Find intersections of roads in QGIS

Question

I want to get points at the intersections of the road network. I have a single shapefile. It contains 100k features and some of them continue each other.

As can be seen from 2 pictures above, the selected feature doesn't have any intersections between its neighbors, while it has them from QGIS's point of view. I've tried the solution from Find intersection point in a road network of line shapefile:

1. Dissolve without attribution the original road network shapefile, which contains 100k features to get one feature containing all roads. It worked perfectly. As a result, I have a new layer with one feature.
2. Multipart the new layer to Singleparts. After this step I get a new layer, but with the same problem: the one road may have several segments. That's why the Line Intersection doesn't work well.

As you can see, QGIS made an intersection point at the selected road segment, while it must not be done. Any suggestions?

Answer 1

I found a better and robust solution. Tested using QGIS 3.14. Run the following tools from the Processing Toolbox

1. Dissolve All (don't select any attribute). You get all features merged as a single multilinestring
2. Multiparts to Singleparts.
3. Line Intersections (select the same singleparts layer for input and overlay)
4. The resulting points layer will have only the line intersections, but there are duplicate vertices at some nodes
5. Delete duplicate geometries
6. Done!

Answer 2

If your road network has a name attribute or similar, you should be able to do this by dissolving based on attribute(s).

As an example, consider the following road network layer. The layer has been symbolized to have points at the first and last vertices. You can see that, like your data, mine is split into many, many short segments.

1. I run the Dissolve tool and choose Name as the dissolve field. The results appear similar to before, but by selecting one of the roads, I can see that these are actually multipart, but still distinct features for each road.

2. I run the Merge lines tool. The output is what I want: connected parts of each road are merged together, but each road is still a distinct feature.

3. With the merged roads layer, I can now run the Line intersections tool.
   NOTE: This is where the process laid out in the question breaks down; with a single merged feature, it won't self-intersect anywhere.

Here's the output, with intersections symbolized as Xs. To help illustrate the result, I've included the original network's start and end points, which you can see are no longer included as intersections.

An additional benefit of creating line intersections this way is that the intersection layer can retain the attributes of both intersecting features from the lines layer, since we did not do away with the attributes in that initial dissolve. This may not be useful to your particular purpose, but it may be beneficial to manage the output data. You'd only be a field calculation away from something like "Intersection of Avenue X and Street Y" for each intersection.

Finally, be aware that this method isn't perfect. If an otherwise continuous line changes attributes, you may end up with "intersections", as shown below. The tiny dead end in this picture technically has its own name, so it kept it separate.