Home Point is top of a Cliff - How would RTH work ?

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I just flew my P4 Plus from the top of a cliff. Elevation at home point was about 420'. I took off and went up to 150'. I set my RTH height to 200'. I then wanted to fly off the cliff and then wanted to descend closer to the river which is beyond the cliff. Elevation of the river is 0' or close to it.

But then something occurred to me- If I lose connection while flying over the river, what is RTH going to do ? If I lose connection while I'm flying over the river at say 150' above the river, the Phantom would have to ascend from the point of lost connection to a height of 470' to get to the the cliff top and the RTH height of 200'. Or would it just ascend 50' from the 150' of the lost connection ?
 
When RTH is initiated, the Phantom will ascend from the currently displayed altitude in DJI GO to the RTH altitude and then fly straight to the home point. If the currently displayed altitude in DJI GO is already above the RTH altitude, then the Phantom will return home at the current altitude. The Phantom does not know where the ground is located, so it's only able to use the takeoff point as its reference. That makes all of the other elevations you mentioned irrelevant.
 
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In other words, the assumption you are making is the currently displayed altitude in DJI GO once fly off the cliff and descend would be negative numbers ? Unfortunately, because I did not descend at all once I flew over the cliff, I cannot confirm that DJI GO would show negative numbers as the altitude. Have you actually seen DJI GO show negative numbers for altitude in such a situation. I'm a little nervous to give it a try unless someone has actually done this.
 
What msinger is correct, no assumptions.
I have seen on youtube phantoms going lower than take off point and 0 is recorded, not negative numbers.
Hope this helps
 
At this rate of going would it be possible to shut down the motors with the full down stick input?
 
I have flown off cliffs several times and descended, no problem. Your home point is elevation zero - the reference used for distance and altitude. Negative altitudes means nothing except you are below the home point altitude. An RTH will go to the altitude you have entered above your home point.
 
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Thx NM_Quad. No disrespect to msinger who always seems to have the correct answers for all of us, but I feel better with what NM_Quad says because he has actually done what I'm trying to do.

FYI msinger: If you have flown off cliffs before, please accept my apologies !
 
FYI msinger: If you have flown off cliffs before, please accept my apologies !
No worries. I have never flown off a cliff, but I have seen negative altitudes in DJI GO before. Cliff or no cliff, RTH still works like I described above.
 

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