GCP's heights in coordinate systems

PIX4Dmapper PIX4Dmapper Questions/Troubleshooting
1

Hi all,

Novice user, just starting out with Pix4d for surveys of stockpiles and general contour surveys.

I am using a dji phantom 4 pro for the photography and Leica RTK GPS for the ground control.

Every project I have undertaken to date, the GCP’s are in the correct horizontal location, but incorrect in the vertical (see pic below).

I am using the autodetect, WGS84 coordinate system, for the pictures.

I am selecting the same for the GCP’s.

I thought it might be a geoid separation, but entering this does not seem to make any difference.

Where am I going wrong please?

2

Hi Adam.

 

You’re doing nothing wrong. The GPS on phantom 4 pro isn’t reliable at all. The altitude it marks on images varies alot. I’ve done multi-battery flight and got different altitude on images in second flight. 

What you can do now is to use basic editor to mark few GCP’s and after that reoptimize the project. It should correct the horizontal shift.

3

Hi Jaakko.

Thanks.

How do I reoptimise after editing a few of the GCP’s ?

Surely the basic GPS in the Phantom 4 pro isn’t 70 odd metres out ? its got to be a coordinate system issue. ?

 

4

Hey Adam,

Re-optimization is under the  Process tab then Reotimize. Yeah GPS in Phantom 4 shouldn’t be off 70 meters maybe couple meters. 

5

Hi there

 

I’ve flown with Phantom 4 pro at the same location multiple times and got almost 50m altitude differences in exif-files (measuring stockpiles). I’m pretty sure that the ground hasn’t shifted up :slight_smile: The vertical accuracy of GPS in DJI drones is horrible

These are taken from Mavic pro. Same day. Almost same location and drone on the ground.

6

Hi everyone,

The Z value that is recorded by DJI equipment is not coming from the onboard GPS, but is actually a height above take off based on barometer readings if I am not mistaken.

7

 

Thank you Selim and Jaakko.  reoptimisation sorted out the problem.

 

8

Hello all,

@Adam: I am glad that the issue is resolved now!

As it was mentioned, the images that are saved on the drone’s SD card are geotagged by DJI. Lat./Long. coordinates are reliable in the image EXIF, however, there might be some inaccuracies for the altitude depending on the location where you are mapping.

The vertical coordinate can be off by several meters that can reach an error of 100 meters. Note that this is just an offset meaning that within the model, the accuracy is not affected, only the absolute location is. 

As a consequence, we always recommend processing with ground control points (GCPs) in order to fix these uncertainties. If you have issues to mark the GCPs in the rayCloud because of vertical shift, click here. 

Regards,