[GIS] Collapsing parallel lines into a single line QGIS

mergeqgisshapefile

I have a shapefile of the Washington metro area, but along much of the route multiple lines run parallel to each other (see picture 1) and I need to collapse them into a single line.

I've tried creating a buffer and dissolving the result, which visually achieves what I'm looking for but ultimately I need a single shapefile line representing the center line that I can then run the qchainage process on but so far haven't been able to achieve that.

Does anyone have a way to achieve this in QGIS?

I believe I'm essentially trying to replicate the 'Collapse Dual Lines To Centerline' functionality in ArcGIS.

Input shapefile:

Dissolve Buffer result:

Best Answer

I have a function that returns a parallel line from three parameters, a line object, a left or right, and a distance. The returned line is parallel to the left or right of the given line a distance of the given distance.

The function takes each node of the given line and computes the center of a tangent circle (90 degrees to left or right of the current bearing) with a radius of the given distance and uses the center of the circle as the node's parallel point.

After the function returns the new line, I reverse order the the nodes of the returned parallel line if its drawn direction does not match the given line.

For street center lines, the original line must accommodate extensions to intersections.

Related Solutions

[GIS] Union, Dissolve, Buffer — multiple layers, all in one step

You can try something like this:

WITH unioned AS
(
SELECT id, ST_Union(geom_4269) AS geom, cost FROM catchment_10304
    GROUP BY id, cost
)
SELECT id, ST_ConcaveHull(geom, 0.8), cost FROM unioned

First we union the shapes each into a single multipoint (the group by allows us to keep the original fields), then we use ST_ConcaveHull (http://postgis.net/docs/ST_ConcaveHull.html) to create a concave hull.

[GIS] Merge adjacent parallel lines and add count as attribute in QGIS

First, you don't really want to make the lines to overlap, but only to reflect in the attributes the number of flights that are using the same path. This can be done following these steps:

Make sure you create a new column for your lines, call it "flights" and use the value 1 for all of them. I used the name "Count" which is not the best.

Then, perform a buffer operation with the desired proximity, you will have to play with the distance that you consider appropiate for your analysis: (my QGIS is in Spanish but I am sure you can figure it out)

Then, you have buffered lines that are most likely touching each other, like sausages. Your last step is to perform a spatial join, under vector, data management menu. Use the options intersect, as follows:

Make sure you add the summary option with the sum. The result will be a new column in every attribute, adding the number of other flights that are intersecting. If the lines have an ID you can join this new column to your original lines, or you can turn the buffered polygons back into lines.

If you want to simplify and remove the extra lines is a different problem, but I hope I could have helped you to start at least.

Best Answer

Related Solutions

[GIS] Union, Dissolve, Buffer — multiple layers, all in one step

[GIS] Merge adjacent parallel lines and add count as attribute in QGIS

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