[GIS] point thinning/point selection based on distance threshold in QGIS/GRASS

grassqgissaga

I need to thin a point vector later based on the distances amongst points. So I have a threshold and wish to omit all records but one that is below a distance D representing a distance between points. Say D points are tree locations D is the canopy radius and I want to select/extract/remove all trees within the canopy of another tree.

I have tried 2 approaches unsuccessfully:

Distance matrix (QGIS) and I can select points below D but can only remove all or none, but I need to have a single remaining one.
Buffering the points with D and applying the Select Within Plugin to select polygons with their centroid within other polygons returns a selection of all polygon because centroids fall inside the other polygons.

How do I remove all records but the first that are closer than D?

Best Answer

The function Point thinning of SAGA plugin for QGIS does this based on a distance threshold as desired. A similar question explains this here

Related Solutions

QGIS – Building Queries to Select Specific Points Within a Buffer

This can be done using the Processing Graphical Modeler to set up a custom chain. Here's a pic:

From left to right we have three inputs - the tree layer (a Vector Layer Input Object), the buffer size (a Numeric Input) and the tree height (a String Input). The tree layer is then selected by its height attribute. The selected trees are then buffered by the value of the buffer_width input - this is saved as a layer treebuff (remove that output if you don't need it, I just used it for checking). Then the Select By Location process sets the selection to those trees inside the buffer region (set the first input to the trees, the second to the buffer, and the predicate to "contains")

Here's the resulting output:

Large trees are drawn large, and you can see the buffer zone and the selection now includes the large trees and their neighbours within the buffer.

QGIS – Cropping Voronoi Polygons to Specific Distance from Input Points

Quick solution / principles

See below for variation with dynamic distance buffer

Create voronoi polygons from your points (as you already did).
Create a buffer around your points and be sure to check the dissolve option.
Intersect buffer with polygon layer.

Voronoi polygons (blue) of the white points; red: dissolved buffer around the points:

Green: voronoi polygons intersected with the buffers - everything outside of the buffer layer is clipped from the voronoi layer:

Variations

Make buffer with a dynamic distance, based on the density of the points (based on the nearest neighboring point). In the buffer tool, for Distance use data driven override and insert this expression (you can modify it to vary the buffer size even further):

length (
    make_line (
        $geometry, 
        array_first (
            overlay_nearest (
                'points',
                $geometry
))))

On the buffer layer, run Multipart to single parts before intersection.
After intersection, on the output layer check if there are any polygons that do not contain points (this is the case where the buffer of a points partially falls inside the voronoi polygon of another point - see screenshot). To select these "empty" poylgons, use Select by expression with this expression, where points is the name of the layer containing the points: overlay_contains( 'points') = false. Delete the selected polygons.

"Empty" polygons, highlighted in yellow; red: buffer with dynamic size, black lines: boundary of the voronoi polygons:

Green: resulting voronoi polygons, clipped with a buffer with dynamic distance:

Result, showing the whole result: voronoi-polygons clipped to a dynamic distance around the points, based on the density of the points (the closer points are, the smaller the voronoi polygons). Of course, there are no overlaps between polygons, but sometimes gaps between them:

Best Answer

Related Solutions

QGIS – Building Queries to Select Specific Points Within a Buffer

QGIS – Cropping Voronoi Polygons to Specific Distance from Input Points

Related Question