What's nice is the way they do 360d view, the 6s, the low drag, the smaller camera (setups with gopro - which is 4k too - or bigger are quite .. massive).. and they also seem to be catching up to the APM/Pixhawk on the software side. Also, lightbridge/HD.

Now price, non-opensource, etc.. of course. Still a nice piece of hardware.

As far as I know both PX4 and APM have half-baked optical flow code in various branches, but no out-of-the-box / off-the-shelf support in mainline (even though 3DR sell optical flow hardware).

Knowing DJI's past I'm not sure their integration is much better, but if it is, that's a big win IMO. Optical flow is awesome for new pilots as it makes the quadcopter's position hold ability in hover much more stable. It's one of the main reasons the AR.Drone works so well despite its relatively imprecise / high-latency phone-based control interface.

See video of a Canadian CT-155 Hawk (Not F-16 as titled) https://www.youtube.com/watch?v=zN_Zl64OQEw

For an airliner like an Airbus A320, a small bird can damage the engine fan blades and require an expensive repair. If the bird goes through the core even minor damage becomes very expensive to fix.

DJI's Phantom quadcopters (with newest firmware) enforce the 400 foot ceiling in software, not sure what their plans are with this one since it is being marketed as much more pro-sumer than than the Phantom line, but if Joe Schmoe Parkflyer flies one of these above 400 feet he is violating FAA regulations and theoretically could be charged large fines currently.

When you turn the Phantom on one of the first things it does is tries to get a GPS lock on its current position which it will use as the "home" point. This is a pretty important piece of data for the device to have (and the bright LEDs on the arms will light up red until it has it, warning you that taking off is dangerous) because if it goes into "return to home" failsafe for any reason (loss of connection to the controller, battery low situation, etc) the device will attempt to fly itself to that position (after first ascending to a programmable return-to-home altitude if needed to minimize risk of hitting obstacles like trees or houses).

The controller on the Phantoms also has a barometer which can be used for (relative) altitude change readings. I have no idea if it uses that at all in the 400 foot ceiling calculations.

It is extremely doubtful that the Phantom carries any sort of WGS 84 to local height mapping because that is a ridiculous amount of data (even at a fairly low resolution) to carry for the entire globe, and it has no link to any kind of lookup service for that, and if you based the height on sea level that would make the device pretty worthless in many places (eg. Denver which is more than 13 times higher than 400 feet above sea level at ground level).