An example showing how to simplify a multipolygon layer, keeping topology between objects

What we want

Simplifying this layer into this: [[image:SPT_dept_sim.png]]

The data

French administrative subdivisions, called "départements", will be used. Data can be downloaded here: ​http://http://professionnels.ign.fr/DISPLAY/000/528/175/5281750/GEOFLADept_FR_Corse_AV_L93.zip

Data projection is Lambert-93, EPSG:2154

Loading the data

shp2pgsql -IiD -g geom -s 2154 DEPARTEMENT.SHP departement | psql

Principle of simplification

Based on the technique described here [[]], we extract linestrings out of polygons, then union and simplify them. st_polygonize() is used to rebuild surfaces from linestrings. Attributes from the initial layer are associated with simplified polygons.

Steps

1. First extract the input multipolygons into polygons, keeping their departement code. This will allow us to associate attributes to each part of multipolygons at the end of the process.

create table dep_poly as select gid, code, st_dump