I think from memory that there’s a small sensor in the drone so it’s set each time. Bit like if you have a garmin watch and it reads the pressure to get heights and if often wrong!!
Following on from the above messaging, I have chosen to use a laser measure to attempt to calculate the building height. The device I have has a 2 (or 3) point Pythagoras theorem equation function which can quickly show you the measurement with very little effort. I just need to find time to go back and locate a clean line of sight to be able to hit the sweet spots at a distance … wish me luck as the wobbling wibble factor of trying to keep a microdot steady over 50mtrs is not something that is easy to achieve!
Does it have a tripod mount option? If not, you can still attach it onto one or even use brumstick or something similar to steady it.
No tripod mount, it’s a small Leica Disto2 handheld unit. I’ll take a bean-bag or similar to squidge it down into for stability. I just know from experience that the further away the target, the greater my circle of wobblishness is 
When you adjust the aim, do it by adjusting the beanbag (squeezing it), not the device. That allows you to make very accurate and smooth adjustments. It’s the same principle as with a scoped rifle.