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GCP marking: Green cross position gets worser the more marks are placed?


I’m having some issues with the GCP marking. I have about 2000 images of a road and some bridges. Images are a mix of nadir and oblique photos. GCPs are spread along the road.

When marking 2-3 GCPs and selecting “automatic marking” the GCPs are detected and marked on most images that show the target. The automatically marked images are mainly the nadir ones plus some oblique ones that were shot with a steep camera angle. The position of the green and yellow crosses are typically very good, right in the middle of the GCP.

However, when checking the remaining images, I often notice that not all GCPs were detected and the green cross is some meters off. This is mainly happening on oblique images shot with a flatter camera angle. So I start to add manual GCP tie points on these images, and the green markers start to move towards the GCPs. Perfect!

Unfortunately, at the same time, the position of the green cross on the automatically marked images starts to move away from its previous (very good) position. Not much, maybe 10 - 50 cm.

What is your recommendation how to handle this?


I would manually improve the marks of the images that were automatically marked and that have their green cross that moved. 
Each yellow mark has a certain weight associated to it, which influences the computation of the green cross. This weight represents the confidence level that you have in your mark. This confidence level is represented by the yellow circle around the yellow cross (which represents the mark). The more you zoom in on the image before adding the mark, the smaller the circle and the higher the weight associated to this mark.  

It could be that the zoom level was not high enough on the images that were marked automatically, which could explain why these marks “lose importance”, when you add marks on the images with a flatter camera angle. 

Tip to mark the GCPs: when you hover over the images in the right sidebar of the rayCloud view, click the space bar. This will put the image full screen. This makes it easier to correctly mark the GCPs (and Manual Tie Points (MTPs)). Click the space bar again to remove the full screen. 

Let us know if that worked out, or if you have found another solution to solve the issue.