Cleaning up layer by deleting features with only one overlapping vertex in QGIS

Question

After applying r.thin and then r.to.vect to a raster image the resulting vector layer contains hundreds of small "branches" that are not supposed to be there:

I've tried selecting and deleting these mini branches using a $length field in the layer's attribute table but that also deleted short segments in the main contours I want to keep.

A solution would be to identify all short paths where nodes_to_points = 2 and number of intersecting vertices = 1 as this would leave out the paths that are part of the contour (and therefore have 2 intersecting vertices).

Does anybody know how to write this as an expression in the Field Calculator or can think of a better way to remove my unwanted artefacts?

Answer 1

You can achieve the result that you are striving for with GRASS toolset v.clean for cleaning topology of a vector element. Probably, you will need to deploy rmdangle option.

rmdangle: remove dangles, threshold ignored if < 0

Important to set up Threshold and snap tolerance parameters.

Check the following references for more details:

• Removing 'tails' from line features?
• Deleting lines in QGIS
• Automated removal of dangles / loose ends in line with QGIS?

Answer 2

Tested on QGIS 2.18 and QGIS 3.4

Additionally, I can suggest using a "Virtual Layer" through Layer > Add Layer > Add/Edit Virtual Layer....

Let's assume we have the following layer "line" with its Attribute table, see image below.

With the following query, it is possible to clean up the line out of "tails"/"dangles".

SELECT l1.*
FROM line AS l1, line AS l2
WHERE st_intersects(start_point(l1.geometry),end_point(l2.geometry))
OR st_intersects(start_point(l2.geometry),end_point(l1.geometry))
GROUP BY l1.id

The output Virtual Layer with its Attribute table will look as following

---

Mind the fact that previous example will erase only "tails"/"dangles" that are not connected to the starting/points of line features. Additionally, you will need to adjust the query accordingly.

Let's assume we have the following layer "line" with its Attribute table, see image below.

With the following query, it is possible to clean up the line out of "tails"/"dangles" that own two vertices.

SELECT l1.*
FROM line AS l1, line AS l2
WHERE ST_NumPoints(l1.geometry) > 2
AND (st_intersects(start_point(l1.geometry),end_point(l2.geometry))
OR st_intersects(start_point(l2.geometry),end_point(l1.geometry)))
GROUP BY l1.id

The output Virtual Layer with its Attribute table will look as following