Please select Into the mobile phone version | Continue to access the computer ver.
Phantom 4 Pro - exif altitude is not WGS84
5969 16 2018-6-12
Uploading and Loding Picture ...(0/1)
o(^-^)o
Chook
lvl.3
Flight distance : 999708 ft
Australia
Offline

Hi all,
On a phantom 4 Pro with the lastest firmware and current Go4 App I find that the altitude that it writes to the exif data in the images to be incorrect.  The height above ground is correct.  I guess that it takes an initial GPS altitude on statup and adds height above ground to determain altitude to write to exif.  The initial altitude should be based on WGS84 ellipsoid height.  In my recent experience the value is no where near this (and will even changes from day to day at the same location). On two recent flights I see values different by around 50m & 120m.


Has something changed in the way the aircraft determines GPS altitude?   What is the abolute altitude based on?


2018-6-12
Use props
Geebax
Captain
Australia
Offline

GPS altitude itself is the issue, it is very inaccurate. The 'height above ground' may appear to be so, but it is not. It is the height above the takeoff point, and if you fly some distance away, it has no knowledge of where the ground is under you.
2018-6-12
Use props
Chook
lvl.3
Flight distance : 999708 ft
Australia
Offline

Geebax Posted at 2018-6-12 19:20
GPS altitude itself is the issue, it is very inaccurate. The 'height above ground' may appear to be so, but it is not. It is the height above the takeoff point, and if you fly some distance away, it has no knowledge of where the ground is under you.

Thanks. Yes, of course the telemetry reports the height above take off point.  The issue is the value it uses for the altitude of the take off point.
2018-6-12
Use props
Labroides
Captain
Flight distance : 9991457 ft
  • >>>
Australia
Offline

Chook Posted at 2018-6-12 19:34
Thanks. Yes, of course the telemetry reports the height above take off point.  The issue is the value it uses for the altitude of the take off point.

GPS is woefully inaccurate for altitude and swings significantly over short time periods.
Try taking a photo close to the sea and see what the exif shows.
Errors of +/- 200 ft and more are common.
It's not just the Phantom, you get similar results with a handheld GPS unit.

2018-6-13
Use props
James Mitchell
lvl.1
Flight distance : 41673 ft

United Kingdom
Offline

Thank you.

So, the GPS height in the image is the height the drone is above ground or the height the drone is above the homepoint?
Does the DJI GO 4 app show the height as the height above the homepoint?

Thank you
2018-6-13
Use props
Labroides
Captain
Flight distance : 9991457 ft
  • >>>
Australia
Offline

James Mitchell Posted at 2018-6-13 06:11
Thank you.

So, the GPS height in the image is the height the drone is above ground or the height the drone is above the homepoint?

The height you see in the app is from barometer.
It's the height above the level of the home point.
The height shown in exif data for each image is from GPS and is nominally the height above sea level but It's horribly inaccurate because GPS is bad for altitude.
2018-6-13
Use props
Oniman
New
Russia
Offline

different by around 50m & 120m
2018-9-14
Use props
solentlife
Captain
Flight distance : 1087530 ft
Latvia
Offline

Altitude by GPS with regard to a DJI unit has always led to comments of its inaccuracies. It is further compounded by the Flight Controller comparing barometer to GPS and if significantly different can initiate ATTI mode in some cases or error warning on screen.

Why DJI use a less accurate GPs system than most others, I do not understand. Others such as Soloshot use the GPS signal of remote tag back to automatic camera base to track vertically and horizontally and it does a remarkable job as long as the tag on the item tracked is not moving fast and erratically.

Here's a good example of it tracking :



The only time the tracking actually fails is when the tag and base are within 20m of each in a horizontal plane - that's because then the GPS coordinates are too close to each other in terms of separation.
But you can see that GPS can in fact provide reasonably accurate height info for this unit to track.

But I agree - DJI units altitude assessment via GPS is woefully poor...... and lets be honest - no camera is going to have super accurate GPS data acquisition ... so exif info is only really good for coordinates.

Nigel  
2018-9-15
Use props
Manxmann
Captain
Flight distance : 1178793 ft
Australia
Offline

Chook Posted at 2018-6-12 19:34
Thanks. Yes, of course the telemetry reports the height above take off point.  The issue is the value it uses for the altitude of the take off point.

Surely the take off point altitude is zero ??
2018-9-16
Use props
Labroides
Captain
Flight distance : 9991457 ft
  • >>>
Australia
Offline

Manxmann Posted at 2018-9-16 01:54
Surely the take off point altitude is zero ??

