since the earth is no perfect sphere.
I don't believe you can for activities, sorry
CW
For those still wondering if the FR735XT is the right watch for them, I did a number of tests on Elevation and the Optical HR. In general I got better results with the elevation than others seem to have. The optical HR generally followed the same path of being good for steady efforts and less good at picking up the changes when doing intervals. Still like having it though. For those interested I've put a pretty detailed review on my website here: http://www.trijedi.com/2016/08/27/garmin-735xt-review/