These are the 3 axis to take into account. Well use a persons head as an example.
Pitch: this us like nodding your head. On your phantom your quad will pitch when moving forwards (downward pitch) or backwards (upwards pitch)
Roll: this is like moving your head to touch your ear to your shoulder. Roll concours when you move your phantom left or right with the right stick on the controller. The quad rolls downward in the direction it's moving.
Yaw is like shaking your head no. Yaw is when you rotate the phantom left and right with the left stick. This doesn't make your quad dip any direction, it just rotates it.
A 2 axis gimbal will level you for pitch and roll. A 3 axis adds yaw to keep you pointed the same way during the smaller yaw movements. A 2 axis gimbal will take care of most of what your looking for. You won't have the camera roll sideways or pitch up and down when the quad moves. If you do smaller yaw adjustments (or if the phantom does them while hovering in GPS mode) the image won't stay centered, it will move. To me, this is not a major issue. Most of the time when I'm yawing, I want to rotate the camera. If it's worth the extra $ to you to add yaw to the gimbal, then go 3 axis, but I think most folks will bad fine with 2 axis.
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