I have seen this topic posted somewhere but I can't find it now, of course.  My P4 has quite a large altitude error most of the time.  It is accurate from takeoff to about 50 feet and from there on, it starts deviating quite a bit.  This is not an issue most of the times but I have seen a point in flying a Litchi waypoint that I wanted to get as low as possible and not have to worry about the altitude accuracy.  An adjustment would be great if anyone knows of one.
Thanks,
Jim
WA5TEF

Perform an IMU calibration on a level surface.

I somehow doubt it having done enough calibrations to turn green in the face with sundry levels.  If I think a new calibration has made any difference, the next time out it's again bad.  Even Autopilot has a means to tighten up the altitude deviations from the default of 15 meters down to 5 meters at best.  I think their advisement is to not depend on anything less than 10 meters as a safety buffer.  Could be the pressure inside the bird, or some thermal drift matter as the electronics warm up, bad or low tolerance hardware, or just buggy software.

Fwiw, on mine I often see takeoffs at zero meters in GO, but landing at -16 meters, or flying around at -3 meters.  Really is awful for all the hardware in it, but I don't get excited by it much anymore.

Annoying, but after three birds I just accept the fact it doesn't work that well and plan around it.  Autopilot told me to fly it initially with a greater buffer, and then lower the altitude on subsequent flights if it works out.  You have to face the fact these aren't military-grade birds we own and fly.

Labroides Posted at 2017-3-19 17:30
Perform an IMU calibration on a level surface.

Yeah, I have not tried an IMU calibration.  I may just have to forget about it.  I also noticed that the RTH function is off a little also.  Not more than maybe 5 or 6 feet which is plenty good enough I guess.  Anyway, I'll try the IMU and see if it makes any difference.
Thanks for your post.
Jim
WA5TEF

WA5TEF Posted at 2017-3-19 18:08
Yeah, I have not tried an IMU calibration.  I may just have to forget about it.  I also noticed that the RTH function is off a little also.  Not more than maybe 5 or 6 feet which is plenty good enough I guess.  Anyway, I'll try the IMU and see if it makes any difference.
Thanks for your post.
Jim

"the RTH function is off a little also.  Not more than maybe 5 or 6 feet which is plenty good enough I guess"

That's within the accuracy that consumer GPS offers.
It's not off at all.
But you don't need to let the Phantom land itself anyway.
You can always choose a better landing spot than blind programming can.
If you really want precision RTH autolanding (it's just an impressive gimmick), get the P4pro.