How does Verify Height work?

I’m confused about how Verify Height actually works. I understand that if I set a waypoint height to 10m and Verify Height is NOT checked that should be 10m above the home location (where the copter was last armed). But if it IS checked it should be 10m above the local ground surface. The barometric altitude at the home location is set on arming and the home location is set by the GPS, but the ground elevation at the home location is not known because the copter is not connected to Mission Planner when it is armed in the field. So there seems to be no way of relating absolute altitude to barometric altitude for the subsequent flight. The only way I can see it working is if the absolute altitude at the home location is assumed to be the same as at the first waypoint. Can anyone explain how it works ?

From what I can see, if you tick verify height and then start plotting waypoints, mission planner uses google maps terrain data to know the height above sea level and will automatically increase the height of your waypoint to account for the changes in elevation.

For example, I just tried making a mission at home with the default altitude of 5 mtr. Waypoint 1 was close to home but I then set waypoint 2 where I know there is a hill and waypoint 3 is then the other side of the hill. The altitude of wp1 is 5mtr but wp2 is 54mtr and wp3 is 25mtr.

Thanks Mark. I have set up a mission with verify height checked and default altitude 10m, starting low down and with waypoints going up over a hill. Viewed as KML in Google Earth the waypoints were all 10m above the local ground surface as expected.

But what concerns me is how does the flight controller turn these heights into barometer readings. The barometer is not calibrated against actual height at the arming site because this could be anywhere at an unknown height. It would need to be connected to Mission Planner to know the height and this would not be the case in the field.

So I wonder if the actual height of the first waypoint is used as the arming point height. In which case should arming be done close to the first waypoint. I suppose the method works but I am reluctant to try it without understanding it.

Because of the abundance of trees where I typically fly, I set my first and last waypoints almost overhead of where I expect my Home or arming position to be.

MP appears to base it’s verify height adjustments from the indicated home position. So when planning a mission at home, drag the home location to where the home location will be ie where the mission will be started from.

That way MP knows your starting altitude (above sea level based on the google data which becomes your 0 barometer level) and varies your minimum altitude for the flight path based on that altitude.

Thanks Mark, that sounds like good sense. I’ll give it a try.