Hi guys,

Can someone 'explain like i am 5' how the Phantom 4 calculates it height.

Say i take off and fly to an altitude of 100ft. I then proceed to fly over a hill say 80ft. The height above the ground the DJI reports is not 20 ft but 100ft.

Because it doesn't report the altitude above the ground (that the P4 is flying over), it reports the altitude above (or below, if a negative number) the takeoff point.

The P4 has a barometric altimeter, GPS altitude data, and a pair of ultrasonic rangefinders that point downwards. I assume it uses all 3 systems when it can to calculate height above takeoff point.

Ok, thank you for the very thorough answer.

It's very important to confirm HOW the phantom 4  calculates the height becouse from this method it depends how to set the "RETURN TO HOME" height if something goes wrong.  

It basically just calculates from take off point not sea level. That's another way some people get above the the 500 meter threshold if you're already on a 3000 foot mountain top and then fly off into the valley

Wingsy Posted at 2016-9-19 00:32
Because it doesn't report the altitude above the ground (that the P4 is flying over), it reports the ...

"The P4 has a barometric altimeter, GPS altitude data, and a pair of ultrasonic rangefinders that point downwards. I assume it uses all 3 systems when it can to calculate height above takeoff point."

Except that it does not use GPS at all for altitude data (it would be much too inaccurate) and it only uses the VPS below 10 metres.
Which means that the Phantom only uses the barometer for altitude data in flight.

Excellent info guys, greatly appreciated, thank you all.

billwish76 Posted at 2016-9-18 21:20
It basically just calculates from take off point not sea level. That's another way some people get a ...

So actually if I'm flying over the valley, up the hills and so and finally I'm lets say 300, 400m above my altitude (out of sight of course but I'm still below 100m AGL) the only reasonable set up of emergency RTH (I don't use RTH but I can loose RC signal or so) is set it to land and climb for it?? Ta

Ben Tichy Posted at 2016-9-20 15:35
So actually if I'm flying over the valley, up the hills and so and finally I'm lets say 300, 400m  ...

If you're at the base of a 400m high hill, then you would set your max height limit to 500m (the maximum height allowed). Fly 400m to the top of the hill. Lets suppose you go over that hill and lose signal. You will have planned for this beforehand and set your RTH height to more than 400m (lets say you set it at 500m). So it loses signal, the plane initiates an RTH, it climbs to 500m, travels horizontally until directly above home point and then lands.

500m is the maximum height above the home point you can fly, so if the hill were 600m and you flew round it then you could well be snookered.

This seems to have been the fate of a few Phantoms going round tall buildings, lose signal , rise to a max' of say 120m and then crash into the balcony on the 40th floor

Aardvark Posted at 2016-9-20 12:11
If you're at the base of a 400m high hill, then you would set your max height limit to 500m (the ma ...

Well said....

Ben Tichy Posted at 2016-9-20 22:35
So actually if I'm flying over the valley, up the hills and so and finally I'm lets say 300, 400m  ...

if you fly in to the vally i understand you will have neg alt. if you are less than 20m it will land where it is not rth. if rth and  bird is lower than rth preset will rise to preset. if at or above preset it will rth at that alt and then land