PostGIS MultiPolygon Conversion – How to Convert from MultiPolygon to Polygon in PostGIS

convertgeometrymulti-polygonpolygonpostgis

I have a table in PostGIS, where the data is currently stored as a MultiPolygon. Each Multipolygon has only a single Polygon in it.

How do I query and get the containing Polygon, and not MultiPolygon? I am ok ignoring the Edge case where there are Multiple Polygons in the Multi-Polygon Geometry. All I need is the first or only Polygon in that Geometry.

Best Answer

If you want to Ignore the edge cases, and get only the First (or only) Polygon, from the MultiPolygon geometry, then you need to use the ST_GeometryN() function.

A query like the following will give you the geometry as Polygons

SELECT ST_GeometryN(geom, 1)
FROM polygons;

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[GIS] Simplifying multipolygon using PostGIS

The Postgis docu suggest to wrap a ST_Multi around it to get a multi-geometry:

SELECT stusps,
       ST_Multi(ST_Union(f.the_geom)) as singlegeom
     FROM sometable As f
GROUP BY stusps

[GIS] Export polygon in geojson as multipolygon

You can convert all your data into multipolygons with ogr2ogr by defining "new layer type" with -nlt MULTIPOLYGON. However, I am not sure if the result is exactly what you believe when multipolygon has only one part.

This is a triangle as polygon

{
"type": "FeatureCollection",

"features": [
{ "type": "Feature", "properties": { }, "geometry": { "type": "Polygon", "coordinates": [ [ [ 95.0, 216.0 ], [ 241.0, 253.0 ], [ 175.0, 138.0 ], [ 95.0, 216.0 ] ] ] } }
]
}

This is the same triangle as multipolygon

{
"type": "FeatureCollection",

"features": [
{ "type": "Feature", "properties": { }, "geometry": { "type": "MultiPolygon", "coordinates": [ [ [ [ 95.0, 216.0 ], [ 241.0, 253.0 ], [ 175.0, 138.0 ], [ 95.0, 216.0 ] ] ] ] } }
]
}

EDIT

However, if the aim is to combine all the separate features, both polygons and multipolygons into one feature which is of type multipolygon, you must build an union. If your mixture of polygons and multipolygons is in file "mix.shp" the command to use is:

ogr2ogr -f geojson union.json mix.shp -dialect sqlite -sql "select st_union(geometry) from mix"

The result "union.json " contains one multipolygon as can be seen from the ogr2ogr report:

Layer name: OGRGeoJSON
Geometry: Multi Polygon
Feature Count: 1
...
OGRFeature(OGRGeoJSON):0
  MULTIPOLYGON (((125 229,185 289,252 234,208 169,125 229)),...

Best Answer

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[GIS] Export polygon in geojson as multipolygon

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