So, I have the maximum altitude on my Mavic set at 400 feet (consistent with FAA rules). That much is clear, but what I don't know is how this is measured. A couple of days ago, for instance, I took off and flew the Mavic to the top of a nearby hill. The drone was 400 feet in the air compared to where it took off from, but much less than that height above the top of the hill (because the hill's summit was several hundred feet higher than where I was standing at the takeoff point). If the hill had been a little higher, I would have been unable to fly to the top, because I would have exceeded what the drone thought was its maximum flying height.

Does maximum altitude refer to height above the takeoff point, the height above the ground at any given moment, or something else? If I took off, say, the edge of a canyon or the top of a very tall building (more than 400 feet tall), what would the drone do when the ground below it was more than 400 feet down? Would I need to override the max height?

Thanks in advance for any advice or info.

this should answer your question: http://forum.dji.com/forum.php?m ... id=75855&page=1

"It's based from where you took off. Most laws/regulations are based of AGL, or Above ground level, rather than above sea level.

For example where I live, I can fly up to 140m (400ft) AGL. Because the way the DJI works, basing its height off starting point I do have to be careful, not to start the drone at a high point, and then fly at max height above somewhere lower than where I started.

For example, If I took off and flew right to 140m above starting point (140m AGL), the flew across somewhere that was 50m lower than my starting point, I would technically be at 210m AGL (140m+50m) which would be in breach of the 140m AGL limit where I am."

it is based off of your takeoff location. So keep in mind the rule is 400ft above the ground or a structure. So if you know you are in an area with changing terrain you should set the altitude limit to something higher than the highest point but keep it under 400ft from the highest point to be within the legal limits.

Just to be pedantic. It is actually based on the point where you power on the drone which is not necessarily your take off point. Usually will be the same.
It records this point as zero altitude and all is reference to this point. Measured by pressure altimeter/s which have no idea if you fly over a canyon or rising ground.

We should all try to observe the proper ceiling height limit of 400ft, but that may be easily broken by genuine mistake rather than by design. If your drone were at a height of 200ft and flew over a short ridge depression of say 220ft, for ten seconds or so, that would technically be in breach of the regs. I'm sure in an instance like that it is plainly obvious you were flying sensibly and with due consideration, as any passenger carrying aircraft would never be in the same airspace as you other than for example, a rescue aircraft like an emergency services helicopter.

Being the law abiding model citizen that you are, if you were to see such an aircraft in the area you would have observed extra diligence and either lowered your height considerably or not flown at all. The FAA/CAA are concerned with idiots that regularly flout the regulations by flying at much higher altitudes and at distances way beyond line of sight. It amazes me how mamy videos are published on publicly open social media sites that are clearly in total contravention of the regulations, not thinking that they are supplying evidence of witness to their own crimes, and stand a good chance that if the authorities were to pursue such contraventions, they could end up in serious trouble.