Hello, is it possible to have the Phantom maintain a constant height above the ground? I would like to be able to perform a grid survey, having the drone survey a grid (taking photos at selected intervals), while maintaining a constant height of ~2-3 meters above the ground. This requires being able to ascend or descend according to the terrain. Is that possible? Is the barometer accurate enough to do this?

Running some calculations, I think actually 3-4 meters should be sufficient for our purposes. Will the Phantom 4 maintain a height of 4 meters above ground using the barometer? Or will it crash if it runs into a hill?

Your height is always relative to where the launch point was.

I notice that often over water the height slowly decreases above the ocean as well.  Also, when you're only about 2 meters above the ground, the UAV has to fight additional upward thrust from the propellers.  This "grid survey" might be one of the few concepts of use that DJI hadn't quite fully adapted into the abilities of the P4P

citronade Posted at 4-20 11:19
Running some calculations, I think actually 3-4 meters should be sufficient for our purposes. Will the Phantom 4 maintain a height of 4 meters above ground using the barometer? Or will it crash if it runs into a hill?

Will the Phantom 4 maintain a height of 4 meters above ground using the barometer?
No it won't.
The barometer has no idea where ground level is.
Launch near the edge of a 1000 ft cliff and fly 5 feet up, then fly out past the edge.
Your drone's barometer shows the same altitude .

Ah, another one of these...
Answer is, with the current software, and your AC, NO.
If the software was written to use a DSM it would allow terrain following but not to the precision you're asking. ±5-10 m is about the best you could expect from a non RTK AC, so trying to fly 3-4m off the deck would be a recipe for disaster.

Don't even try it !  As stated in previous posts the wee-beastie only has reference to the departure altitude/pressure.  
Was a good idea though