OGR2OGR Polygon Dissolve – Dissolving/Aggregating Polygons with OGR2OGR or GPC

aggregationogr2ogr

I have a large number of polygons representing the boundaries of US counties which I need to merge into as few polygons as possible.

Is it possible to do this with non-commercial software, or software of a reasonable price?

PostGIS is not available, nor is Java, but C is.

Best Answer

With ogr2ogr (GDAL >= 1.10 with SpatiaLite support):

ogr2ogr output.shp input.shp -dialect sqlite -sql "SELECT ST_Union(geometry), dissolve_field FROM input GROUP BY dissolve_field"

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If you have MySQL 5.7.5 or later, you can use MySQL's ST_Union function from ogr2ogr:

ogr2ogr -sql "SELECT ST_Union(geo) FROM counties" -f KML counties.kml MySQL:"..."

If you have GDAL/OGR built with SpatiaLite support, then you can let GDAL/OGR do the unions with the SQLite dialect. As a special case, "Union" is called GUnion:

ogr2ogr -dialect SQLITE -sql "SELECT GUnion(geo) FROM counties" -f KML counties.kml MySQL:"..."

There are a few options for the KML driver to check out. Also, to make a KMZ file (e.g. countries.kmz) it needs a small trick to the datasource name:

ogr2ogr ... -f KML /vsizip/counties.kmz/counties.kml ...

Best Answer

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