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Orthomosaic Accuracy

I’m working on a conceptual redesign of an intersection, and we usually use orthomosaics to set our alignments and LiDAR data that is available to us statewide for a DTM. The orthomosaics available to us statewide are pretty low quality, so I would like to start using orthomosaics created from Pix4D on all of our projects that require preliminary design. I flew a 63 acre area at 375’ with a Mavic Pro, and the orthomosaics created were very high quality. I used 5 GCPs spread around the edges of the project that were spaced 500’-800’ apart. My issue is that when I compare common points on the Pix4D generated orthomosaic to the orthomosaics available to us statewide there is a 3’-10’ discrepancy. Should I be trusting the Pix4D generated orthomosaic? Is there anything wrong with the process from which I generated the orthomosaic that I described above? I trust the accuracy of all GCPs used and they were all present in at least 15 images.

Could you share the quality report?

5 GCP’s in 63 acre (~25ha area) is too low in my opinion. Few days back I did a job 30ha job and used 43 GCP’s and 15 of the as checkpoints. That gave me enough information to verify the correctness of my otrho and DTM. 

https://support.pix4d.com/hc/en-us/articles/115000140963-Difference-between-a-ground-control-point-and-a-checkpoint

https://hmbpe-my.sharepoint.com/:b:/p/jmeyer/EUyrm6GRhItInY1V29wYGtIBnE8DAZw_XE-0lcxZ-J6hSg?e=oXdFlX 

Here is the quality report for the aforementioned project. I worked with Brad on the project and actually processed the data on my machine.

Can I asked what you used for GCP’s on your project? Setting 43 CP’s with targets would take quite a long time. 

You were flying Mavic Pro and it has rolling shutter. Try to run the project again with rolling shutter effect corrected

https://support.pix4d.com/hc/en-us/articles/208460436-How-to-correct-for-the-Rolling-Shutter-Effect

I used spray paint to mark GCP’s and RTK-GPS