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Can't Match GCP Marker with Images. GCPs Necessary?

I am looking to obtain the volume of a stock tank. In the field, I took coordinates from 8 GCPs with an RTK unit and took 761 drone images with a Phantom 4 Pro from varying heights and gimble pitch angles. When I import my GCPs into the project, I can not match them with their corresponding images (at least not if I try to match them as imported). If I manually lower the z value of every GCP by 60 meters (in this case) I can make all the necessary matches. Why do I have to lower the GCP marker elevations to make matches and does doing so ruin my eventual volume calculations? Is this because I am running flights at multiple heights? Is it just from inaccuracy in the drone vs the RTK unit?

I am also wondering if GCPs are even necessary for calculating volumes for these stock tanks. The only information I am trying to gather from these stock tanks is the volume, their coordinates and exact height above sea level isn’t important. If the elevations calculated from the drone are coming out so drastically different from the GCPs (over 60 meters in this case) then would I be better off just going with the relative elevations that come from the drone? OR, as long as I don’t need the exact elevation, does using this method of fixing the GCP z values still work better than not using GCPs?

The following image shows the issue. I had already processed this imagery using my “decrease the z value of the GCPs by 60 meters” method but removed those GCPs and imported the original ones. You can see that I am highlighted on a GCP named “sims tank 7” and only those 3 images are showing up for me to match with.

Any help on this issue would be greatly appreciated!

Hi @vsutton,

I believe our article on Vertical shift between the model and the GCPs / Cannot mark the GCPs in the rayCloud will help you understand the origin of the issue and help you fix it.

Let me know how it goes.

Best!