Generating dangle point for polyline in QGIS

Question

How do I generate the dangle points (into shapefile or something) (blue dots in picture below) of any polyline in QGIS? These are not start/end vertices.

Answer 1

1. Extract the vertices from road layer
2. Explode the lines
3. Join each line to the vertices, end points will get line count 1 and all other more than 1
4. Extract them

Answer 2

The specification of any polyline is a little optimistic (given the possibility of duplicate geometries), but for a topologically correct network it can be done using a "Virtual Layer" through Layer > Add Layer > Add/Edit Virtual Layer....

This query first collects all start and end points into one table, then selects those points which intersect only once with the lines layer.

Assuming the line layer is called 'edges' and its geometry field is called geometry:

SELECT *
FROM (
    SELECT ST_StartPoint(geometry) AS geom
    FROM edges
    UNION 
    SELECT ST_EndPoint(geometry) AS geom
    FROM edges
    ) AS startend
JOIN edges
    ON ST_Intersects(startend.geom, edges.geometry)
GROUP BY startend.geom
HAVING COUNT(*) = 1

Note: The seemingly missing point towards the lower left of the image is actually a cul-de-sac and not a dangle!

Answer 3

To just visualize the dangles, use Geomtetry generator with this expression (see below to create actual geometries):

difference (
    collect_geometries (
        array_foreach(
            array (start_point($geometry), end_point($geometry)),
            with_variable (
                'point',
                @element,
                if (
                    array_sum(
                        array_foreach(
                            overlay_nearest(
                                @layer, 
                                $geometry,
                                limit:=100,
                                max_distance:=0.1  -- change this distance to fit your needs
                            ),
                            intersects (
                                @element, 
                                buffer(@point,0.1)  -- change this distance to fit your needs
                            )
                        )
                    ) = 0,
                    @point,
                    make_point(0,0)
                )
            )
        )
    ),
    make_point(0,0)
)

---

To create acutal geometries that can then be saved as Shapefile, you can't use this expression, but - based on the expression above - must proceed in three steps:

1. Extract all start- and end-points using Geometry by expression with this expression: union (start_point ($geometry), end_point($geometry))

2. Run Multipart to singleparts

3. On the resulting layer, once again use Geometry by expression, this time with this expression (based on the solution above with Geometry generator):

if (
    array_sum(
        array_foreach(
            overlay_nearest(
                'network', -- change the layer's name accordingly
                $geometry,
                limit:=100,
                max_distance:=0.1  -- change this distance to fit your needs
            ),
            intersects (
                @element, 
                buffer($geometry,0.1)  -- change this distance to fit your needs
            )
        )
    ) = 1,
    $geometry,
    NULL
)

Answer 4

Let's assume there is a polyline layer called 'lines_test2' with several features in it, see the image below.

Step 1. Proceed with "Split with lines" tool to split the line layer with itself.

Step 2. Apply the "Extract specific vertices" geoalgorithm to get the first and the last points.

Step 3. Make use of the "Extract by expression" tool together with the following expression:

count(1, group_by:=geom_to_wkt($geometry)) = 1

and get the output