Find the NW vertex of a polygon

Question

I am using QGIS 2.18 and I am wondering how to find the NW corner of a polygon and create an attribute of that point number. See the attached image.

For example, please node 8 is the NW corner for Grid #1, 13 is the NW corner for Grid 5 and so on. any help with this would be wonderful. The two files I am working with is

Grid = 'grid'

and

Vertex = 'nodes'

Answer 1

QGIS 2.18.20

(Latest QGIS versions have issues with Virtual Layer)

Add a Virtual Layer with below syntax:

SELECT grid.*, nodes.id
FROM   grid, nodes
WHERE  st_touches(grid.geometry, nodes.geometry) = 1                  -- (1)
       AND
       st_x(nodes.geometry)-st_x(st_centroid(grid.geometry)) < 0      -- (2)
       AND
       st_y(nodes.geometry)-st_y(st_centroid(grid.geometry)) > 0      -- (3)

Footnotes:

The idea is really simple. As you need northwest corner, the x coordinate at the vertex is less, while the y is greater, than the centroid of the polygon.

(1) This helps to choose touching four corner points around each polygon.

(2) x-difference (dx) between the anticipated node and the centroid becomes negative.

(3) y-difference (dy) between the anticipated node and the centroid becomes positive.

So, if you want to find NE corner, (2) should be >0.