QGIS – Ordering Polygons from North to South

Question

In QGIS 3.10 I want to order 22 polygons north to south based on their position using their centroids. I have used the centroids tool to determine where their middle is. But now I'm not sure now how to automate figuring out their north to south order.

Answer 1

You can create a new field with this expression :

array_find( -- find the current feature id in the list
    array_reverse( -- reverse the order of the list, so greastest Y coordinate (= Northest) is first
        array_agg( -- create a list of features id ordered by Y coordinate of the centroid
            expression:=$id,
            order_by:=y(centroid($geometry))
        ),
        $id  -- current feature id
    )
) + 1  -- position starts from 0, so add 1

Using features ID instead of a list of Y coordinates is to eliminate the case when 2 features have exactly the same Y coordinate (even if it isn't very probable).

The expression y(centroid($geometry)) can be directly written as y($geometry) because of the definition of the y() function :

Returns the y coordinate of a point geometry, or the y coordinate of the centroid for a non-point geometry.