Divide grid in smaller grid QGIS 3.16

Question

I have a grid of 1000x1000m (which covers a whole department) and I would like to redivide this same grid again but to 500x500 m, so to obtain in a 1000mx1000m cell, 4 small 500m cells.

I would have this :

To do this, I recreated a grid based on the initial 1000m grid. Except that I get a shift in relation to this grid whereas I want there to be an overlap.

This is the result (in purple: initial grid: 1000mx1000m and in red: result: grid 500x500m). We can see the geographical shift between the 2 grids...

I tried another solution using the "Polygon Divider" tool but without success because I always have shifts.

Do you know what is the problem ?

Answer 1

As the initial layer you have creates many problems due to the fact that it was created in a geographic CRS and is thus not really apt to be subdivided based on a fixed distance, this is a completely different approach. You can split the existing cells in the middle, in this way avoiding the different CRS units.

You can download the grid that splits each of the initial cells in the middle in four equal parts here: filename subdivide.gpkg.

How to implement it

To avoid problems with different CRS, proceed everything in EPSG:4326: the same CRS the initial layer is in.

1. Create a line from the centroid of each cell to the centroid of each neighboring cell (9 totally: 8 neighbors + 1: the current cell). Use this expression with geometry by expression:

collect_geometries(
    array_foreach (
        overlay_nearest( 
            'MGRS_1km_31T_unprojected', 
            centroid($geometry),
            limit:=9
        ),
        make_line (
            centroid($geometry),
            @element)))

2. Run Menu Vector / Geometry Tools / Multipart to singleparts

3. Run Menu Processing / Toolbox / Delete duplicate geometries

4. We want to keep only horizontal and vertical lines and delete diagonales. Thus we select all lines that are not within a certain tolerance (5 degrees) of 0/90/180/270/360 degrees. Use Select by expression with this expression:

   90-(degrees (azimuth(start_point ($geometry),end_point ($geometry)))%90)>5

   Delete the selected lines.

5. Extend the remaining lines so that they cross the whole cell. Use this expression (in EPSG:4326): extend ($geometry,0.01,0.01)

6. Apply Menu Processing / Toolbox / Split with lines, set your initial grid as input and the lines from step 5 as Split layers

7. The resulting layer contains a lot of tiny sliver polygons we want to delete. Use select by expression with this expression: area (transform( $geometry, 'EPSG:4326', 'EPSG:2154'))<100000 and delete the selected polygons.

You now have the initial cells (blue) with the smaller cells in red:

Answer 2

There are at least two options - to create the new grid correctly (1) or to shift the grid that does not fit (2).

1. Be sure to select the same CRS and to set the same extent: best use the grid-layer already created for the extent of the new grid, see screenshot:

2. Select the red grid in your screenshot, select all features, enable snapping, toggle editing and use the Move feature tool to place (snap) an angle of the red grid to the purple grid.

Edit

As you mentioned in your comment, both solutions did not work for you. Based on the data you provided, I can see that you mixed up different CRS: the department is in EPSG:2154, the 1000 m grid in EPSG:4326. This cannot work if you than want to fit a 500m grid in EPSG:2154.

When I tested it with creating a grid with the extent of your department-polygon in the same CRS, all went fine. I created a 1000m- and a fitting 500m-grid covering the the department-polygon - find the project and all layers for download here.

To reproduce it yourself:

Create a new 1000m grid with extent of the department and EPSG:2154.

Than use select by expression: grid / disjoint / department and delete the selected cells to keep only those that overlap the department:

Than create the 500 m grid with the extent of the 1000m grid:

Now, both grid fit perfectly - black: 1000m grid, red: 500 m grid:

Finally, to join attributes from the old grid to the new one, use Menu Processing / Toolbox / Join attributes by position and be sure to set Join type to Take attributes of the feature with largest overlap only (one-to-one) - see screenshot for details. However, be aware that some attributes might not be valid any more (those refering to the geometry like length, area, coordinate values etc.).

Screenshot with attributes from the old grid layer (blue) joined to the new one (red), illustrated with the field OBJECTID: