QGIS 3.16.11 shortest distance between point and line via field calculator and overlay_nearest

Question

I was searching for the shortest distance between a point (point layer with about 22000 points) and the nearest point of a line from another layer (MultiLineString). I tried Distance Matrix, nearest neighbour, distance to nearest hub. None with the expected result. But then I found something interesting on GIS SE, but only displaying the shortest line. (Find nearest line feature from point in QGIS by @babel) It goes:

make_line( 
    $geometry ,
        closest_point( 
            geometry(   get_feature_by_id ('bayern_25832gis_osm_waterways_free_1',
    array_to_string( overlay_nearest ( 'bayern_25832gis_osm_waterways_free_1', "osm_id" ) 
  )
   )
   ),
    $geometry ) ) 

It looked perfect, so I tried copying it in the field calculator by exchanging make_line by 'distance' and voila.

Could it be thus simple?

I am just a biologist and have really no clue about expressions, their functions and so on.

How can I verify my results?

By now I only looked at the data, labeled the distances and measured by hand but cannot do this for 22.000 features.

Answer 1

To get the length of the line, you must not replace make_line by distance, but add the function length to the rest of the expression, then use it in field calculator to calculate the length of the shortest line from between each point and the nearest point on a line.

Thus the whole expression will look something like what follows, where you have to replace the [...] part with the settings that work for you (your initial expression):

length ( make_line ( [...] ) )

Don't forget to add another closing bracket ) at the end when adding the length function.

Screenshot: the dotted lines are created with the same expression as the one you see here, just without the length() part. Be sure to apply this on the points layer and make the right settings: get thecorrect name of the line layer, set Output field type to a numerical format (integer or decimal) and check if the preview at the bottom shows the correct result:

Answer 2

The way @babel and @biologist showed did not work. Using get_feature instead of get_feature_by_id and aligning the reference field with the field referenced by overlay_nearest worked.

In @biologist's method, overlay_nearest returns the value of the "osm_id" field, which is probably a different value than the "id" that get_feature_by_id takes as an argument. babel's method uses "$id" as the expression for overlay_nearest, but does this not return the geometry ID of the reference geometry, rather than the geometry ID of the search target?

like this:

distance( $geometry,
    closest_point(
        geometry(
            get_feature('line_layer','osm_id',
                array_to_string( 
                    overlay_nearest( 'line_layer',  "osm_id"
                    )
                )
            )
        )
    , $geometry)
)