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We have a very large shapefile with over 30000 features and want to merge it with a script on a server. Currently we try it with QGIS, but the processing takes way too much time.

Is there a way to do it with the GDAL/OGR Tools or Python?

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By merge, do you mean to say, change the 30000 features to just one multipart feature? – R.K. Sep 15 '12 at 13:30
yes, thats what I want to do. – leak_my_brain Sep 15 '12 at 14:54
What are those data by the way? How would they be merged? Do they have common fields etc? – R.K. Sep 15 '12 at 15:14
We are working with the OSM-Coastline Polygons, which come from here: link – leak_my_brain Sep 15 '12 at 16:23
We want to render them in TileMill, but get artefacts at the cutlines, which we want to eliminate by merging the polygons. – leak_my_brain Sep 15 '12 at 16:25

3 Answers

up vote 4 down vote accepted

In GIS context, the better is organize your data in a database (!).

Then, many kinds of operations will be simple. With PostGIS (using PostgreSQL as database) the command that solves your problem is

  SELECT ST_Collect(geom) FROM your30000features_table;

And the commands are standard, see standard OGC functions ST_Collect and ST_Union.

Using as input "OSM-Coastline Polygons", you are using POLYGON OGC types in the database. The ST_Collect(geom) of a set of POLYGONs will return a GEOMETRYCOLLECTION, but you can convert it into a MULTIPOLYGON, or produce ir directally by ST_Union function.

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This seems to be the most viable solution, as I wanted to try PostGIS anyway. I'll try it out right now. Thanks! – leak_my_brain Sep 18 '12 at 6:59
You're welcome; good decision, try it! PS: if you are new user in the Stackexchange, remember to click at the check icon in your accepted answer. – Peter Krauss Sep 18 '12 at 11:18
What would be the complete SQL-String for doing this with ST_Union? I just dont get it... – leak_my_brain Nov 6 '12 at 23:09
Hum... Sorry, I "dont get" your question, both are aggregate functions, use the same: 'SELECT ST_Union(geom) FROM table'. Perhaps you must check CPU time with 'EXPLAIN SELECT ST_Union(geom) FROM table', and compare with ST_collect... And read the guide to check what you need (check also ST_UnaryUnion) about "dissolve boundary" efects. – Peter Krauss Nov 7 '12 at 6:42
Sorry, I was a bit undefined. How do I write that into a new table. Yesterday I thought I worked it out, but the resulting table was empty after this: CREATE TABLE water_polygons_merged AS SELECT ST_Union(geometry) AS geometry FROM water_polygons; -- Update the geometry_columns table SELECT Populate_Geometry_Columns(); – leak_my_brain Nov 7 '12 at 9:43
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Not quite Python or GDAL but maybe you could try GRASS' v.extract function. It should let you extract and dissolve the features. You can access it through QGIS' SEXTANTE Toolbox.

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I think trough GRASS I add a lot of overhead to the whole thing. But other things don't work, i'll try it out. – leak_my_brain Sep 18 '12 at 6:57
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i dont know whether arcmap Python scripting works for you or not but you can check out this topic about batch merging script here

Beside this in python there is a library with the name of pyshp about reading and writing shapefiles.

i hope if helps you...

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Thanks so far. I don' use ArcMap, but give pyshp a try. – leak_my_brain Sep 18 '12 at 6:56
Just a warning though, pyshp is a pure python library so it might be very slow considering the number of features it will have to process. – R.K. Sep 18 '12 at 7:05

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