QGIS – Numbering Polygons from North-East to South-West

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I'm working with polygon outputs of an algorithm, where each polygon has a (seemingly) random ID (that is due to some internal logic of the algorithm).

I'd like to assign spatially meaningful IDs:
A unique ID for each polygon that is based on the location of each polygon within the dataset, going from North to South and East to West (i.e. the Northeastern-most polygon gets the ID 1, the next in Southwest-direction gets ID 2, 3, … and so forth).

Is there a tool in QGIS for "spatially sensitive sequential numbering"?

I have some ideas how to do it "programmatically" (e.g. extract coordinates of each polygon extent (upper left corner) and sort by these coordinates, then do a sequential numbering), but I'm hoping there are built-in functions for this…

Best Answer

You can create a Virtual Layer with the query:

SELECT ROW_NUMBER() OVER( ORDER BY ST_MAXY(geometry) DESC, ST_MAXX(geometry) DESC) AS id, *
FROM patch

Which is numbering the rows in the order of maxy -> miny and maxx -> minx using ROW_NUMBER function. The output will be temp so you need to save it to file.

You can also try order by centroid x/y coordinates.

Related Solutions

QGIS – Ordering Polygons from North to South

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.

QGIS – How to Convert South and West Coordinates to Align with X/East and Y/North

You can do this without using the georeference tool as you have the information to write your own world file. From wikipedia the world file structure is as follows

Line 1: A: pixel size in the x-direction in map units/pixel
Line 2: D: rotation about y-axis
Line 3: B: rotation about x-axis
Line 4: E: pixel size in the y-direction in map units, almost always negative[3]
Line 5: C: x-coordinate of the center of the upper left pixel
Line 6: F: y-coordinate of the center of the upper left pixel

You should be able to find the the size in pixels and therefore calculate how big each pixel is. From your coordinates it looks like there is no rotation so 0. The last two lines require a bit more maths as you have to calculate the centre of a pixel.

World files should be saved with the same name as the image but a different extension such as .wld.

Here is an example

0.50000000
0.00000000
0.00000000
-0.50000000
438007.16150000
193255.10440000

Best Answer

Related Solutions

QGIS – Ordering Polygons from North to South

QGIS – How to Convert South and West Coordinates to Align with X/East and Y/North

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