He was talking about the altitude value shown in the exif data that is part of each image file.
This comes from GPS while the altitude shown in the app comes from the barometer.
The numbers can be very different as GPS gives a nominal altitude above sea level but with an error of +/- 200 ft or more.
2018-9-16
Use props
Manxmann
Captain
Flight distance : 1178793 ft
Australia
Offline

Ooops,  sorry Labroides .......  that's all above my pay grade !
2018-9-17
Use props
Danimal0017
lvl.1
Flight distance : 2045164 ft
United States
Offline

I am having the same issue, Through the app my hight typically is fine. Though last week I had issues of this keeping proper height on a sloped yard. I was taking surveying shots for a client at different heights and just from flying 25ft in 5-foot increments, The measurements were off by 10ft (+or-). Just wildly inconsistent. I was required to land and take off to recalibrate the "home" altitude for every photo. On top of this, my altitude embedded in my files are all negative...... (see photo attached)
How can this be? After takeoff, the photos all show this very odd pattern of height.

Height Seen on screen (+/-)          ~15ft            ~ 20ft          ~ 25ft          ~30ft           ~35ft         ~ 40ft
Embedded data Height              -95.14m        -93.54m      -88.37m     -86.97m     -85.37m     -84.37m
converted into ft                           -312ft            -306ft         -289ft          -285ft          -280ft       -276ft

So my metadata is stating that my P4P2.0 is a submarine??????

I guess the only good thing out of this is I can create a formula to covert the offset but shouldn't the number displayed on my screen be sent to my metadata?
This just makes for extra work in the field

However, it looks like there are no true fixes yet. I just wanted to share my findings.


2018-9-19
Use props
Geebax
Captain
Australia
Offline

Danimal0017 Posted at 2018-9-19 18:02
I am having the same issue, Through the app my hight typically is fine. Though last week I had issues of this keeping proper height on a sloped yard. I was taking surveying shots for a client at different heights and just from flying 25ft in 5-foot increments, The measurements were off by 10ft (+or-). Just wildly inconsistent. I was required to land and take off to recalibrate the "home" altitude for every photo. On top of this, my altitude embedded in my files are all negative...... (see photo attached)
How can this be? After takeoff, the photos all show this very odd pattern of height.

It is hard to tell from your explanation, but you said you were working in a sloping yard. If you took off at the top of the slope, then flew down the slope, maintaining your height above the ground, then yes, the displayed height will be negative, as you will be below the launch point. The launch point is always zero.
2018-9-19
Use props
Danimal0017
lvl.1
Flight distance : 2045164 ft
United States
Offline

Geebax Posted at 2018-9-19 18:08
It is hard to tell from your explanation, but you said you were working in a sloping yard. If you took off at the top of the slope, then flew down the slope, maintaining your height above the ground, then yes, the displayed height will be negative, as you will be below the launch point. The launch point is always zero.

That's not the point here. Even when I maintained the height and flew towards the water my elevation did not change. just the height above ground level. If the altitude is determined from my home point. Then I should be able to crash the drone into the ground if I fly straight into the upward slope with a reading of 5ft.
There just was a discrepancy in the height reading. I moved downslope (lower altitude) and was hovering just over the ground (again at a lower point from my home / take-off point). The screen was indicating I was 5ft above my take-off point when I was clearly 3-4ft lower. That is not my underlining issue.

The main issue is the negative altitude reading in my metadata. What is up with that?
2018-9-19
Use props
Labroides
Captain
Flight distance : 9991457 ft
  • >>>
Australia
Offline

Danimal0017 Posted at 2018-9-19 19:25
That's not the point here. Even when I maintained the height and flew towards the water my elevation did not change. just the height above ground level. If the altitude is determined from my home point. Then I should be able to crash the drone into the ground if I fly straight into the upward slope with a reading of 5ft.
There just was a discrepancy in the height reading. I moved downslope (lower altitude) and was hovering just over the ground (again at a lower point from my home / take-off point). The screen was indicating I was 5ft above my take-off point when I was clearly 3-4ft lower. That is not my underlining issue.

The main issue is the negative altitude reading in my metadata. What is up with that?
Read post #4 & #6 above for the explanation

2018-9-19
Use props
Richpasq
United States
Offline

I was asked to fly drone above a home and get the altitude so that they can determine the views for a potential roof top deck. I did not realize the exif data would be so innaccurate. Is there a way to calculate the correct altitude using the exif data? The controller said about 24 feet above ground. The exif data says 58.1? Any help would be appreciated?
2021-7-24
Use props
Labroides
Captain
Flight distance : 9991457 ft
  • >>>
Australia
Offline

Richpasq Posted at 7-24 10:29
I was asked to fly drone above a home and get the altitude so that they can determine the views for a potential roof top deck. I did not realize the exif data would be so innaccurate. Is there a way to calculate the correct altitude using the exif data? The controller said about 24 feet above ground. The exif data says 58.1? Any help would be appreciated?

Accurate height (above your launch point) will be recorded in your exif data.
But exif data contains two different heights.
One will be the height you saw on your screen.
Post the original image here and I'll find both heights for you.

btw ... because the drone has a very wideangle lens, the view it would show from 24 ft above the ground would look almost identical to the view from the ground.

2021-7-24
Use props
Advanced
You need to log in before you can reply Login | Register now

Credit Rules