Giving specific naming of points within quadrant in QGIS

Question

In QGIS 3.18.1 I am doing sampling to estimate deer densities.

I would like to know which tool I can use to specifically name points within quadrants. There are four points in a quadrant, each point must have a specific name. South-West point = 1, North-West point = 2, North-East point = 3, South-East point = 4.

The name of the point will then be made up of the name of the quadrant and the name of the point.

Example for point name in quadrant: 440_82_(1); 440_82_(2); 440_82_(3); 440_82_(4)

Here is a sample design for our data.

Answer 1

You can create a virtual layer. The query will calculate the azimuth from each polygons centroid to all intersecting points. Replace "centroids" and "grid" with the names of your layers:

select pnt.*, 
case 
    when degrees(azimuth(centroid(poly.geometry), pnt.geometry)) between 40 and 50 then 3
    when degrees(azimuth(centroid(poly.geometry), pnt.geometry)) between 125 and 145 then 4
    when degrees(azimuth(centroid(poly.geometry), pnt.geometry)) between 215 and 235 then 1
    when degrees(azimuth(centroid(poly.geometry), pnt.geometry)) between 305 and 325 then 2
    else 999
end as 'bearing'
from centroids as pnt
join
grid as poly
on st_intersects(pnt.geometry, poly.geometry)

Answer 2

Check for each point the azimuth (angle clockwise from north) of a line from the centroid of the grid cell and allow for a certain tolerance - like in this visualization: which of the four red wedge-buffers contains the point?

Use this expression to create the numbers from 1 to 4 depending on the quadtant. The expression calculates the azimuth (direction) from the centroid of the grid cell to the point and assignes values 1 to 4 as desired.

Then combine this result with the name of the grid cell using concat() function or pipes ||:

with_variable(
    'azimuth',
    degrees(
        azimuth (
            centroid(
                overlay_within(
                    'grid',
                    $geometry
                )[0]
            ),
            $geometry
        )
    ),
    case
        when @azimuth > 10 and @azimuth < 80 then 3
        when @azimuth > 100 and @azimuth < 170 then 4
        when @azimuth > 190 and @azimuth < 260 then 1
        when @azimuth > 280 and @azimuth < 370 then 2
    end
)

For demonstration purpose, the expression used as dynamic label to create numbers from 1 to 4 to label the points: