Hello,

With my P3, it is possible to fly forward perfectly at 1 or 2 meter without loosing any altitude.

With my P4, it is impossible ... it always lose altitude and its close to hit the ground if I don't adjust the altitude ....

I know that many P4 have this problem, any solutions ?

Try it with Downward Vision Positioning being disabled. It may not be able to deal with the structure of this field.
You might encounter the same effect when flying low over water.

Thanks    Any other ideas ?

We are waiting for an update with terrain following, but I don't know if it will help over that field.

Check the heights shown for the VPS sensors, see if they are sensible.   It is very possible that the ultrasonic waves don't bounce off that crop to give a nice clear reflection, since there isn't a nice hard surface!  If they are looking sensible then the terrain following will help, currently it makes no adjustment for changes in ground level and there is no way around it other than for the pilot to continuously adjust it, or fly a waypoint mission with the points set either end of the field in which case it will follow the slope as long as the slope is constant.

fanse25e50b1 Posted at 2017-4-26 08:15
Thanks    Any other ideas ?

You could try calibrating the VPS sensors using DJI Assistant 2 from Here

Could be something as simple as the field actually slopes at such a small angle and the crop is varying heights. It may look straight but it doesn't mean it is. And Leo as another poster said VPS will try to keep height using sensors but needs a good surface. Do t forget GPS isn't 100% accurate. Altitude is relative to your take off point.

As mentioned, try flying a waypoint box pattern over that field with the waypoints all at the same height.  I've done it with Autopilot and a P4 and the mystery to me is that the P4 will fly the correct height outbound, but on a return leg it will be higher, maybe 15 feet.  If I allow it to continue, the outbound drops down to the correct height again, and the inbound climbs up again.  Very odd, but seems direction oriented too.  The GO screen number are all over the place so I cannot rely on them compared to what the video shows with the climbing height.  It often lands -40 feet showing in GO while takeoff was at zero.

Autopilot has a setting where you can control the flight altitude tolerance.   I think default was 45 feet and the tightest was 15 feet.  I set it to tightest and it still shows return leg higher than outgoing.  Dunno.

In addition, do you use an after marked support bracket across the landing gear?

If the ground is rising and you may not realise that , read your altitude to check.