QGIS – Creating Connection Lines Between 2 Layers with Geometry Generator

arrayexpressiongeometry-generatorqgis

I would like to know if there is any way to generate a connecting line between 2 points layers (blocks-points representing a watershed, concpoints-points representing the concentration location of the flow from that watershed) through the geometry generator in the style panel of QGIS, with the specification that in some cases there is a relationship of several basins concentrated in a single point.

In these specific cases, the attribute "ID_UC" that indicates which basins contribute to a concentration point is a string that separates the basin ID's ("ID_QE" attribute of the layer blocks) using a comma, as in the following image:

As you can see in the following image, I was able to accomplish the task for simple cases where there is a ratio of 1 basin to 1 concentration point, using the expression

make_line(
   start_point($geometry),
   geometry(
       get_feature(
           'concpoints',
           'ID_UC',
           "ID_QE" 
       )
   )
)

I imagine that there is a way within the geometry generator to transform the string field "ID_UC" into an array, where each element would be the ID of a different basin, and thus be able to make this connection between the blocks layer and the concpoints layer, or another way to generate these connection lines.

Download example project and layers

Best Answer

You can do this using array_foreach() function

collect_geometries(array_foreach(string_to_array( "ID_UC" ),

Use this expression on the 'concpoints' layer.

collect_geometries(array_foreach(string_to_array( "ID_UC" ),
make_line(
    start_point($geometry),
    geometry(
        get_feature(
                   'blocks',
            'ID_QE',
            @element
        )
    )
)))

Prerequisite

For this solution to work, the point layer must have a field (in this example called buffer_id) that contains the id of the joined buffer polygon. The solution was implemented in QGIS 3.30.3.

Label values

The values for the labels are generated with this expression:

to_int(                                           -- optional cast to integer
    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:='id',   -- 'id' in single quotes is the `id` field name in the buffers layer
                        value:="buffer_id"        -- "buffer_id" in double quotes is the field value of the **current point** in the joined layer
                    )
                )
            )                                                                        
        )
    )
)

The expression finds the feature in the buffers layer where the buffer_id matches the joined field in the points layer, makes a line from the buffer centroid to the current point and gets the length of that line.

The expression is entered by clicking the button next to the Value text edit. The mode is changed to Offset from point and the lower center quadrant is selected.

Label placement

The labels are placed at the midpoint of the connecting lines using this expression:

centroid(                                 -- get the centroid (mid-point) of the line joining the point with the centroid of its associated buffer
    make_line(
        $geometry,      
        centroid(
            geometry(
                get_feature(                  
                    layer:='buffers', 
                    attribute:='id',      -- 'id' in single quotes is the `id` field name in the buffers layer
                    value:="buffer_id"
                )
            )
        )                                                                                
    )
)

The expression is entered by checking Geometry Generator and clicking on the ... button next to the text edit area. Point / MultiPoint is chosen as the geometry type.

Label rotation

The labels are rotated parallel to the line using this expression:

main_angle(                              -- get the angle of the line joining the point with the centroid of its associated buffer
    make_line(
        $geometry,      
        centroid(
            geometry(
                get_feature(                  
                    layer:='buffers', 
                    attribute:='id',     -- 'id' in single quotes is the `id` field name in the buffers layer
                    value:="buffer_id"
                )
            )
        )                                                                                
    )
) + 90                                 -- add 90 to make the label text parallel to the line

The expression is entered as a Data defined override by clicking the button next to Rotation in the Data defined section

QGIS Geometry Generator – Creating Horizontal Lines Connecting Points

Easiest is probably

make_line (array_agg ($geometry, group_by:=$y))

Or, extending your version, you can add a geometries_to_array() function: it is available since QGIS 3.28, so for older versions use the expression above:

make_line( 
    geometries_to_array (
        collect ($geometry, group_by:=$y)
    )
)

Best Answer

Related Solutions

QGIS Geometry Generator – Creating Labels Between Associated Features in Different Layers

Prerequisite

Label values

Label placement

Label rotation

QGIS Geometry Generator – Creating Horizontal Lines Connecting Points

Related Question