Hi All,
yesterday, during a flight with my P4, it suddenly popped up a doubt.
The home point was in a church located in top of a hill. I sent the P4 horizontally backwards, without altering altitude with the left stick.
Due to a little cliff under the church the bottom sensor soon measured an altitude of 95 meters, which was very close to the maximum altitude I previously set (100 mt). At that point a doubut suddenly raised up to my mind :
What would have done the P4 if the cliff was higher and, suppose, the measured altitude was 150mt? It would have start "sinkinking" down to keep the altitude from the ground at the maximum limit of 100 mt regardless the home point is at 0mt?
I need to kill this doubt because I have programmed a flight in a mountain that has nice grassy fields on the top, but with a sharp vertical rocky cliff going down for approximately 600-800 mt. It's a wonderful panorama, but I want to be aware how the P4 would "behave" once it is pushed far over that cliff. I am afraid it would start to drop down vertically attempting to reach the maximum altitude set in the dji go app, which could eventually end with the P4 loss.
Someone may suggest to simply increase the maximum altitude in the app, but from a beginner's eye that appears more a workaround than a solution.
Thanks.

Any altitude you see is in relation to the take off spot, if you fly over the edge of the cliff it will not drop. If you descend down after going over the cliff you will see a negative altitude reading. If you set an RTH height of 100 feet and it were 200 feet below edge of cliff (-200 ft) and lost signal then the aircraft would rise to a height of 100 feet above home point before flying home. But bear in mind RTH can fail  at times, mostly due to height not being set correctly. Best to keep it under manual control for getting home.

"What would have done the P4 if the cliff was higher and, suppose, the measured altitude was 150mt?"

The VPS sensors on your P4 have an effective range of only 10 metres.
They can't measure any height above the ground beyond that.
But none of that matters.
Your Phantom won't plummet just because ground level falls away.
Your Phantom would act just like any plane when it flies over a valley or cliff and keep going at whatever altitude the pilot wants it to.

Aardvark Posted at 2016-9-26 12:47
Any altitude you see is in relation to the take off spot, if you fly over the edge of the cliff it w ...

Thank you Aardvark !!
Thank you Labroides !!
Understood.