I have been using Pix4D since 2012 with the original Sensefly Swinglet, moved on to the Ebee and now use the DJI Phantom 4 Pro (P4P) to make survey accurate contour maps. My experience in this field numbers in the thousands of projects flown and mapped.
I have come across an issue lately processing jobs with the P4P where the Quality report will show all green arrows, the GCP residuals will be 20-50mm but in the ray cloud editor you can visually see that the 3D GCP are not passing through the point cloud surface, and 3D check points in the middle of the data set are up to 500mm below the point cloud. Even the residuals for the check points report they are within 20mm but clearly are not.
This is further proven with comparison to conventional ground survey using the same control, and comparing the TIN surface with Pix4D cloud surface and find the surface ‘bends’ from the ground control through the middle and back again. Where the GCP check within 20-200mm of the point cloud (but report shows 20mm) and the check points are withing 500mm (but the report shows 20mm).
Clearly this is not supposed to occur, there should be a flag or error somewhere but on the surface it is a successful project, but to a qualified surveyor like myself it fails the simple test of simply not agreeing with independent checks. This is at least the 10th time I have seen this happen, in the past I have manually edited the cloud in a crude manor to make it fit for purpose in our survey software, or re-flown the site with the exact same specification and it has worked. Not an ideal solution.
I would like to upload and share the result in hoping to uncover the issue that systematically appears. Below is a screen shot of what Pix4D consider within 20mm