The altitude reading gets higher than the actual altitude.  The longer I fly the more the difference.  After a couple of minutes of flying the reading will be about 10 feet higher than it should be, after about 5 minutes the reading gets to about 20 feet higher than it should be.  I keep waiting to see if any of the FW updates correct this, but so far nothing.  I have updated the FW each time, calibrated everything after each update but the altitude remains an issue.  Any thoughts?  Thanks.

Try changing to metric see if you get the same results....just a thought

I have same issue. I have asked it from DJI and this is what they said:
-The barometer has 2cm/s of error. This is barometer's feature.
-If the flight time is 25min = 1500s, then the error can accumulate approximately to 30m = 2cm/s * 1500s
-The alt also relative to HP alt. If there is alt different between HP and landing location, it will also add in the alt.

My altitude show 20 meter after 17 min flight (ground level, on the home point).

jorski, that does sound like the same issue.  I haven't thought too much about it until now.  With the new intelligent flight features out, you could be flying up to 25 feet lower than posted values.

The P3 uses a barometer sensor for altitude. Could be faulty.

The difference in pressure between between a few feet in altitude is very small, yet these things can detect it. That is how sensitive they are.

I have the same issue after latest firmware updates. My dif. is arround 40 ft.

I'm a newbie, flying a P3A since August 1st, 2015, but I have noticed the same issue; after about an 18 minute flight I will land in approximately the same location but my indicated altitude will be around + 30ft instead of 0 ft. I've logged 44 flights and it always showing a positive altitude shift with my flights.
You can go back and check your flight log data to see how often the altitude has shifted during a flight.
It's a problem, a 30 foot shift is scary. I don't rely on the indicated altitude.

My theory on why the shift occurs: At the beginning of the flight the sensor is at ambient temperature;
after about 10 minutes of flying the altitude sensor's temperature has increased due to localized heating inside
the aircraft; this increase in temperature is affecting the output of the altitude sensor. I'll bet that the shift will be smaller during my up and coming winter flights! I wonder if anyone at DJI can comment on my theory??