As I understand it, the P3 measures altitudes above homepoint (using barometric pressure). Thus with a waypoint recorded at, say, 100 feet altitude the P3 would know to fly to that x-y coordinate point at an altitude of 100' above the start point. However, if your homepoint for the start of a waypoint flight was LOWER than on the 'training' run, the actual height at x-y would also be lower. If you started 101 feet (vertical distance) down a hill, the P3 would try to plough into the ground at x-y. Do I have that right? Which begs the question, why does DJI use barometric pressure to altitude? GPS provides three dimensional positioning. This is absolute positioning, not relative to a starting point. The x-y precision is obviously extremely good; I assume its altitude reading would be similarly accurate. At the very least, even if there is some additional benefit from a barometric measure, why not use both?