Maybe I was not clear enough. I created the arch with 2 circles and the plane in between them. Then i cut them with lines. And because i want to add some solids on top of the arch, which have to follow the form of the arch, i need the outer surface. But when i copy it, it has a gap to the arch, so the green line on picture 2 is the top edge of on of the stones, but still has a gap.
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