Creating regularly spaced points inside polygon based on attribute value using QGIS?

Question

I have a polygon layer with an attribute value attached representing the count of individuals inside a polygon. I'm using QGIS 3.0.

I need to visualize the counts as a grid of points inside a polygon (i.e. 1 point = 100 frogs, colored by specie). The points do not necessarily need to occupy entire polygon, and preferably be drawn around a centroid with pre-defined spacing (i.e. 300 m), so that they can represent a neat grid at a specific resolution.

Current workaround I found is to use "Regular points", with counts parameter, to create grids inside extent drawn on a map, then delete the extra points created by algorithm (algorithm rounds to a grid and you might get 20 points instead of 17 in input). This produces desired result, but requires drawing the extent of area for regular points for each polygon, as well as manual attribute input and clean up of points created. Furthermore, since I'm running "Regular Points" with count specified, instead of spacing, each polygon gets differently spaced points.

In a nutshell: Regular points (defined # of points, drawn extent) for each polygon (batch) > Delete extra points > assign attribute values to the # points

Alternative (avoids the irregular spacing, but requires even more manipulations):

Regular points (defined spacing i.e. 300m) in the extent of the entire layer > Clip to the polygons extent > Delete extra points in each polygon until you get desired # of points > assign attribute values to points.

The main problem with my workarounds is the polygon-by-polygon processing, which makes it hard to update the data and work with larger number of polygons.

Analogous question was asked as Creating regularly spaced, defined # of points within polygon in QGIS, but specifics are different.

Answer 1

May be a other way inspiring by the answer of Sarath SRK :

1. Centroid of polygons
2. Buffer employing an expression with the square and to adapt with your scale : i.e (1000*(sqrt("Frogs"+"Cats"+"Diplodocs" )/2))+1 and Segment=1, End cap style=Square
3. New Grid point with a spacing coherent with the scale of the buffer.
4. Delete point that don't intersect with the buffer layer (select by location)
5. join attribut by location (Attribut of the buffer or the original polygons)

Yet, you will have something like this

6. Use à graphical modeler to create a new field with the commande "@row_number"

7. Execute the model with an iteration on the buffer layer

8. Merge all new layer

9. Delete all the point with an ID_pt field bigger than the sum of your field ("Frogs"+"Cats"+"Diplodocs" )

10. Categorise with rules (i.e.
    • "ID_pt" <= "Frogs"
    • "ID_pt" > "Frogs" and "ID_pt" <= ("Cats" + "Frogs" )
    • "ID_pt" > ("Cats" +"Frogs") and "ID_pt" <= ("Cats" + "Frogs"+ "Diplodocs" )

Answer 2

QGIS 3 comes with a new displacement method in the cluster renderer called grid. Looks like this is pretty much what you need. Just create the number of desired points at the centroid of your polygon.

I am not aware of a method to generate the points only with a gui tool, but a relatively simple python script should do that.

with edit(point_layer):
    for polygon_feature in polygon_layer.getFeatures():
        point_feature = QgsFeature(point_layer.fields())
        point_feature.setGeometry(polygon_feature.geometry().centroid())

        point_feature['type'] = 'Frog'
        for i in range(polygon_feature['Frogs']):
            point_layer.addFeature(point_feature)

        point_feature['type'] = 'Cat'
        for i in range(polygon_feature['Cats']):
            point_layer.addFeature(point_feature)

        point_feature['type'] = 'Diplodoc'
        for i in range(polygon_feature['Diplodocs']):
            point_layer.addFeature(point_feature)

If you want to go crazy, wrap that in a custom processing algorithm.

Answer 3

You could use geometry generators for this.

1. Add 3 sublayers in the symbol, one for each color/attribute.
2. Create an expression that generates a multipolygon for each sublayer.
3. Now the hard part, generate square parts for the multipolygon based on the attribute. I guess this will require a custom python expression function.
4. Optional, use the rendering variables for map-scale to determine the scaling of the polygons and spacing.

Answer 4

One solution could be to create a point layer with points at the centroid of each polygon. So for exemple you got at the centroid of Edmonton 25 superposed point : 5 with a frog attribute, 20 point with cat and 0 with diplodocs.

Then using the point displacement renderer with the Placement method set as grid you should be able to get the effect.

The difficulty is to create the point layer....

Answer 5

A very simple alternative might be to use the Pie chart available in the layer's properties ? Attributes and sizes can be dictated by an expression.

Answer 6

You could generate the points on a spiral path starting from the polygon centroid. The following Stack Overflow post has python code to create equidistant points but I can't test it at the moment given my lack of free time.

Answer 7

In your case,

1. Create regularly spaced points using given distance for entire AOI
2. Create centroid points for all polygons(input).
3. Create buffer for that centroid points using Fixed distance buffer with segments value '1'. It will create square buffer instead of circle(Use rotate tool in Qgis if you want to rotate buffer polygons).
4. Create a selection of points falls inside buffer polygons using vector -> Research tools -> Select by location (points within polygons rule).
5. Switch selection and delete points outside buffer polygons. So now you got regularly spaced points around centroid of polygons.
6. Give Majority attribute for whole points and the rest of the points need to be classified manually.Use QuickMultiAttributeEdit tool to update attributes easily.