QGIS – How to Merge Lines with Same Name Attribute and Intersection

attribute-joinsintersectionmergeqgis

I have a polygon layer that has lines showing rivers. A river with the same name attribute might be consisted of a few lines touching each other. For example, in the image below, the river consists of 4 separate lines (with the same name in the name attribute column). The small line shown in red belongs to another river with a different name.

I need to merge the lines with the same names together and I know ways such as merge, join by attribute etc, however I cannot simply use merging by same name because it can happen that a line belonging to the same river (thus having the same name attribute) does not intersect with the rest of the river and cannot be merged into other lines.
Is there a way in QGIS 3.22.5 to unite the lines belonging to same river together (having same name attribute), but only those ones which touch in one point?

Dissolve window in QGIS 3.22:

Advanced window:

Best Answer

Use the QGIS Dissolve tool, set the parameters this way:

Input layer: your lines layer
Dissolve field(s): the river name
Keep disjoint features separate: True (this is in the advanced parameters section)

Optional if you don't see the Keep disjoint features separate option, dissolve and then run the Multipart to single parts tool.

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QGIS Dissolve – How to Merge Lines with the Same Name and Intersect Each Other

I can describe how to do this in spatialite. You'll probably be able to adapt it to sql server. First import the shapefile into spatialite

.loadshp "roads" roads (code page) (srid)

Suppose the roads table now has columns: id as primary key, name, and geometry. Create a duplicate, empty table for the merge:

CREATE TABLE roads_merge (id INTEGER PRIMARY KEY AUTOINCREMENT, name TEXT);
SELECT AddGeometryColumn('roads_merge','geometry',(srid),'LINESTRING','XY');

Now insert into the new table the columns that intersect with identical names:

INSERT INTO roads_merge (name, geometry) 
SELECT r1.name, GUnion(r1.geometry, r2.geometry)
FROM roads AS r1, roads AS r2 
WHERE r1.id <> r2.id AND ST_Intersects(r1.geometry, r2.geometry) AND r1.name = r2.name;

And now insert all the single roads:

INSERT INTO roads_merge(name, geometry)
SELECT r1.name, r1.geometry
FROM roads AS r1, roads AS r2
WHERE r1.id <> r2.id AND Disjoint(r1.geometry, r2.geometry) AND r1.name <> r2.name;

I would probably first create a spatial index and use it in above the query.

HTH, Micha

[GIS] Merge lines with the same ‘name’ attribute on QGIS

Vector->Geoprocessing Tools->Dissolve does work for lines and will return a multi-geometry where the lines do not have coincident vertices.

Yet another option is available from PostGIS or SpatiaLite using the ST_LineMerge function.

Best Answer

Related Solutions

QGIS Dissolve – How to Merge Lines with the Same Name and Intersect Each Other

[GIS] Merge lines with the same ‘name’ attribute on QGIS

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