How to connect lines in a network?

Question

I have a shapefile of lines and another of polygons. When two or more lines intersect a polygon they should be connected (snapped), but sometimes during manual digitization this doesn't occur. I need to find and correct all these errors. I used v.clean from GRASS GIS and it automatically snapped a lot of vertices, which was good, but some of those disconnected lines are further from the tolerance I gave and if I increase the tolerance too much it begins to generate other types of errors.

Any ideas?

Answer 1

My understanding of the problem is as follows:

If a polyline endpoint intersects a polygon then the polyline needs to be connected (by adding or adjusting vertices) to all additional polyline endpoints that intersect the same polygon.

Some polyline endpoints don't intersect a polygon, being undershoots, but these should be connected as above.

This answer builds on some already given, but isn't complete as I'm not aware of all the QGIS commands - perhaps someone else can add to this?

1. For each polyline, extract the endpoint coordinates (Vector > Geometry Tools > Extract nodes then tidy up) and calculate whether or not they are within a polygon (Vector > Research Tools > Select by Location).

2. For polyline endpoints that are within a polygon, extend that end of the polyline to the centre of the polygon (Extract the polygon centre using Vector > Geometry Tools > Polygon centroid, not sure about the line modification, though you might be able to work with the polyline vertices as points and add additional ones with appropriate numbering before reconstructing the modified polyline using the Points to Paths plugin)

   You mention that there may be situations where some or all of the lines have been correctly connected, in these situations (identifiable by coincident polyline end points) you might choose to use the coincident end point location instead of the polygon centroid as the point to extend any additional unconnected polylines to for that polygon.

3. For polyline endpoints falling outside a polygon, extend that end of the polyline to the centre of the nearest polygon. Again, I'm not sure about calculating the nearest polygon, though the GRASS v.distance module looks to do the trick .

Answer 2

I think that I could do it with some custom algorithm... Note sure how you would like to implement it, but I think it would work.

For each line, for each end point, do a point in polygon for the vertex at the endpoint and link it to the polygon.

Then, for each polygon, if there is more than one line's end point linked, I'd merge (move) them all to the average coordinate, or to the centroid of the polygon perhaps.

That is pretty simple. But not available in standard operators. I think it could be done in ArcGIS with a days coding time.

Another note, in the image you posted, one of the lines is outside the polygon... You could handle this in a simpler way by simply finding the closest vertex to each end vertex, ensuring that the closeness is mutual (I think that is necessary, so that you have vertices snapping only in one way), then you can move them to the average position of the vertices.

Answer 3

Interesting problem! I can't think of a way to automatically perform the snapping, but here's a step in identifying areas to at least look at for manual correction:

• Add a new column to your line layer, and set it equal to '1' for all features.
• Under Vector > Join Attributes by Location, set your Target layer to your polygon layer, and the line layer as the join layer. Then select "Take Summary of Intersecting Features" and click "Sum." Give a location for the output file, and "OK."
• The resulting polygon file will be your original polygon layer, but with some additional fields. The SUM field is what you want; when it's 2, it means that the then you can zoom to it and manually snap the features.

Depending on the situation (number of features, number that are in this situation vs. those that aren't, etc.) this may save you some time.

EDIT: You may also want to use the polygons with a sum of 2 to create a sub-set of your line layers, then run the cleanup on those with a larger tolerance so as not to change the other features.

Answer 4

once you have the endpoints, I would do a spider diagram taking polygons centroids as facilities and endpoints of nearby lines as targets. You perform the spider diagram analysis and as a result you would have a segment for each polyline.

This would work even for those ending outside the polygons. How to do that in QGIS I don't know, sorry, but I would do this in ESRI's products that I know well. That's what I understood when whuber, in the first reply, suggested to build a "star".

I'm pretty sure there are people here who know how to perform such analysis in QGIS