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GCPs Slightly Off in Pix4Dmapper

Hello,

I am processing a small project with 4 GCPs I created in GIS using existing features on the ground. After I mark the GCPs and run step one my GCP accuracy error is roughly 1m. When I go to edit the GCPs before reoptimization my GCPs appear to be shifted in different directions. I have attached images showing how the software determined GCP is inconsistently shifting.

I have double checked the accuracy of my points in GIS and they look good.

Specifics:

  1. Imagery was gathered with a DJI Phantom 4

  2. Image coordinate system is WGS 84

  3. GCP coordinate system is WGS/Pseudo-Mercator

  4. Output coordinate system is WGS/Pseudo-Mercator

Screenshot 2021-11-05 134148
Screenshot 2021-11-05 134207
Screenshot 2021-11-05 134226

Hi,

May I ask if in the quality report the RMS error is bigger than expected? If not I would not worry. Have in mind that different elements are displayed on the images depending on the object / point that is selected.

  • For Manual Tie Points / GCPs :
    • Green cross : Projection of the 3D point computed from marks of the selected Manual Tie Point or GCP.
    • Yellow cross : Position where the selected Manual Tie Point or GCP has been marked.
    • Yellow circle : Its radius indicates the zoom level at which the selected Manual Tie Point or GCP has been marked. It defines the area in which the point has to be reprojected when estimating the 3D position. The higher the zoom level, the more influence the mark has on the reconstruction.
    • Blue circle : The position corresponds to the projection of the input 3D coordinate of the selected GCP / Check Point.

For more:

I hope this helps.

Best,

To clarify, the blue circle represents what coordinates are of the input GCPs from the .csv?

Hi,

The blue circle represents the projection of the coordinates (from .csv) of the selected GCP before marking the GCPs and before processing. Does this make things clearer?

Best,

That makes it a little more clear for me. So if I mark the images with the green X in the spot that I believe is the true GCP location the error in my report is the difference between the green X and blue circle?

I am still trying to understand why the blue circles shift after I reoptimize the project.

Hi,

The yellow cross is the position where you marked the selected GCP.
The green cross is the computed position of the GCP.
The blue circle changes position after marking the GCP and reoptimizing as the reprojection is changing.
The Theoretical Error S(X,Y,Z) and Error to GCP Initial Position are computed after Step1 and they give a good estimation of the accuracy of the block.

The Theoretical Error S(X,Y,Z) informs us about the relative accuracy whereas the Error to GCP Initial Position informs us about the absolute accuracy .

For more: https://support.pix4d.com/hc/en-us/articles/202559139-What-is-the-difference-between-theoretical-error-and-the-error-to-GCP-initial-position

Best,