Combining point and polygons layers using QGIS

Question

I'm currently trying to compare species sample points vs their actual geographic range.

I have:

• a points layer with species names and each species has multiple corresponding points/records
• a polygon layer with the same species names showing the area each occupies

To avoid too many clicks from switching and toggling between vectors, I'd like to combine these layers so that when I select the species name, it shows both the points and polygon in one layer (e.g. third attached image).

Any suggestions?

Answer 1

You can use pyqgis to 'link' the check state of category legend nodes for categorised layers by catching the styleChanged signal.

Copy the script below and paste it into an editor in the QGIS Python console. With this approach there are two layer name variables which need to match the names of your point and polygon layers. I used your layer names based on your comments below; just double check that they match. Click the Run button to connect the styleChanged signal of both your point and polygon layers to a slot function which will sync the visibility check state of the category child nodes.

from functools import partial

# The two lines below are the only ones you may need to edit    
lyr1_name = 'Occurrences'# This needs to match your points layer name
lyr2_name = 'IUCNRange'# This needs to match your polygon layer name

project = QgsProject.instance()
lyr1 = project.mapLayersByName(lyr1_name)[0]
lyr2 = project.mapLayersByName(lyr2_name)[0]

tree_model = iface.layerTreeView().layerTreeModel()

def on_change(v_layer):
    ltl = project.layerTreeRoot().findLayer(v_layer.id())
    lln = tree_model.layerLegendNodes(ltl)
    states = dict()
    for node in lln:
        sp_name = node.data(Qt.DisplayRole)
        check_state = node.data(Qt.CheckStateRole)
        states[sp_name] = check_state
    vector_layers = [l for l in QgsProject.instance().mapLayers().values() if l.type() == QgsMapLayer.VectorLayer]
    for layer in vector_layers:
        if layer.renderer().type() == 'categorizedSymbol':
            if not layer.id() == v_layer.id():
                ltl2 = project.layerTreeRoot().findLayer(layer.id())
                lln2 = tree_model.layerLegendNodes(ltl2)
                for n in lln2:
                    n_name = n.data(Qt.DisplayRole)
                    if n_name in states:
                        if n.data(Qt.CheckStateRole) != states[n_name]:
                            n.setData(states[n_name], Qt.CheckStateRole)
                    
conn1 = lyr1.styleChanged.connect(partial(on_change, lyr1))
conn2 = lyr2.styleChanged.connect(partial(on_change, lyr2))

See the short screencast below for expected results:

To disconnect the signal/slot connections, run the two lines below (you can simply run them in a different editor (see end of screencast above).

QObject.disconnect(conn1)
QObject.disconnect(conn2)

For reference I found a couple of similar questions/answers here and here. My answer incorporates some of the code presented therein.

Answer 2

A layer can only contain one type of geometry: either points or polygons (or lines), but not both.

However, using Geometry generators, you can create symbolization (visualization, not actual geometries) of another geometry type.

Let's say you have a polygon layer named poly with an attribute polygon_species and a point layer point with an attribute point_species.

On the polygon layer, add a new symbol layer of type Geometry generator / Geometry type: point and insert this expression. When you then (manually) select a polygon, all the points containing the same attribute value are created as additional symbols on the polygon layer (see screenshot):

with_variable (
    'selected',
    attribute('polygon_species'),
    if (
        is_selected( ),
        aggregate( 
            'points',
            'collect',
            $geometry,
            filter:=point_species=@selected
        ),
        ''
    )
)

Polygon layer: blue, initial point layer: light blue dots; selected polygon (B) highlighted in yellow, points with the same attribute (B) highlighted in big red dots (style can be changed manually as you like. If you toggle visibility of the point layer, only the red B_points will be visible (as long as polygon B is selected):