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2 Locations for 1 GCP?

I have marked 35 images for a single GCP location and applied them. The GCP shows up in an additional 7 images and the location is off by around 1ft/0.3m in those 7 images. (Attached is a screenshot of some of the marked and applied GCP locations next to some of the location discrepancy images). This is not the only GCP that is doing this and I am wondering why there is such a large discrepancy? I did fly at two different AGLs in this location because of the elevation change in the area. Any help or thoughts would be appreciated. Thanks!

Hi John

Could you share the quality report? It seems that pix4d is having difficulties trying to match images from different flights. What I have done in similar situations is to use “all prior” in internal parameters optimization in step 1

That kind of mismatch usually leads to multiple blocks whitch is not a good thing



Thanks very much for the insights.


I have attached the report for reference. Could you explain further about using “all prior” in internal parameters optimization in step 1? Will I have to reprocess step one for that?

If you want to use “all prior” you will need to process step 1 again:

Here you can see that pix4d hasn’t been able to align photos properly and has made 11 different blocks. That usually means that there is some kind of vertical or horizontal “shift” which can be seen when marking GCP’s

@John, I had a close look at the Quality Report and, overall, the project looks well reconstructed. The difference between the initial and optimized camera parameters is not significant and you can still process the project with the default settings (3D maps).

Since the project consists of multiple flights, it might happen that not all of them are properly linked to each other (horizontally and vertically). For example, I could see in the first screenshot that the GCP is visible in two different flights, DJI_iv and DJI_ii, and that they are not very well matched between each other. 

As Jaakko already pointed out that there are also several blocks created in the project that might cause the issue. Adding Manual Tie Points between the blocks and reoptimizing can connect the blocks together. For more information: 360000395103.

At this point, I would recommend marking the GCPs in additional images and adding more MTPs, especially if you see some misalignments like in the first screenshot. After marking the points the project needs to be re-optimized in order for the changes to be taken into account.