Hi ,
I did a 138 km hilly ride yesterday . The original elevation is about 1800 m where as my Garmin 830 recorded it as 893 m . Very disappointing after such a hilly spin. Many a time the gradient was recorded as 0% when it is about 7 or 8% .
I have to correct it in Strava base map . I am uploading a screenshot of the activity here. Can you please let me know how to rectify it in the future ? 
I do calibrate every single time. But when you climb 10% - 15% and garmin gives 0% or sometimes negative grade. That is not about calibration
Find a location where you know the elevation. I use survey markers and topographic maps. Then save tha location in the Edge 830 and edit the elevation to ensure the correct elevation is saved to that location…
You should try calibrating the altimeter. I think that is all you can really do to improve the elevation that is recorded.
Garmin says the altimeter on the Edge 500 is accurate to 50 ft. I assume that means…
