# Cartometric Blog

A scrapbook of GIS tricks, with emphasis on FOSS4G.

## PostGIS: query all multipolygon parcels with at least one hole

I was writing some code to iterate over Well Known Text expressions for polygon features, and I decided I needed to test the most complex edge-case I could think of–multipolygon geometries where at least one of the bound polygons has a hole (i.e. an interior ring).

I ended up with the following query. This seems like the kind of thing I’ll want to reuse later, so I’m noting it here. For good measure, I also use a rudimentary technique to sort the output with the most complicated geometries in the table at the top of the list. Basically, the more “text” it takes to describe the geometry using Well Known Text, the larger and more complex I figure it must be!

```SELECT
SomePrimaryId,   /* your primary key, i.e. ogc_fid, etc. */
SomeUniqueId,    /* your descriptive id, i.e. a parcel number */
ST_NumGeometries(wkb_geometry) AS num_geoms,
ST_NRings(wkb_geometry) AS num_rings,
ST_AsText(ST_Centroid(wkb_geometry)) AS center,
Char_Length(ST_AsText(wkb_geometry)) AS len,
ST_AsText(wkb_geometry) AS wkt
FROM SomePolygonTable
WHERE
ST_NumGeometries(wkb_geometry) > 1
AND
ST_NRings(wkb_geometry) > ST_NumGeometries(wkb_geometry)
ORDER BY Char_Length(ST_AsText(wkb_geometry)) ASC ;```

Just for the sake of promoting caution, I’m not certain this is a definitive approach for identifying the largest geometry in a table, as the length of the binary representation and the length of the readable text representation do not correspond one-to-one. Moreover, a feature could have more vertices that required less precision to express (fewer decimal position), than a geometry with fewer vertices that needed more precision, and then you have to ask, which is bigger, fewer vertices and more text, or more vertices that coincidentally did not require as much text? My conclusion is, the “most complicated geometry” is probably relative to the one asking the question. However for my purposes, this was close enough to put the most complicated stuff at the top of the list.

Written by elrobis

November 8th, 2013 at 10:26 am