How to count polygons in multipart features?

Question

I'm trying to count the number of polygons in a multipart feature. My aim is to have a 'count' attribute to quantify fragmentation of multipart land parcels.

I've looked at the field calculator and for suitable plugins, but can't see anything that would help. There are lots of ways to count points, lines and vertices, but not polygons.

Answer 1

4 years later it seems as if QGIS has integrated this feature into the field calculator. The expression syntax is:

num_geometries($geometry)

Answer 2

I know that you're looking for an answer that uses QGIS but if you're willing to use the open-source GIS Whitebox Geospatial Analysis Tools instead, then here is a complete answer that will take any shapefile, and create a new attribute field in its table that contains the number of feature parts. You simply need to open the Scripter window in Whitebox, change the script language to Groovy, and paste the following code in:

import whitebox.geospatialfiles.ShapeFile
import whitebox.geospatialfiles.shapefile.*
import whitebox.geospatialfiles.shapefile.attributes.*

def inputFileName = "/MY_DIRECTORY/MY_FILE_NAME.shp"
def shape = new ShapeFile(inputFileName)
AttributeTable table = shape.getAttributeTable()

DBFField field = new DBFField()
field.setName("NUM_PARTS")
field.setDataType(DBFField.DBFDataType.NUMERIC)
field.setFieldLength(5)
field.setDecimalCount(0)
table.addField(field)
for (int i = 0; i < shape.getNumberOfRecords(); i++) {
    ShapeFileRecord record = shape.getRecord(i)
    int numParts = record.getGeometry().getParts().length
    recData = table.getRecord(i)
    recData[recData.length - 1] = new Double(numParts)
    table.updateRecord(i, recData)
}

println("I'm done")

You'll need to update the value of the inputFileName variable near the top to point it to your file. You could also use Python to achieve the same thing, but I wrote this in Groovy. If you like, I can also have the script exclude hole parts if you are working with polygons.

Here's an example of a world country map that has been rendered to show the number of parts each country's polygon feature includes:

Answer 3

If QGIS is the only tool available The following python code in the Python Console might do the trick:

layer = iface.activeLayer()
for x in layer.getFeatures():
    geometry = x.geometry().asMultiPolygon()
    print len(geometry)

Of course this is not a complete solution, it will just print a bunch of numbers without indicating which feature they pertain to, but it's a start. You could have it also print the 'NAME' field or some other preferred key attribute. Or you could modify to write the len(geometry) integer to some other field in the layer.

Answer 4

Another non-QGIS solution is to use OpenJUMP. It has a special tool in Tools - Stastistics - Feature Stastistics. The tool creates a new layer with these attributes:

• nPts (number of vertices)
• nHoles (number of holes)
• nComponents (number of components)
• area
• length
• type (type ot geometry)

If you aim is to count the number of polygons in multipolygons this tool should do the job fine. However, if you want to count how many polygons there are in GeometryCollections which contain also other sort of geometries you must explode the collections first.

Answer 5

You can try to dissociate multi polygons into unique polygon and then open the attribute table and watch the number of lines (which represents the number of polygons).

To separate multipolygon : Vector->Geometry Tools->Multipart to single parts

Answer 6

This information is now in the information pop up. If you right click the layer and select properties, the window that pops up shows the feature count.