference is 7 meters. Just to illustrate it, that is more than 2 storeys in a house.

So, if the 'elevation correction' calculates on static, given altitude values, how is the above thing possible?

0 trux over 5 years ago

The "elevation correction" calculates the elevation from cartografic data for the route detected by the GPS in your watch. However, since there is a certain sampling rate, and certain tolerance/incertitude (especially in urban environment), the individual points of the route are never the same even if you run it always in exactly the same way. The algorithm can so easily pick a wrong altitude value from the map, distorted by buidlings or vegetation in your proximity. So I am afraid the results will never be 100% perfect. The algorithm and the data may get better in future, but for the moment you will have to live with it as it is.

0 Former Member over 5 years ago in reply to trux

Thank you. I could accept it if the route the watch records would be that much distracted, however they are pretty much overlapping, what means the difference shouldn't be this big.

0 trux over 5 years ago in reply to Former Member

The watch does not read the GPS position continuosly. The final track you see on the map is just an interpolation through some key points. By default most Garmin watches use so called Smart Recording. It records key points where the device changes direction, speed, heart rate or elevation. Since the barometric sensors for elevation are rather slow and not very sensible, it means, that the distance between the key points may easily be quite important. In other words, at each of your activities at the same location, the key points may be at completely different positions, where the map data may give quite important differencies of altitude.

Additionally, it is enough that you run on a slope, close to a ridge, stairs, wall, buidling, etc., or simply on an unequal terrain, and due to the theoretical and practical limits of accuracy of GPS (~1-15m, and rather at the higher numbers for civilian use), you can easily get an altitude reading from the map that is several meters different from another pass through exactly the same position. In urban environment or under vegetation, the accuracy will be even worse.

If you don't mind the higher power consumption, you can activate GPS+GLONASS or GPS+Galileo for better accuracy. And you can also change the GPS reading mode from the "Smart Recording" to "Every Second Recording" (it will also drain your battery faster), but still there is little chance that the altitude data would be always identical.

You can read more about GPS in Garmin watches, and find how to tune the settings, in this document: https://support.garmin.com/en-US/?faq=xQvHXbfaT27Zr4hxZDvjv5