QGIS Select by expression - buffer selection

Question

I want to select points 2 and 3 with a 'selection with expression' in QGIS 3.4. I want to do this by using the buffer function in the expression builder.

I already managed to create a separate buffer and then selecting points that are overlapped by the buffers.

I expect that the expression will select points 2 & 3. The expression buffer( $geometry, 5000) is not giving the result i expect.

Answer 1

Thanks to the suggestion by @she_weeds, I found a much better solution. This method can be performed entirely in the Select by Expression tool, without creating any additional layers.

Use the collect function to combine the church points into a multipoint geometry. Use this multipoint geometry in the intersects function. Because we're using collect on a different layer, we have to put it inside the aggregate() function, like so:

intersects($geometry, buffer(aggregate('churches','collect',$geometry),5000))

Answer 2

After posting my previous answer, I found a way to do this selection without creating any additional layers.

1. Install the RefFunctions plugin. This adds several new functions to the expression builder, including geomdistance().

2. Use the geomdistance() function to select points in the parking lot layer that are within 5000 distance units of any point in the church layer. See example expression below. In the example, the church point layer is called 'churches'. It has a field called 'field', where all the values in that field are 1.

   geomdistance('churches','field',5000)=1
   

Notes:

• You can use a field already in your church point layer, as long as every feature has a value, and you can write a true/false expression that is true for every feature. Eg, if you have a numerical field where every feature has a value greater than 0, write the expression as geomdistance('churches','yourfield',5000)>1. For some reason, it doesn't work with the criteria is not null.

• The distance is calculated using the project settings for distance calculations.

Answer 3

Unlike the Buffer tool, the buffer function in the expression builder can only accept a single feature as input. It can't buffer an entire layer. So you'll have to merge all of your church points into a single multipoint feature before you can do this process using the select by expression tool.

1. Use the Dissolve tool (processing toolbox) to merge the church points into a new layer with a single multipoint feature.

2. Assuming that layer is called 'church_points_dissolved', it has an attribute field 'ID', and the only feature in that layer has 'ID' = 0, you can use this expression in the Select by Expression tool to select parking lot points that fall within a specified buffer distance of a church point:

   intersects($geometry, geometry(get_feature('church_points_dissolved','ID',0)))
   

Note: This method is at least as much work as simply creating a separate buffer layer of the church points, and using the Select by Location tool to select parking lot points that intersect the buffer layer.