[GIS] Merging features (lines) in shapefile with same name using Python and QGIS

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I am working with the NHD flowline dataset (USGS) and some of the line features have the same name. I want to merge the line features that have the same name. For example: There are three features that have the name "Kings River" and four features that have the name "Boulder Creek" how do I write a python script that merges these features based on the name? The question How to merge multiple overlapping lines using python? deals with a merge that is based on geometry – I need to merge the line features based on a non spatial attribute (name of flowline).

Best Answer

I would use the two following functions:

Vector > Data Management Tools > Merge Shapefiles to One... in order to make one shapefile from the different ones you have (it seems that they are all line geometries so this will work).
Vector > Geoprocessing Tools > Dissolve in order to merge the features based on the name attribute (this function asks for a merge attribute).

I have not tried this on a sample but it should work. 'hope I understood your problem correctly.

Related Solutions

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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