Generating lines from one polygon to group of polygons (one-to-many network) using QGIS

Question

I have a polygon layer where every polygon has a unique "ID", and another polygon layer where those unique "ID"s can be found in a column in a one-to-many relation.

I would like to have QGIS draw lines from the polygons in the first layer to the polygons in the second layer in a way that reflects the one to many relation.

Any advice on tools?

This sounds like a spider diagram. You might start by looking at http://gis.stackexchange.com/questions/2823/ and its Related questions on the right side of the page. — Chris W, Nov 09 '15 at 17:58

score 1 · Answer 1 · answered Oct 23 '20 at 09:19

Let's assume there are two polygon layer "poly_unique"(orange) and "poly_groups"(green), with its corresponding attribute tables accordingly, see image below.

Proceed with Plugins > Python Console > Show Editor and paste the script below

# function that converts a string into a list
def to_list(var):
    output = var.split(",") if isinstance(var,str) else ['']
    return output
function that converts an integer into a string
def to_string(var):
    output = str(var) if isinstance(var,int) else ''
    return output
accessing two layers with polygon features by their names
poly_from = QgsProject.instance().mapLayersByName("poly_unique")[0]
poly_to = QgsProject.instance().mapLayersByName("poly_groups")[0]
defining attributes that establish relations between two layers
attr_unique = 'id' # attribute with unique ID, exists in both polygon layers
attr_group = 'Connect' # attribute that shows the one-to-many relations
creating a virtual output layer with line strings
line_layer = QgsVectorLayer("LineString?crs={}&index=yes".format(poly_from.crs().authid()), "Spider", "memory")
adding fields for the virtual output layer
provider = line_layer.dataProvider()
provider.addAttributes([QgsField("from", QVariant.String), QgsField("to", QVariant.String)])
line_layer.updateFields()
looping over each feature from two layers and making all possible connections between them
for f1 in poly_from.getFeatures(): # looping over original polygons
    for f2 in poly_to.getFeatures(): # looping over destination polygons
        if to_string(f1[attr_unique]) in to_list(f2[attr_group]): # condition checks if unique id is in a column with relations
            connect = [f1.geometry().centroid().asPoint(), f2.geometry().centroid().asPoint()] # connection between two points
            line = QgsGeometry.fromPolylineXY(connect) # creating a line string from connection
            f = QgsFeature() # creating Qgs feature
            f.setGeometry(line) # setting new geometry as a line string
            f.setAttributes([f1[attr_unique], f2[attr_unique]]) # setting new attributes as from and to
            provider.addFeature(f) # adding Qgis feature to the output layer
adding the virtual output layer to the map
QgsProject.instance().addMapLayer(line_layer)

Press Run script and get the output that will look like

References:

Generating lines from one polygon to group of polygons (one-to-many network) using QGIS

1 Answers1

function that converts an integer into a string

accessing two layers with polygon features by their names

defining attributes that establish relations between two layers

creating a virtual output layer with line strings

adding fields for the virtual output layer

looping over each feature from two layers and making all possible connections between them

adding the virtual output layer to the map