How do dissolve polygons from shapefile using open source tools?

Question

In my current project I'm visualizing data on a map representing some values for each administrative areas of a country. I'm given a shapefile of this country, which I'm converting to GeoJSON for use with PolyMaps. Now I've got a requirement to cluster multiple areas into bigger ones, since there is no data for some of the smaller parts, but aggregated data for big ones can be acquired. Those clusters are predefined (e.g. A+B+C=D), so I figured it would be easier to produce a clustered shapefile and convert that to GeoJSON afterwards.

But how do I create such a cluster, preferably automated and with open-source tools? Any other sensible way is of course welcome too.

Answer 1

You can always use an open-source tool like QGIS to open your shapefile and merge features using the built-in "merge selected features" tool (just select features, click merge, select attributes to inherit to the new merged feature, done).

This of course doesn't make much sense if we're talking about hundreds or thousands of merges. I myself was wondering the other day if it is possible to merge features "by attribute" in QGIS?

EDIT: Ok, I'm stupid, if you have a common attribute for all features you're going to "cluster" (like a province or county name) this would be a case for the dissolve tool. Open your shapefile, choose dissolve (in QGIS it's located at Vector > Geoprocessing Tools > Dissolve), select the column with the above mentioned common name, and start the process. This should hopefully (if I haven't misunderstood you) result in what you were looking for.

Answer 2

For less manual work, you could have a look at SpatiaLite. You can import shapefiles easily using e.g.

spatialite> .loadshp ~/maps/areas areas iso-8859-15
spatialite> UPDATE areas SET Geometry = SetSrid(Geometry,4326);
spatialite> SELECT RecoverGeometryColumn('areas','Geometry',4326,'POLYGON',2);

Then for the dissolving part:

I'd like to be able to define those clusters (e.g. via NUTS codes: merge FR413 and FR411 into something called _FRX1)

The function you need here is GUnion(). You might want to define your "clusters" in a second table. Then you can join cluster and area table and use GUnion() with GROUP BY to create the final union_table.

You can then export to shapefile using .dumpshp

spatialite> .dumpshp union_table Geometry new_shapefile iso-8859-15 POLYGON

If you need GeoJSON, you can convert the file now.

You can write a short script in e.g. Python that handles all these loads, joins and dumps.