Merging polygons only if three or more are on top of each other in QGIS

Question

I am working on a layer with many polygons (circles). I would like to create a polygon containing only the areas where 3 or more circles overlap (in my example there would be only 1 area in the east). Each circle has a specific "ID".

I don't know if QGIS allows such specific conditions.

Answer 1

You can use QGIS expressions with either Geometry generator or Geometry by expression (see here for details and differences between these two options). Use this expression with Geometry generator (see below for Geometry by expression) and replace @layer with the name of your polygon layer:

with_variable (
    'poly',
    array_foreach (
        case
        when 
            array_length(
                overlay_intersects(
                    @layer, 
                    $geometry, limit:=-1
                )
            ) = 2
        then
            overlay_intersects(
                @layer, 
                $geometry, limit:=-1
            )
        end,
        intersection (
            $geometry,
            @element
        )
    ),
    intersection (
        array_get (@poly,0),
        array_get (@poly,1)
    )
)

If you use the expression with Geometry by expression, use this expression, where you have to replace poly in lines 8 and 14 with the name of your polygon layer:

with_variable (
    'poly',
    array_foreach (
        case
        when 
            array_length(
                overlay_intersects(
                    'poly', 
                    $geometry, limit:=-1
                )
            ) = 3
        then
            overlay_intersects(
                'poly', 
                $geometry, limit:=-1
            )
        end,
        intersection (
            $geometry,
            @element
        )
    ),
    intersection (
        array_get (@poly,1),
        array_get (@poly,2)
    )
)

Red hached area created with Geometry generator:

Answer 2

Let's assume there is a polygon layer called 'test' with six features in it, see image below.

Solution #1. Getting the feature with three and more overlaps

This solution was partially derived from @johns's comment

Step 1. Perform an intersection by means of the SAGA's "Polygon Self-Intersection" geoalgorithm

Step 2. Use the "Extract by expression" geoalgorithm with the following expression:

array_length(string_to_array("ID", delimiter:='|')) >= 3

and get the output

Solution #2. Dissolving only features that contain a feature with three and more overlaps

Step 1. Create a field "test" with this expression:

array_sum(overlay_intersects(@layer, 1))

Step 2. Use the the GDAL's "Dissolve" (GDAL > Vector geoprocessing > Dissolve) from the Processing Toolbox (Ctrl+Alt+T) with field "test" from the Step 1, and do not forget to tick the Produce one feature for each geometry in any kind of geometry collection in the source file.

Press Run and get the output

---

References:

• Dissolving overlapping polygons without common attribute in QGIS

Answer 3

So, if I understand your question correctly, follow me.

My initial data is a layer called "buf" type - multipolygons;

1. Polygons in simple polygons - Vector->Processing geometry->Split composite objects with saving layer "buf_parts", see figure 1;

Figure 1

2. Next, polygons into lines, see Figure 2;

Figure 2

3. Then analysis tools->Vector-Overlay->Cut with lines, see figure 3;

Figure 3

Result

Figure 4

4. Then create centroids for each of the split parts and save the result, named as "center_parts", see the result shown in figure 4;

Figure 4

5. Next, Layer->Create Layer->New Virtual Layer, import the results of steps 1 and 4 and run the SQL script, see figure 5 (it will help you count the number of points that fall into the composite circles);

Figure 5

SELECT count(*) cnt, b.geometry from "center_parts" a JOIN "buf_parts" b ON st_intersects(a.geometry, b.geometry) group by b.geometry 

6. Next, run the following SQL-script on the "virtual_layer", with saving the result in the layer "buf_sel" , see figure 6 (in this case, we work with copies of points, based on the number of intersections);

Figure 6

SELECT cnt, geometry from "buf_sel" WHERE cnt>='8' 

7. Next, on the layer "buf_sel" run the SQL-script, which combines the circles in common polygons and this is your final result, be sure to save it and then act yourself :-).

Figure 7

SELECT cnt, st_union(geometry) geom FROM "buf_sel"

Result

Figure 8

Original spatial solutions...

Answer 4

1. Convert the polygons to lines with Polygon to lines.

2. Split the polygons with the lines from 1: Split with lines.

3. Create a Buffer with a very small, negative distance from the output of step 2.

4. Check within how many of the initial polygons the output geometries of step 3 are: create a new field called within using Field calculator with this expression (replace poly with the name of the initial polygon layer): array_length (overlay_within ('poly', $id)).

5. Go back to the layer created in step 2. Use Select by expression with the following expression: array_first (overlay_contains ('Buffered', "within")) >=3 (adapt the number for how many overlapping polygons you want to have).

Now the parts that have 3 or more overlaps are selected - you can copy and paste them to a new layer. Don't forget that you have duplicate geometries: use Delete duplicate geometries.

Area where 3 or more polygons overlap (hatched in pink):

Answer 5

Using the Union Geoprocessing tool is probably the fastest way:

1. Run Menu Vector / Geoprocessing Tools / Union

2. On the output of step 1, use Select by expression with this expression to select those parts where 3 or more polygon overlap: array_length (overlay_equals(@layer, $id))+1 >=3

Blue circles = output of Union. The labels are created by the expression above (minus the >=3 part) and shows how many polygons overlap in each part. Highlighted in yellow: polygons selected as described in step 2: