Take off altitude 0 feet - landing altitude at same place +41 feet. why??

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I take off from a "landing pad" and always return to that same exact spot. My take off altitude shows 0 feet BUT when I return and land it shows my altitude as higher than 0 feet ... sometimes 40 plus feet higher.
I therefore have "trust" issues with the shown altitude while in flight. I have done a "cold" IMU calibration and the IMU numbers are great when i check them, I also compass calibrate B4 every flight. I would appreciate your help in correcting this issue. I know the P4 uses barometric pressure to determine altitude BUT there can't be that much pressure change every flight I take, after all flight time is only 15-20 minutes. I purchased my P4 directly from DJI ... do i need to send it back or is there a "fix" available for this issue that I can do on my own?? Thanks in advance for your help,
Garry
 
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Wow, yours sounds just like what mine was doing. Take-off zero, but almost -50 feet on return to landing in same spot after about one mile flying distance.

What I found was my calibration table was about 1/4 bubble off from being level against a longer carpenter's level. Might be about one degree, but I think the trig works out to be about -50 feet or so over that length of flying. Once I switched levels, the matter seems to have resolved itself (Nose had to come up about 1/4 bubble.). The tilt of the drone uses the IMU as you fly about.

There was also scattered talk about the feet not being parallel with the circuit board internally throwing it off maybe due to a rough landing, and maybe the board verse the motors too. Some suggested placing a 12" square piece of glass on the tops of the motors to level the drone. Haven't tried that route yet since I seemed to have a bum level and seems to be okay for now.
 
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I found the ideal size glass that I push up out of a coffe table inlay Now I have to give a days notice of my intentions to do a IMU cal due to an unfortunate indecident involving a hot tea cup and a table that wasn't actually acting as a table.
There are many ways to lose your P3
 

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