Drawing lines to nearest points in QGIS

Question

I have two point layers, one with telecom towers and another with secondary schools. I have used the NNJoin Plugin to find the nearest tower from each school, and calculated the distance between. The result is in a vector layer, with separate columns for the tower and school names.

How can one draw lines between the pairs of points?

Answer 1

This should be possible without any plugin using the default Processing tools, particularly Distance to nearest hub:

https://docs.qgis.org/testing/en/docs/user_manual/processing_algs/qgis/vectoranalysis.html#qgisdistancetonearesthublinetohub

Answer 2

There is a possibility using a "Virtual Layer" through Layer > Add Layer > Add/Edit Virtual Layer...

Let's assume we have eight features in 'school' (blue squares) and five in 'tower' (orange circles) accordingly, see image below.

With the following Query, it is possible to draw lines between the pairs of points.

SELECT s.Name AS SName, t.Name AS TName,
       make_line(s.geometry, t.geometry),
       ROUND(MIN(ST_Distance(s.geometry, t.geometry)),2) AS distance,
       s.id || '_' || t.id AS uniqueid
FROM school AS s, tower AS t
GROUP BY s.Name
ORDER BY distance DESC

The output Virtual Layer will generate shortest lines between schools and towers including the following attributes "school name", "closest tower name", "distance to the closest tower" and "unique id".

---

References:

• Finding point closest to point on near feature?
• PostGIS nearest points with ST_Distance, kNN

Answer 3

Nowadays, exactly since QGIS version 3.16, there is another possibility: use the overlay functions of QGIS expressions, either with Geometry Generator or Geometry by expression. Use this expression:

make_line (
    $geometry,
    overlay_nearest (
        'towers',
        $geometry
    )
)

Screenshot: red= schools, white=towers; the black lines (arrows) are created with the expression from above:

Answer 4

There are several ways to accomplish this. I used the following method.

1. Use the NN_join plugin to find the shortest distance between a school and a tower.
2. Use the vector/geometry tools/add geometry attributes to add coordinate values for the point pairs to the file.
3. Export as .csv, and open in Excel. Sort the file and add a row number as "ID".
4. Split the file in two, one with school points and another with the associated tower. Keep the sorting sequence, and add row number as "ID" in the second file.
5. Now search for the following plugin https://plugins.qgis.org/plugins/connect_points/
   Note: This plugin is not updated for QGIS 3.x, so I used QGIS 2.8 for this step.
6. Import the schools and towers layers and run the plugin.

The result is a shp-layer with lines from each school to the nearest tower.