Vertical Height Discrepancies Between Garmin & Phone Apps.

watch out for barometric mishaps as atmospheric pressure can "confuse" them

The Strava phone app corrects elevation using map data, whereas your 810 uses a barometric altimeter (air pressure basically). This is significantly more accurate than GPS-only based elevation measurements, but still prone to occasional screw ups.

Strava provide a "correct elevation" button on your Garmin recorded rides that lets you get the same map based elevation correction as the Phone app. It's pretty handy when you get a duff ride from the Garmin.

More info here for the curious
https://strava.zendesk.com/entries/20965883-Elevation-for-Your-Activity

In Japan its the other way round. The Garmins show half the climbing height compared to those with Strava on their phones. Sometimes the phones show treble. What gives me some comfort in the Garmin is when I plan the routes on RidewithGPS or Mapmyride they tend to be 10-25% more than the Garmin reading at the end of the ride (which I account for the website measuring every small bump on the map that gives incline, whereas the Garmins work on a pressure change averaging approach). Something is madly wrong with Strava and the way it uses the map to calculate altitude on a ride in Japan.....

I take the elevation reading on my Garmin with a pinch of salt, one recent reading was -186ft as I rode along the river.

My garmin always seems to over estimate the elevation gain. isn't there an option to allow it to correct if you upload to there site - using a base map elevation data I guess.

I've recorded the same ride on my garmin & on strava & then put them both onto OS map (tracklogs) & garmin was over by 50m & the phone 50m under. was only a shortish mtb ride though with 300m or so of climb.

My Garmin told me 7' the other night when I had done over 1200'. Must be the cold weather.

excellent Question.

I use the 810 and last ride it said 900M of climbing which felt right. However when I downloaded the file onto Strava (syncing does not work for me at the moment) it gave it as 750M of climbing from the same data file. I trust the 810 ....