Can someone explain this ATTI Mode behavior?

[yawnalot29]

Let me preface it by saying that my understanding of ATTI mode is that it will maintain altitude. This is what I understood from manual.

Last month, I was flying my P2V+ from a house that's sitting on top of a hill (about 650 feet above sea level) looking out to Pacific Ocean (Southern California).

I took the P2V+ up to about 150 feet AGL from where I was at and flew my P2V+ (in GPS mode with 13 GPS lock) toward the Pacific Ocean and reached there fine.

I toggle over to ATTI mode (on the control) and flew back manually (not RTH mode). And notice the drone is now at maybe 500 feet above sea level. So it had dropped about 300 feet in height between taking off in GPS mode and coming back in ATTI mode.

I am taking the height based on the height of the location I was at. And how high I flew is based on what the DJI App is telling me.

Is this a normal behavior? Or is my understanding of ATTI mode not correct? ...

 

[Mark The Droner]

It's my understanding that there is a sensor in the Phantom that measures barometric pressure, and that's what keeps it level in ATTI (small changes is air density). So my guess is the air pressure on top of the cliff was more dense than at the same horizontal level over the ocean.