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Vertical coordinate issue Phantom 4PRO

Hey!

In my latest projects I having problems witd vertical coordinate. The recordet hight during the flight is very of. Causing huge problems in geo.reffering the point cloud. My GCP points are above the recorded EXIF photo hight. flying altidude 65 meter. 

Any quick fix here?

Use a geotagging software to edit the altitude in the EXIF…I have seen others with issues on DJI equipment.

Or you can export GPS-coordinates to a .cvs file. Edit the altitude values with find/replace function in Excel . Import the edited .cvs back to Pix4D. DJI should really do something about this known problem

Hi Kjetil,

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. We made testing in our office here and we found that the vertical coordinate is off by several meters that can reach an error of 100 meters. Some users noticed the same. Note that this is just an offset meaning that the within the model, the accuracy is not affected, only the absolute location is.

As a consequence, we always recommend to process 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. If you do not have GCPs, please refer to this procedure.

Hope it helps!

Hi Julie,

I am having an issue with vertical offset of my GCPs as well.  DJI Phantom 4 Pro user. However, the vertical offset was not enough that I couldn’t mark my GCPs in the ray cloud.

After marking the GCPs in the image, the vertical was corrected and the points are on the ground where they are supposed to be.

However, my Quality Check flagged the georeferencing for high error (RMS error = 0.912m), and my GSD is 2.4 cm.

Am I ok to continue steps 2 and 3 of the processing with no issues or should I somehow address the vertical offset differently before continuing?

Thanks!

Phantom 4 Pro doesn’t record the drones true GPS altitude properly.  It is heavily weighted toward the barometer’s elevation values. I’ve seen it differ greatly when flying the same site a week apart when the barometric pressure had changed.  

This doesn’t cause me any real issues now that I know what’s going on.  I can choose to export the image location values from pix4d to a .csv file and adjust the altitudes to what they should be, and reimport the image locations into pix4d. 

If you have GCPs like Julie mentioned above, you can let pix4d adjust the data to the GCPs. I normally just pick 3 GCPs and mark them in the basic editor and reoptimze.  This will shift the data into the right location.  I then mark any remaining GCP and checkpoints in the raycloud editor and reoptimize once more and regenerate my quality report. 

RMS values should be good if your GCP locations are accurately marked.  I typically see RMS values of .02’ - .04’ when using centimeter grade located GCPs. The elevation inaccuracy should not affect your RMS values.  Your RMS value indicates an inaccurately marked GCP or non survey grade accuracy of your GCP points. 

  

Two (easy) ways to deal with the DJI altitudes in the image exif - 

 

  1. Use the basic edit to mark  ground control points,  3 images each.  Then run “reoptimize”.  After reoptimizing, mark the rest of your control in the raycloud editor and continue as normal.  

 

  1. Use the “image properties editor” in Pix4d to change your image elevations to something ~200ft above your control elevations.  You can now mark them in the raycloud editor.  

 

*Number 2 does work fine without introducing any other issues, but I don’t like using method two because I think the potential exists for future software changes affecting that workflow.