Hi all, I am new with my p3p but noticed yesterday when landing  on the same spot my home point was set to I was 2 feet above ground and the dji go app showed I was -13 feet at that time...the 4' x 4' pc of wood did not move and I flew for about 12 min. is this normal? will a descrepency like this cause a problem with RTH function?

It won't make any difference to RTH because your Phantom will descend until it can't descend any more.
(btw why are you letting some blind programming land for you?)
It's not uncommon for the barometer altitude to vary slightly over time.
Doing an IMU calibration on a level surface may reduce the amount of error your Phantom shows.

Correct as above, an IMU calibration may help.
Also, when you power up the aircraft let it sit for a minute or two so it can acclimated to the pressure.

Thanks for the reply and info, I am not letting it land on its own, never have, just want to know if it has to on its own (because of low batt) it can and this error wont be a problem. Been flying flybarred helis for 10 years, this thing is to very easy to fly and land almost no skill needed.

labroides@yahoo Posted at 2016-7-28 06:47
It won't make any difference to RTH because your Phantom will descend until it can't descend any mor ...

There is a lot more than just landing we let some 'blind programming' do while flying our AC's, so why not let it land ?
I use RTH and auto landing 99 % of the time, just like our big brothers from Airbus and Boeing are trusting their 'blind programming' to land them safely, also in 99 % of the cases.

PhanFran Posted at 2016-7-29 07:58
There is a lot more than just landing we let some 'blind programming' do while flying our AC's, so  ...

Because not all flying is done from flat parking lots.

DJI-Ken Posted at 2016-7-28 10:27
Correct as above, an IMU calibration may help.
Also, when you power up the aircraft let it sit for a ...

If you REALLY want to minimize the errors in height-measuring the best you can do is
1) NOT do the IMU calibration with a COLD AC
2) Take off, let the AC hover in place for one minute or so and land it again (so that the actual 'zero'level is reset, but this time with a warm IMU) before taking off for your actual flight.

Not a procedure you need to do for every flight but absolutely necessary when you are flying in mountainous regions with big differences in height.

PhanFran Posted at 2016-7-29 06:21
If you REALLY want to minimize the errors in height-measuring the best you can do is
1) NOT do th ...

I've been doing a cold IMU calibration for several years (shorten the IMU warming up time) and have never once had any kind of altitude errors.But correct, hovering for a minute and landing then taking off again may help.

DJI-Ken Posted at 2016-7-29 10:45
I've been doing a cold IMU calibration for several years (shorten the IMU warming up time) and have ...

Well, you will indeed shorten the warm-up delay but you will diminish accuracy of the barometer significantly.

PhanFran Posted at 2016-8-7 13:58
Well, you will indeed shorten the warm-up delay but you will diminish accuracy of the barometer si ...

Can you please explain how that is?  The barometric altimeter works off atmospheric pressure.  At sea level this is 14.7 lbs per square inch.  Temp. lower than ambient should not change pressure.  Or am I missing something?

PhanFran Posted at 2016-8-8 04:58
Well, you will indeed shorten the warm-up delay but you will diminish accuracy of the barometer si ...

That's what my friend thought too because he saw it on the internet. I showed him a cold calibration then flew at different heights and hovered next to objects like a football field light post and then did a ambient air calibration and the altitude reading was exactly the same.But I would like to see your finding, if you can do the same test and show it changes the altitude reading at the same height.

Kneepuck Posted at 2016-8-8 18:14
Can you please explain how that is?  The barometric altimeter works off atmospheric pressure.  At  ...

I'm not an engineer and I can't give you the details. This warning doesn't come from me either; it's a warning that circulates on the Litchi-forum and in the Litchi-FAQ's.
And I must say, I'm not an engineer but an MD and so I am not completely ignorant of the laws of physics. One of these laws is the gas law and this says that gasses will expand at higher temperatures so if you 'calibrate' an instrument that measures the presuure of the air in a temperature that is significantly colder than the ambient temperature and then you take off and tell it before take-off: this is ZERO AGL, it doesn't sound strange to me that readings will become less precise when the altitude is calculated at a higher environmental temperature.

DJI-Ken Posted at 2016-8-8 22:09
That's what my friend thought too because he saw it on the internet. I showed him a cold calibratio ...

I beg to differ on this one: I did indeed do the test that I described: ambient temperature IMU calibration AND the one minute hover and relanding before taking off again.
That procedure compared to just a cold IMU-calibration gave me a more accurate reading of height: with the procedure described the gimbal tilt was at 0 ° and I had the center of a rotor of a windmill right in the middle of my video. The cenbter was at 100 meters and the barometer showed between 99.8 and 100.1.
Same test with cold calibration and immediate take off: gimbal angle varied between -12 and + 8 and when put at 0° the center of the rotor was never in the middle of my screen.

I know, just one test (done three times though) and this means nothing but still I think the reasoning behind this isn't wrong.

PhanFran Posted at 2016-8-8 23:55
I beg to differ on this one: I did indeed do the test that I described: ambient temperature IMU cal ...

You said cold calibration then immediate takeoff.
Have you tried cold calibration then later on doing the test when the aircraft has reached ambient temp them powering it up and flying to the center of the rotor.I know that if you immediately do the cold calibration then power up and fly it may be off because the barometer is not at ambient temp.

The beauty of the part 107 study process is we will have people that learn the difference between AGL,ASL, and AMSL.  In addition a good study program will cover Pressure Altitude, Indicated Altitude, True Altitude, and Density Altitude.
Still that will leave the issue of the Phantom FW, and the algorithm for setting the zero point reference, with differences for manual take off,auto takeoff, and different versions.