This morning I took my P4 for another set of tests and I noticed that at some point my altitude readings were completely off.

  

  

I usually fly on a large baseball fields area (4 fields altogether) so the terrain is as flat and grassy as it can get.

  

  

I did a few left to right flyby's (flying sideways). When I go right to left my P4 climbs 1-2 meters no matter what and sinks the same amount when I stop

  

When I fly left to right it maintains height and I did not notice any sink after a stop.

  

  

However, today, after different flight patterns, my readings were totally off. I was about a real 10 meters of the ground but my app showed 3-4 meters.

  

When I carefully lowered the bird, reading went pass 0 (=ground) and started reading negative numbers. Almost like if the barometer was totally off or the VPS

  

got tricked somehow. I was temped to use RTH to check if the auto land would use the info on the bird or the actual real height but I did not wanted to risk.

  

  

On the attached pics you can see what I mean. I started with a 0 meter reading (=Home) and after landing on the same spot it reads as -3 Meters (same location)

  

  

Has anyone experienced these readings? I will keep an eye on these until further upgrades but there is definitely a bug somewhere.

  

The Phantom series aircraft use a barometer to measure altitude, and it is measured with your take-off point as zero. Contrary to what people may think, the GPS system is not used to measure altitude, as it is hopelessly inaccurate.

But the barometer is also subject to variations as well. And this is probably the main reason DJI introduced the VPS system to control altitude at low levels, rather than rely on the barometer. The P4 should maintain altitude in the region up to 3 Metres, but there is also a suspected bug where it sinks down when stopped. This will probably get addressed in a future firmware update.