[postgis-users] st_intersection() outputs multipolygons

[Obe, Regina]

Dylan,

Are you trying to make them all the same or all into POLYGONS?  If you
end up with POLYGONS that don't touch (which is quite common with
st_intersection even n non-sliver cases) I think they have to be
represented as MULTIPOLYGONS so the best you can do is row creation and
break each out into a separate record or make your POLYGON a multi by
doing

ST_Multi(ST_Intersection(...))

so you can put constraints on it.

Hope that helps,
Regina
 

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
Dylan Beaudette
Sent: Tuesday, April 22, 2008 6:04 PM
To: PostGIS Mailingliste
Subject: [postgis-users] st_intersection() outputs multipolygons

Hi,

I have two imput tables with all polygon geometry types, all polygons
are 
reported as valid. After performing an intersection, I am left with
mixed 
polygon and multipolygon geometry types:

CREATE TABLE dwr.mapunit_dau as
SELECT m_polys.areasymbol, m_polys.mukey, d_polys.dau_id, 
ST_Intersection(d_polys.wkb_geometry, m_polys.wkb_geometry) as
wkb_geometry
FROM
	(
	-- subset map unit polygons to certain survey areas
	-- 6.540 s
	SELECT wkb_geometry, areasymbol, mukey
	FROM mapunit_poly
	-- results in 21682 map unit polygons
	WHERE areasymbol in ('ca653', 'ca654', 'ca113')
	) as m_polys
JOIN	
	(
	-- subset DAU polygons that overlap with specific survey areas
	-- 	512 ms
	SELECT dwr.dau.wkb_geometry, dau.dau3_d_id as dau_id
	FROM dwr.dau JOIN mapunit_bound_poly
	-- results in 58 DAU polygons
	ON mapunit_bound_poly.areasymbol in ('ca653', 'ca654', 'ca113')
	and dwr.dau.wkb_geometry && mapunit_bound_poly.wkb_geometry
	) as d_polys
-- join condition: only those polygons which completely intersect
ON ST_Intersects(d_polys.wkb_geometry, m_polys.wkb_geometry);


--
--
--
Looking at the results, I have several geometry types with several total

geometries per original feature:

SELECT DISTINCT GeometryType(wkb_geometry),
ST_NumGeometries(wkb_geometry) 
from mapunit_dau;

 geometrytype | st_numgeometries 
--------------+------------------
 MULTIPOLYGON |                2
 MULTIPOLYGON |                3
 MULTIPOLYGON |                4
 MULTIPOLYGON |                5
 MULTIPOLYGON |                6
 MULTIPOLYGON |                7
 MULTIPOLYGON |                8
 MULTIPOLYGON |                9
 MULTIPOLYGON |               10
 MULTIPOLYGON |               11
 MULTIPOLYGON |               12
 MULTIPOLYGON |               13
 MULTIPOLYGON |               14
 MULTIPOLYGON |               15
 MULTIPOLYGON |               16
 MULTIPOLYGON |               18
 MULTIPOLYGON |               21
 MULTIPOLYGON |               24
 MULTIPOLYGON |               26
 MULTIPOLYGON |               27
 MULTIPOLYGON |               32
 MULTIPOLYGON |               36
 MULTIPOLYGON |               41
 MULTIPOLYGON |               52
 MULTIPOLYGON |               56
 MULTIPOLYGON |               69
 POLYGON      |                 

I have checked, and these multipolygons represent large features, not 
sliver-like artifacts that are commonly left-over from an intersection.
Also, 
they contain useful information (non-overlapping polygons) as I traverse
the 
geometry tree with GeometryN()...

Any ideas?

Cheers,

Dylan 

-- 
Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341
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