I am using QGIS 2.18.13.

I have a polygon layer with building footprints, and a point layer with a given number of points. I would like to draw lines (randomly) between two points and see how many buildings the line crosses. How I can do that with QGIS?

  • I am not sure its random, but would this work for what you are after as a starting point: gis.stackexchange.com/questions/92751/…… If you are able create the linking column as a list of unique random numbers its worth a try. I gave it a shot, but it needs a further step to remove duplicates. Got to get back to work :).
    – ZrSiO4
    Oct 31, 2017 at 18:41

1 Answer 1


One approach is through Virtual Layers / SpatiaLite syntax.

An example with 12 building and three points.

enter image description here

For illustration purpose add connecting lines between points with the Virtual Layer SQL:

select p1.id as id1, p2.id as id2, MakeLine(p1.geometry, p2.geometry) as geometryline
from point p1
cross join point p2
where p1.id <> p2.id

Use the Add / Edit Virtual Layer button:

enter image description here

In the QGIS Create a virtual layer dialog it looks like:

enter image description here

It will give you:

enter image description here

Add another virtual layer doing the complete query. Remember to rename the layer so you won't overwrite the first virtual layer:

    crossinglines(geometry) as (
        select distinct MakeLine(p1.geometry, p2.geometry) as geometry
        from point p1
        cross join point p2
    select distinct b.geometry 
    from building b
    inner join crossinglines cl on st_crosses(cl.geometry, b.geometry)

In the layer control context menu Show feature count for the last virtual layer - showing the number of buildings with a line crossing.

enter image description here

The lines of the first SQL are doubled. This is handled in the second SQL.

Notice that the virtual layers are dynamic to their base layers. Adding a point will recalculate the virtual layers when they are refreshed with the QGIS refresh button.

enter image description here

enter image description here


Now for adding the building id to the building being crossed add a new virtual layer with the following SQL.

select v2.geometry, b.osm_id
from virtual_layer2 v2
inner join building b on st_equals(b.geometry, v2.geometry)

Where virtual_layer2 is you joined building layer that crosses the lines. Change the column name osm_id to your building id.

enter image description here

  • Thank youuuuuu a lot for your answer.. Now I can display the selected buildings an dcount the number of buildings between 2 points. Is it possible to display the Id of buildings as well ? Nov 2, 2017 at 17:24
  • I updated the answer to include the building id.
    – Jakob
    Nov 7, 2017 at 8:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.