Calculating distance between multiple points within buffer using QGIS

Question

I am very new to QGIS.

Attached is an image of my QGIS project so far. In this, you have the pinkish points of the "Point_centers" layer and green points labeled "Point_trees" which are tree species that I am interested in. What I want to do is calculate the distance of the Point_centers to all the Point_trees within the 15 meter buffer layer I created. I want it to calculate the distances regardless whether or not the Trees overlap in different buffers.

Answer 1

To get the distance from the center point to each tree inside the buffer, use this expression:

array_foreach (
    overlay_nearest( 
        'Point_trees', 
        $geometry, 
        max_distance:=15, 
        limit:=-1
    ),
    length (
        make_line (
            $geometry,
            @element
        )
    )
)

This generates an array of distances from the center to each tree inside the buffer. The get the sum of all distances, enclose the expression in an array_sum() function.

The expression calculates the lengths of the black lines. These lines here are created based on the same expression for visualization purpose:

Answer 2

To calculate the distances in a table:

1. First ensure that your buffers layer and trees layer have a field with a unique identifier. I used tree_id and buffer_id.

2. Perform a spatial join between the trees and the buffers using a one-to-many join type. This will give you a new points layer where points are duplicated when they are within more than one buffer.

Tree T23 now appears 3 times, in buffers 2, 3, and 4.

3. Then use this expression in the Field Calculator on the joined layer:

length(
    make_line(
        $geometry,      
        centroid(geometry(get_feature(                  -- get the centroid of the buffer that matches the buffer_id of the joined point. 
                              layer:='buffers', 
                              attribute:='buffer_id',   -- 'buffer_id' in single quotes is the field name
                              value:="buffer_id")))     -- "buffer_id" in double quotes is the field value of the current joined point
                                                                             
        
    )
)

Now each tree point has a distance to its corresonding buffer centroid:

Example:

(The lines are for visualisation purposes only)

Answer 3

You can use a virtual layer to compute the distance and optionally draw a line to the center point.

Go the the menu layer / add layer / add-edit virtual layer and enter the following query. You can add any field from the point or polygon layer.

select make_line(pt.geometry, st_centroid(poly.geometry)) as geometry, 
       st_distance(pt.geometry, st_centroid(poly.geometry)) as dist
from myPolygonLayer poly
join myPointLayer pt
 on st_intersects(pt.geometry,poly.geometry)

PS: See how accurate is my buffer polygon?! That's why it is usually better NOT to select point within a buffer, but rather to select points within a distance of another point.