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How accurate is altitude reading for P4P?
5155 20 2017-2-4
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piowoc73
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Hi,

let's assume I am taking off of the rooftop, which is 50 ft above ground. I would get 20 ft above the rooftop hover for a minute and then start flying over a flat terrain below. What would P4P altimeter show? Would it be 70 ft, or maybe 20 ft? What would be recorded as RTH altitude?

How accurate is the altitude reading when flying P4P over water?

Any tips on improving its accuracy?

Thanks!
Peter
2017-2-4
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hallmark007
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Height is accurate on p4p , it's measured from the home point ( the spot you took off from) so if you flew down the side of the building it would read a minus figure. Altitude is measured from mean sea level, which if you need you will have to go to the beach if you are looking for altitude.
2017-2-4
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Labroides
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Your Phantom has no understanding of any altitude except Home = Zero.  All other heights are relative to that.
2017-2-4
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piowoc73
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Labroides Posted at 2017-2-4 15:06
Your Phantom has no understanding of any altitude except Home = Zero.  All other heights are relative to that.

Thanks! It's clear now.
2017-2-4
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BashP4P+
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Good question I was about to ask same. Also got the answer. Thanks Labroides
2017-2-4
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sd603
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Labroides Posted at 2017-2-4 15:06
Your Phantom has no understanding of any altitude except Home = Zero.  All other heights are relative to that.

Does it measure height using GPS?
2017-2-7
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Labroides
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sd603 Posted at 2017-2-7 15:23
Does it measure height using GPS?

No ... GPS is very good for location but is not very accurate for altitude.
The Phantom uses a barometer to calculate altitude
2017-2-7
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KevDrones
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So if I launch off the wing of an airplane at 5000 feet and fly down will it show negatives?
2017-2-7
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fans40289743
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KevDrones Posted at 2017-2-7 18:30
So if I launch off the wing of an airplane at 5000 feet and fly down will it show negatives?

I believe so, and I also believe that altitude limits apply for negative altitudes too, so you couldn't descend any further than the distance you could ascend. 120 meters up or 120 meters down, in the US, unless you override the limits.
2017-2-7
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KevDrones
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It think it is 500 meters at the max. I dont even try that.
2017-2-8
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KevDrones
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today, when flew the altitude went up to like 1 meters when I was flat on the ground? What to do?
2017-2-8
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piowoc73
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KevDrones Posted at 2017-2-8 17:09
today, when flew the altitude went up to like 1 meters when I was flat on the ground? What to do?

It could be inaccurate barometer reading. 1m can be the margin of error for it.
2017-2-14
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Antonio76
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hallmark007 Posted at 2017-2-4 13:01
Height is accurate on p4p , it's measured from the home point ( the spot you took off from) so if you flew down the side of the building it would read a minus figure. Altitude is measured from mean sea level, which if you need you will have to go to the beach if you are looking for altitude.


Watching at pictures taken with P4P, I noticed something weird. The GPS and altitude data for the most recent pictures showed a definitely wrong altitude. Older pictures, taken BEFORE I reset all the DJI GO 4 settings to "Factory" were more consistent with actual altitude reached by the craft, even if those also showed the Altitude Reference as "below sea level"

Data for picture "flagpole.jpg" (takeoff from 11 meters above sea level, and top of flagple is actually 13 meters above ground, so 24 meters above sea level -air temperature +4ºC):

Altitude: 169.771 m (557.0 ft)
Altitude Reference: below sea level
GPS Version: 2.3
Latitude: (shows correct value)
Longitude: (shows correct value)

Data for a control picture taken with craft sitting on my desk(about 12 meters above sea level -room temperature +20ºC):

Altitude: 28.879 m (94.7 ft)
Altitude Reference: below sea level
GPS Version: 2.3
Latitude: 0° 0' 0" N
Longitude: 0° 0' 0" E

Latest ( as for today) software for both the controller and the craft, latest DJI GO 4 on iPad Air 2. No anomalies shown at startup, craft flies smoothly, and during flight the altitude shown on the screen are consistent with reality.

Any ideas as why is this happening? Should I worry?
flagpole.jpg
2017-2-17
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PhanFran
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Antonio76 Posted at 2017-2-17 04:06
Watching at pictures taken with P4P, I noticed something weird. The GPS and altitude data for the most recent pictures showed a definitely wrong altitude. Older pictures, taken BEFORE I reset all the DJI GO 4 settings to "Factory" were more consistent with actual altitude reached by the craft, even if those also showed the Altitude Reference as "below sea level"

Data for picture "flagpole.jpg" (takeoff from 11 meters above sea level, and top of flagple is actually 13 meters above ground, so 24 meters above sea level -air temperature +4ºC):

