Welcome to PhantomPilots.com

Sign up for a weekly email of the latest drone news & information

Altitude Accuracy

Discussion in 'Pro/Adv Discussion' started by jimerb, Feb 21, 2016.

  1. jimerb

    Joined:
    Nov 29, 2015
    Messages:
    178
    Likes Received:
    37
    Today I was flying in a an open field at 40ft. I decided to buzz over myself while shooting a video.

    I noticed as the p3 was getting close that it was more
    Ike 12 feet high, not 40!

    That instantly made me nervous because earlier, I had been flying at about 120 feet thinking I was safe above some trees.

    I live in a very flat low altitude area, no hills anywhere.

    Gps lock was no problem. I was not in atti mode.

    What could be going on here?
     
  2. Phantom751874

    Joined:
    Mar 13, 2015
    Messages:
    995
    Likes Received:
    269
    Land and reset power and test altitude again. The barometer in these P3's can be wonky like that. I imagine it's not a high quality barometer. You will find on some flights it's pretty accurate while on others it's grossly off. I just make a mental note when it's off. I don't want to bother with landing and resetting power.
     
    jimerb likes this.
  3. shorttimer

    Joined:
    Jan 15, 2016
    Messages:
    364
    Likes Received:
    120
    Location:
    Rocky Mountains
    Your altitude isn't determined by your GPS, only your position. Your altitude is determined by the atmospheric pressure at your take off spot. and can/will change some.
     
  4. Meta4

    Meta4 Moderator
    Staff Member

    Joined:
    Jan 8, 2014
    Messages:
    7,713
    Likes Received:
    3,434
    Temperature change will affect the barometer.
    If you are taking off cold and the machine warms up in flight this might be responsible.
    Keep an eye on it with a few test flights to see if it's consistent.
     
  5. CactusJackSlade

    Joined:
    May 29, 2014
    Messages:
    598
    Likes Received:
    112
    Many threads on this...... the altimeter is not that accurate, unfortunately. As Meta4 pointed out temps can cause errors. The first time I was doing a lot of low level stuff (creek wash) I noticed about a 30 foot difference on the P3P from the time I landed, did a few passes and cam back in front of me. I was 15 feet in the air, yet the ALT said 45...
     
  6. Noble 1

    Joined:
    Jan 8, 2016
    Messages:
    316
    Likes Received:
    70
    Location:
    Oregon
    I personally have found the altitude to be pretty darn accurate but depending on the day and temperature I'm sure it has a range of accuracy +/- 30 ft which is only bad if you like low level flights.
     
  7. jimerb

    Joined:
    Nov 29, 2015
    Messages:
    178
    Likes Received:
    37
    The scary part of inaccurate altitude is not being aware it can be off. When I fly in my neighborhood I have been setting it to 120 feet and not worrying about it because I've scouted by home turf carefully. Now I have to reevaluate my game plan.
     
  8. exit 4

    Joined:
    Jan 19, 2016
    Messages:
    373
    Likes Received:
    97
    Location:
    Southern New Jersey
    Also consider any preplanned flights you may have. You set the altitude for your waypoints at say 20 meters, then the reading is off and you think you are safe when you are really heading straight into a row of trees. I cannot feel safe running any type of preplanned flight until I can be sure the altitude is more accurate. I had planned to do a few zip line shots at 25' but fear I must now do it by hand and try to fly and do the video together. Scary.
     
  9. Air Ontario

    Joined:
    Jan 11, 2016
    Messages:
    1,271
    Likes Received:
    493
    Location:
    U.S./ Canada
    Preflight programming doesn't prevent one from flying manually.

    Many times during a waypoint mission, I adjust the altitude and speed however it's always on distant flights, not videoing the house in front of me.

    I find the P3P barometer extremely accurate however it does drift with atmospheric variables but so does the altimeter in the Cessna. That's why I am closely scanning it and I reset/retune the altimeter a minimum of every 100 KM of flight.
     
  10. olof Ekbergh

    Joined:
    Sep 30, 2015
    Messages:
    252
    Likes Received:
    137
    Location:
    Mt Washington Valley
    Barometric pressure is constantly changing. Even instruments in an IFR (Instrument flight rule, flying in the clouds, no visual reference) certified airplane only has to be accurate to within 50' I seem to remember. That is why ILS class I (the lowest class glide slope landings, no ground proximity radar) have a minimum 200' AGL (hight above ground) or you have to climb up and abort landing and go somewhere that is a above minimums.

    Flying a GA airplane you are constantly adjusting your altimeter as you fly, the local pressure is transmitted by airport ATIS (automatic recorded airport information systems you tune your navcom radios to) and announced by the tower as you approach an airport. It is also part of local weather reports.

    Even a gust of wind will change the pressure. The pressure sensors in the P3P are pretty cheap. I have a couple NAZAs that you have to block sunlight to one side in order to not have the pressure sensor give a false reading, a lot of NAZA users crashed when the sun hit the side of the unit, this would happen when the sun was low and a direct sunlight hit the sensor. So I would not be surprised if even in the P3P light intensity actually effects the sensor. The effect was quite dramatic the MR would climb or descend quickly smashing into the ground or the low branches of a tree. I still have 2 NAZAs with black rubber tape on the sides, with that they work fine.

    All this said. Flying low and near objects/ people, you should always fly by hand and be very close so you can really see what is going on. And definitely not fly FPV w/o a safety pilot keeping a close eye on things.
     
  11. Fat City

    Joined:
    Sep 16, 2015
    Messages:
    144
    Likes Received:
    37
    Location:
    near Boston, MA
    The pressure sensor is designed to compensate for temperature changes, but it's not perfect. Lots of other things will affect the readings, including regular barometric pressure changes due to weather systems. But over a short period of time, I think the development of up or down drafts can have the greatest effect. Down drafts compress the air, so the QC will think its altitude is lower than it is, and up drafts do the opposite.

    In an open field solar heating can cause an up draft over an area that's heating up faster than its surroundings. I've seen this often in agricultural areas where some fields were irrigated and other were not. The irrigated fields were darker and heated up much more quickly. If you see birds soaring in thermals, you know something like this is happening.
     
  12. AlexSP

    Joined:
    Oct 30, 2015
    Messages:
    295
    Likes Received:
    237
    I only fly low-altitude when the aircraft (any aircraft) is on sight, that is close enough so I can gauge it by eyesight with some accuracy. If you're unsure you can turn VPS on, it's there exactly to aid in such situations (though mine's OFF all the time so not to interfere during landing mostly). Maybe someone can confirm or deny this but from what I could get the altitude on the P3 is taken by both the barometer AND the GPS, like a Garmin or other similar positioning system. It's pretty accurate in my experience, at least enough to avoid problems. But yes it can and do get weird at times.
     
    #12 AlexSP, Feb 23, 2016
    Last edited: Feb 23, 2016
  13. Fat City

    Joined:
    Sep 16, 2015
    Messages:
    144
    Likes Received:
    37
    Location:
    near Boston, MA
    GPS is pretty useless for altitude, so I'm sure the aircraft depends on its barometer, unless close to the ground and VPS is enabled.