ANT Forum

Hi all,

Does anyone know how ascent and descent is calculated? If I simply add up all positive values that I can derive from the record message the result is way too high. Not only as compared to the calculated values that I get from my device, but also compared to reality. Where I ride in the Netherlands it is really flat ...

If you look at the raw data you can imagine that the algorithm that is being used contains some smoothing mechanism to filter out “noise”. But how is this done?

Thanks in advance!

Frank      

Sorry if this obvious, but are you perhaps looking at the raw values without converting them first?

Profile.xlsx in the SDK mentions scale 5, offset 500 for altitude in gps_metadata/160, so the real value should be something like: ENHANCED_ALTITUDE / 5 - 500. If you are using values in the csv generated by the sdk tool, those usually need further processing.

If it's due to outliers in the processed data, then I'm not sure. Perhaps, e.g. the barometer and/or gps are temporarily getting erratic readings somewhere?

EDIT: sorry, you mentioned "record", but the conversion is the same. Also, I believe altitude readings in general are somewhat tricky and not very exact (partly due to the model hinging on air pressure?), so it could simply be that if the values are in the correct magnitude, but still off.      

Good question Jens, this could be a good reason.

But no, I already did the conversion that is prescribed in Profile.xlsx. The resulting values are "correct", i.e. around 0, which corresponds to the area where I live.

The issue is that the "noise" in the resulting values must be smoothened. I know that this can be done by replacing the measured values by values from a "DEM" map. This can lead to different outcomes (compare Garmin Connect and Strava).

A Garmin device must calculate the ascent & descent from the raw values - it does not have an elevation map - so it must apply some sort of smoothing algorithm. The thousand-dollar question is: what is that algorithm?