polygonqgisvector-grid
I used the research tool "Regular Points" to produce a certain number of points within a polygon, but it forms a rectangular grid of points rather than only within the shape of the polygon. Since I need a certain number of points within the polygon, is there any way to do so without trial and error?
E.g. right now only 8 of the 25 points I want are within the polygon, but I want 25 points, so I could increase to 50 points and see how many then appear within the polygon
Here is a set returning function ST_CreateFishnet that creates a 2D grid of polygon geometries:
CREATE OR REPLACE FUNCTION ST_CreateFishnet(
nrow integer, ncol integer,
xsize float8, ysize float8,
x0 float8 DEFAULT 0, y0 float8 DEFAULT 0,
OUT "row" integer, OUT col integer,
OUT geom geometry)
RETURNS SETOF record AS
$$
SELECT i + 1 AS row, j + 1 AS col, ST_Translate(cell, j * $3 + $5, i * $4 + $6) AS geom
FROM generate_series(0, $1 - 1) AS i,
generate_series(0, $2 - 1) AS j,
(
SELECT ('POLYGON((0 0, 0 '||$4||', '||$3||' '||$4||', '||$3||' 0,0 0))')::geometry AS cell
) AS foo;
$$ LANGUAGE sql IMMUTABLE STRICT;
where nrow and ncol are the number of rows and columns, xsize and ysize are the lengths of the cell size, and optional x0 and y0 are coordinates for the bottom-left corner.
The result is row and col numbers, starting from 1 at the bottom-left corner, and geom rectangular polygons for each cell. So for example:
SELECT *
FROM ST_CreateFishnet(4, 6, 10, 10) AS cells;
row | col | geom
-----+-----+--------------------------------
1 | 1 | 0103000000010000000500000000...
2 | 1 | 0103000000010000000500000000...
3 | 1 | 0103000000010000000500000000...
4 | 1 | 0103000000010000000500000000...
1 | 2 | 0103000000010000000500000000...
2 | 2 | 0103000000010000000500000000...
...
3 | 6 | 0103000000010000000500000000...
4 | 6 | 0103000000010000000500000000...
(24 rows)
Or to make a single geometry collection for the full grid:
SELECT ST_Collect(cells.geom)
FROM ST_CreateFishnet(4, 6, 10, 10) AS cells;
You can add the x0 / y0 origin offsets (these defaulted to zero).
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.
Best Answer
What QGIS is referring to in terms of the layer boundary is actually the envelope or bounding box of the geometry.
I've documented a solution which is probably the closest you will get to automating this is (short of filing a bug / feature request).
The process works based on the ratio between area of the two objects: the feature geometry and it's bounding box).
Check the following steps below:
Select the layer you want to work on.
Select the feature you wish to work on. Make sure nothing else is selected.
Run the following snippet within the Python Console.
Be sure to push enter as the last line will not automatically return.
Run the
Regular Pointstool again, but enter the number of points that you want inside the box multiplied by the output given by your script.In my sample below, the result I received from the output was a value of ~2. I requested 20 dots to be output by the process where I actually only wanted 10. I was given 8 in return, which is fairly close. Depending on the actual shape you're using the results will vary.
If you were so inclined, you could automate (recursively) the Regular Points process modifying the variables until you arrived at your desired number.