You can reset the elevation. Settings => GPS => Elevation will allow you to set a known elevation for the point you are at. After that, when you start the unit at that point, it will use the elevation you saved.
However, this will not affect the overall elevation gain and may even make things worse. I have noticed the elevation on mine takes a while to stabilise after the unit is powered on. The only way to deal with this is to switch on the unit a few minutes before you start; it should then settle as the unit will calibrate its elevation from multiple satellites.
+/- 50 m is not that big a error in my experience, especially as changes in air pressure due to the velocity of the wind across the barometric sensor are interpreted by the unt as elevation changes. Different versions of the firmware can also give different total ascent figures; anything after version 2.8 should be fairly consistent.
Thanks, I'll give that a try. It's not crucial, but lately having ridden over near the port (road right next to the water) or running along the coast, when I upload the ride to garmin it shows 60+m when it should be 3-4m and in some spots 2m.
Speaking of which, I've been seeing those small posters all over the city "elevation of this point is XX above sea level" - I wonder how accurate that data is, and which exact part of the poster is at the claimed elevation I guess the most accurate way would be to go to Yokohama pier and use one of those huge rulers they have which show water level. Afair, Garmin can store 10 elevation points (that also dupe as waypoints), but I've never used them for anything.
On the Edge 810, at least, there are a couple of choices.
At any time, if you have accurate elevation data you can set your elevation and in doing so manually calibrate the altimeter.
If the elevation is entered manually for a Location (waypoint), riding through that Location after starting the ride timer will calibrate the altimeter. I can't remember whether there's a setting to do this, but it works for me every time I leave home.