Connect unconnected networks in QGIS

Question

I have a network project in QGIS:

Where arcs is a LineString layer, but all the arcs must be connected. I was able to detect subnetworks using the Disconnected Islands plugin which populates a NetworkGrp field.

The nodes layer is just the output of Extract Vertices algorithm, and wanted layer is what i need.

The conditions are:

• If there are 3 subnetworks, i need 2 lines connecting them. In general, one line less than the number of groups is needed.
• The lines can go from nearest points of each subnetwork group, without the need of a node there (preferred way, but they can go from one node to another one). Next to find the lines, i can clean the topology to add the required nodes and break the arcs accordingly.
• I need to solve this for a lot of unconnected networks, so i can't do it manually.
• I want to solve it in QGIS (using 3.22 right now), but can use GRASS GIS if needed. Python solutions are also welcome.
• There isn't a distance threshold, i just need to connect all groups in a layer. But i don't want to connect arcs or nodes of the same network subgroup.
• I need new arcs, but if a snap method is the only solution i can accept it.

---

The arcs layer to test can be downloaded from this link.

Answer 1

1. Aggregate your lines to create multipart lines, based on subnetworks: each subnetwork consists of one feature. Use Menu Processing > Toolbox > Aggregate for this.

2. For each line, find the nearest line and on this, create the closest point to the current feature.

3. Create the shortest line from this closest point to the current geometry. You're done.

Steps 2 and 3 can be done at once using the following expression with Geometry generator or Geometry by expression.

Initial lines and the small red lines created by the expression, connecting the gaps:

This is the expression you can use for the data you provided with settings made for your layer and attribute names:

with_variable (
    'point',
with_variable (
    'group',
    "NetworkGrp",  -- use your attribute name
array_foreach (
    eval ('overlay_nearest (
        ''arcs'',  -- use your layer name
        $geometry,
        filter:= NetworkGrp<>' || @group || ')' -- use your attribute name
    ),
    closest_point (
        @element,
        $geometry
    )
))[0],
make_line (
    @point,
    closest_point (
        $geometry, 
        @point
    )
))