Export one point with shared origin value in attribute table

Question

I am trying to find a way to export unique points from a pointlayer in QGIS. Using the tool ‘Extract vertices’, I generated corner points from polygons in the input layer (first image). I now only need one point per polygon to remain in the pointlayer. The ‘Extract vertices’ tool conveniently generates a column in the attribute table containing the vertex_part_ring value, portraying from what vertex the point is originated from (second image). I am looking for a way to (automatically) export one point from a shared vertex_part_ring series.

Below is the polygon the points are originated from. (Sample project)

Origin polygon

Below is a screen capture of the attribute table as a result of the extract vertices tool. The column ‘vertex_part_ring’ gives the origin of the point. I need to keep only one point from each origin series.

Extract vertices result

• Erik: One point per polygon, or one point per ring?
• Gabriel: It's not clear if the points even need to be on the ring boundaries. If not, it would probably be easier to polygonize them then make centroids.
• Jonathan: @Erik I would like to keep one point per ring
• Jonathan: @GabrielC. I need to keep one point per ring boundary.
• Stu Smith: Is your input polygon layer singlepart or multipart?

Answer 1

You could use the Extract by expression tool on your vertex layer with the following expression:

"vertex_part_index" = 1 AND "vertex_part_ring" <> 0

This will extract all rings' first vertex except for the main polygon perimeter.