QGIS MultiPolygon – Combining Multiple Polygons into One MultiPolygon in QGIS

multi-polygonmultipartpolygonqgisqgis-processing

QGIS 3.34.3 has a processing function "Multipart to singleparts". But what about the other way around? What is the easiest way to create a MultiPolygon from several polygons?

Best Answer

"Collect geometries" processing algorithm is what you are looking for.

Takes a vector layer and collects its geometries into new multipart geometries.

One or more attributes can be specified to collect only geometries belonging to the same class (having the same value for the specified attributes), alternatively all geometries can be collected.

All output geometries will be converted to multi geometries, even those with just a single part. This algorithm does not dissolve overlapping geometries - they will be collected together without modifying the shape of each geometry part.

Related Solutions

QGIS – How to Convert Huge Multipolygon to Polygons Using Python

Shapefiles have no type MultiPolygon (type = Polygon), but they support them anyway (all rings are stored in one polygon = list of polygons, look at GDAL: ESRI Shapefile)

It is easier with Fiona and Shapely:

import fiona
from shapely.geometry import shape, mapping

# open the original MultiPolygon file
with fiona.open('multipolygons.shp') as source:
    # create the new file: the driver and crs are the same
    # for the schema the geometry type is "Polygon" instead
    output_schema = dict(source.schema)  # make an independant copy
    output_schema['geometry'] = "Polygon"

    with fiona.open('output.shp', 'w', 
                    driver=source.driver,
                    crs=source.crs,
                    schema=output_schema) as output:

        # read the input file
        for multi in source:

           # extract each Polygon feature
           for poly in shape(multi['geometry']):

              # write the Polygon feature
              output.write({
                  'properties': multi['properties'],
                  'geometry': mapping(poly)
              })

Convert Multiple Polygons and MultiPolygons into Single MultiPolygon in GeoJSON

You could do this using Javascript Topology Suite which will work with Node.js. Start with an empty MultiPolygon (or the first geometry in your collection) and union this with each (Multi)Polygon in your collection. You can only have one format for the whole collection, obviously, as properties are one to one with the geometry in the GeoJSON. Here are some jsts/node.js examples.

There is also a GeoJSON parser in jsts, which will return a jsts geometry. So putting it all together, you would do something along the lines of:

var jsts = require("jsts");

var reader = new jsts.io.GeoJSONParser();

//read your geometries
var geoms = reader.read(geojson);

//grab the first one
var multipolygon = geoms[0];

//union with all the others
for (var x=1; x < geoms.length ; x++){
    multipolygon = multipolygon.union(geoms[x]);
}

This is untested, but the general idea should work. You would have to recreate the GeoJSON with properties afterwards, but this should be easy with the built in JSON methods in Javascript.

Best Answer

Related Solutions

QGIS – How to Convert Huge Multipolygon to Polygons Using Python

Convert Multiple Polygons and MultiPolygons into Single MultiPolygon in GeoJSON

Related Question