Altitude-data for the EXIF-data of your pictures is probably taken from the GPS which is VERY inaccurate. Putting the 'Height' data in the EXIF would be a bit useless
The height-readings of your Phantom are calculated onboard by a barometer.
The altitude (= above or beneath sealevel) of the pictures taken with my Nikon D800 with GPS are wildly ridiculous sometimes. But also the height ( = relative towards the homepoint) of your Phantom is only approximative. Problem is that there is no technology to measure height more accurately.
The Airbus A 480 or the Boeing 777 use the same barometer. Different there is that a few meters higher or lower is not that important while flying at 30,000 Ft while a few meters difference for a Phantom can make the difference between crash or a nice flight.  When the airbus is landing other instruments and beacons are taking over to let the big 'real' aircrafts 'glide' to a soft landing on the ground, wind and weather permitting.
2017-2-17
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Antonio76
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PhanFran Posted at 2017-2-17 08:37
Altitude-data for the EXIF-data of your pictures is probably taken from the GPS which is VERY inaccurate. Putting the 'Height' data in the EXIF would be a bit useless
The height-readings of your Phantom are calculated onboard by a barometer.
The altitude (= above or beneath sealevel) of the pictures taken with my Nikon D800 with GPS are wildly ridiculous sometimes. But also the height ( = relative towards the homepoint) of your Phantom is only approximative. Problem is that there is no technology to measure height more accurately.

Thanks! So after all I don't have to worry nor trust too much the exif data. Just out of curiosity, however, I'll check the EXIFs from my Nikon P900, but I don't think they give altitude data.
More happy learning tomorrow :-)
2017-2-17
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Antonio76
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PhanFran Posted at 2017-2-17 08:37
Altitude-data for the EXIF-data of your pictures is probably taken from the GPS which is VERY inaccurate. Putting the 'Height' data in the EXIF would be a bit useless
The height-readings of your Phantom are calculated onboard by a barometer.
The altitude (= above or beneath sealevel) of the pictures taken with my Nikon D800 with GPS are wildly ridiculous sometimes. But also the height ( = relative towards the homepoint) of your Phantom is only approximative. Problem is that there is no technology to measure height more accurately.

OK, so I checked the EXIFs from my camera on a few pictures taken one after the other at the same location, with 14 satellites, and the altitude varied from 12 to 38 meters ASL.  Cohordinates also varied, although very slightly. Thanks again for the exhaustive answer :-)
2017-2-17
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Falun
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I would like the answer to the above question - is it 20 feet or 70 feet?
2019-12-6
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1Eagle
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Falun Posted at 12-6 10:14
I would like the answer to the above question - is it 20 feet or 70 feet?

The answer is the height displayed is relative to your home point. Therefore, no matter how high you are off the ground when you take off the height displayed is the height above home point.
Answer: Height displayed is 20ft.
So if you landed on the ground below the roof where you took off the height displayed would be -50 ft.
Cheers,
2019-12-6
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Mark The Droner
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My P3P does have an AMSL column in the log, so I'd imagine the other DJI drones do too.  It's seen in PhantomHelp's "verbose" csv log under the column OSD.altitude [m]. It's in meters and it's an estimate.  Granted, it's not seen in the app that I know of, but it's there in the log.  And I've found the DJI version is not very accurate.  It's a calculation based mostly on air pressure but I believe temperature and maybe humidity is involved?  The height above home point's column is adjacent to it and, of course, starts at zero.  My flytrex device also has an AMSL column in its log, and it's quite a bit more accurate than the DJI log's version.  Either column's running data can be displayed on a drone video using an overlay program such as Dashware.  Meters can be converted to feet with Dashware.  Or with Excel.  

Obviously the height above homepoint data will be much more accurate than the AMSL data since the former has an actual physical starting point.  
2019-12-6
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Falun
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I recently did a similar maneuver. It will reveal the home point altitude - which is zero plus height about the home point. The RC will reveal 20’ even if you flew over a chasm. What confuses me is the P4P has a barometer which measures pressure above sea level. So if you are at any altitude above sea level you should know your altitude because pressure changes with elevation. The pressure reading can be converted into feet above sea level. Learn the elevation of the highest features in your path and fly above them.
2019-12-11
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Labroides
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Falun Posted at 12-11 12:22
I recently did a similar maneuver. It will reveal the home point altitude - which is zero plus height about the home point. The RC will reveal 20’ even if you flew over a chasm. What confuses me is the P4P has a barometer which measures pressure above sea level. So if you are at any altitude above sea level you should know your altitude because pressure changes with elevation. The pressure reading can be converted into feet above sea level. Learn the elevation of the highest features in your path and fly above them.

What confuses me is the P4P has a barometer which measures pressure above sea level. So if you are at any altitude above sea level you should know your altitude because pressure changes with elevation. The pressure reading can be converted into feet above sea level.
Your assumption that atmospheric pressure measures height above sea level is what's confusing you.
It doesn't.
As well as changing with altitude, atmospheric pressure changes with the weather.
Try looking at a detailed weather report some time.

There's no way to relate barometric pressure to height above sea level.
2019-12-11
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