Hello,

I have a Phantom 4 Advanced, being in used for the last 6 months. For the last couple of mission, the results are quite inaccurate in terms of the altitude it shows in the image captured.

For example - My MSL is 95 meters, AGL is 120 meters, the image captured should show up the altitude of 215 meters. Whereas, it shows an altitude of around 260 meters and even varying.

Now this change in altitude comes even after calibrating IMU as well as Compass.

This error makes my processing of the survey done totally wrong and flying it all over again does not solve the problem.

Coming to the question.....
1) How to solve this error of the altitude variation?
2) If it is not possible to eleminate this error, can anyone please tell me how to fix this issue before I go for Data Processing?

Thanks for your help.

That's just the nature of GPS altitude data.
It's commonly quite inaccurate and there's nothing you can do about it.
If you are calculating contour lines, they will be accurate but not tied in to the local datum.
If you use surveyed ground control points, you can make everything correct.

Labroides Posted at 3-6 04:40
That's just the nature of GPS altitude data.
It's commonly quite inaccurate and there's nothing you can do about it.
If you are calculating contour lines, they will be accurate but not tied in to the local datum.

Thank you so much for your quick response.

Yes, I see, its GPS Altitude and it is quite inaccurate.

Yes, that's what I face about Contour lines.

Yes. We use GCPs. So, how to make everything correct for surveys?

hardrone Posted at 3-6 04:56
Thank you so much for your quick response.

Yes, I see, its GPS Altitude and it is quite inaccurate.

If you are using Agisoft, the GCPs are used to correct your whole survey to match local datum.
The Agisoft Help explains how to do that.

For example - My MSL is 95 meters, AGL is 120 meters, the image captured should show up the altitude of 215 meters.  

I've read this 3x.  I still don't understand what you're saying.  

Mark The Droner Posted at 3-7 03:21
I've read this 3x.  I still don't understand what you're saying.

I am sorry. I do not know why this statement was not understandable.

Anyway, its just that, that my drone photos gives out Exif data with Above Sea Level - Altitude. We Fly a drone with respect to Above Ground Level. Now, the value of home location is taken as Above Mean Sea Level. So, Home location Altitude + The Altitude at which I fly my drone = MSL + AGL and which is = Above Sea Level.

MSL = 95meters (sea level to home location distance)
AGL = 120 meters (Home Location to Drone altitude distance)
So, AMSL = 215meters (From Sea level to Drone flying altitude)

Labroides Posted at 3-6 04:40
That's just the nature of GPS altitude data.
It's commonly quite inaccurate and there's nothing you can do about it.
If you are calculating contour lines, they will be accurate but not tied in to the local datum.

You're right that GPS-derived Z is an order of magnitude worse than XY, precision-wise. But unless you have RTK, drone altitude is not using GPS, its using barometric pressure (which is why it is AGL and not HAE; the sensor zeros at Home location and gives altitude relative to that).

As pointed out, OP can use his GCPs in Agisoft or Pix4D or wherever to tie his project to the desired coordinate system and datum.