I realize AGL is relative to where the drone is, and not where the pilot is, so with that in mind here is my question. If you fly out over a valley at 200 ft AGL at your lacation, and the valley is 300 feet deep, you are now by definition 500 ft AGL from the valley floor. If you lower your drone to 200 AGL from the valley floor, will your drone interpret the negative altitude as crashing or will it read the AGL at valley level? And if you use RTH at that point with a RTH AGL of 100 ft, will the drone rise to 100 ft AGL relative to the Home location, or will it now drop to 100 ft AGL valley floor level, and crash as it gets to the edge of the valley on return? This is a serious question, as I would like to fly from a park that sits on the rim of a river valley, and want to stay under the legal height of 400 ft AGL.

The RTH height you set, is relative to the point of take-off. If you fly down into the valley, lower than the take-off altitude, the drone won't "think" it has crashed, and will continue your flight.
But if you hit the RTH button, the drone will rise to the preset height with reference to the point of take-off. And in that scenario you may possibly exceed the 400ft height limit.
It should be possible to calculate, prior to your flight, the relative heights involved, and set an RTH value which would not cause a transgression.  (Unless you're considering descending into a massive canyon or somesuch)

Blériot53 Posted at 3-19 13:33
The RTH height you set, is relative to the point of take-off. If you fly down into the valley, lower than the take-off altitude, the drone won't "think" it has crashed, and will continue your flight.
But if you hit the RTH button, the drone will rise to the preset height with reference to the point of take-off. And in that scenario you may possibly exceed the 400ft height limit.
It should be possible to calculate, prior to your flight, the relative heights involved, and set an RTH value which would not cause a transgression.  (Unless you're considering descending into a massive canyon or somesuch)

Thanks for the info.

mcvet57103 Posted at 3-19 13:36
Thanks for the info.

You're very welcome
Enjoy the hobby.

From my personal experience, the drone won't interpret the negative altitude as crashing. The sensor on the drone should prevent any crashing per say...

Hi, I am relatively new to drone, used to have a Mavic air and now bought a Neo, I always wonder what would happen if I live on 20/F and fly out from my balcony, will this AGL thing applies?? will my drone still flies once it leave my balcony?