Integrate grid and polygon of municipalities keeping the biggest municipality per cell with QGIS (or another)

Question

I have a 9x9 grid and a polygon of municipalities of an extensive area of Brazil. My goal is to have a field indicating the biggest municipality per grid cell.

I've been trying with intercept without good results. I know there are similar questions but I can't figure it out how to handle it in QGIS.

Here is a screenshot of what I'm talking about:

I was thinking that maybe with intersection and then spatial union joining with maximum but it didn't work.

I want to get the final grid with a field indicating the biggest municipality inside each grid cell.

Answer 1

Here's another way of doing it, I couldn't take my mind off of this thing and at the office I made some time to solve this; these are the steps I took in Kosmo:

1.- Create the grid 10x10 km and concatenate Row and Column into a key field

2.- Intersect the grid with the map of municipalities the new map is called IntGridMun

3.- Recalculate AREA for IntGridMun

4.- Sort the shapefile IntGridMun using sortshp (FWTools) "sortshp IntGridMun IntGridMunAREADesc AREA descending" this will create a new shapefile IntGridMunAREADesc

5.- Generate stats for IntGridMunAREADesc grouping by Row and Column key field and obtaining the First municipality name

6.- Join the stats table with the original grid and you should have your map

Hope this helps

Juan José Del Toro

Answer 2

Here's a possible database solution. If you import both the municipalities and the grid polygons into Spatialite, for example, and each polygon spatial table has an "id" column, then a query like this:

SELECT m.municipal_id
FROM muinicpalities AS m JOIN grid AS g ON ST_Intersects(m.geometry, g.geometry)
GROUP BY g.grid_id
ORDER BY ST_AREA(ST_Intersection(m.geometry, g.geometry) DESC LIMIT 1;

should give you the id of the municipality with the largest intersection within each grid cell. Now you can use that to update the grid table as follows:

ALTER TABLE grid ADD COLUMN municipal_id INTEGER;

UPDATE grid SET municipal_id=(
SELECT m.municipal_id
FROM municpalities AS m JOIN grid AS g ON ST_Intersects(m.geometry, g.geometry)
WHERE g.grid_id=grid.grid_id
ORDER BY ST_AREA(ST_Intersection(m.geometry, g.geometry) DESC LIMIT 1;
);

HTH