[postgis-users] Intersection errors

[Ethan Alpert]

The problem is if it's a multipolygon you can have 3 consecutive outer
rings with no inner rings which will look the same as your example.

I opened up the shp2pgsql.c code and sure enough it computes the area of
the polygon for each polygon. If the area is negative it's an inner ring
if it's positive it's a new outer ring.

-e

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Sean
M. Montague
Sent: Tuesday, June 07, 2005 4:25 PM
To: PostGIS Users Discussion
Subject: RE: [postgis-users] Intersection errors


I may be wrong, but I believe it's the order.  If a poly has two inner
rings, There will be a list of 3 polys, the first of which is the outer
poly, the two inner come next.  At least this been my experience
exporting shape files.

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
Ethan Alpert
Sent: Tuesday, June 07, 2005 2:05 PM
To: PostGIS Users Discussion
Subject: RE: [postgis-users] Intersection errors



While we're on the topic of multipolygons. I've noticed that the
shapefile specification doesn't have any convention for communicating
outer and inner rings. That said how do apps like shp2pgsql and ogr2ogr
determine whether the next polygon in a list is an inner or a new outer?
I understand that the convention is that inner rings are counter
clockwise. But still how is the direction of the ring determined
efficiently?



-e

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
Markus Schaber
Sent: Tuesday, June 07, 2005 2:02 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Intersection errors


Hi, Sean,

Sean M. Montague schrieb:
> I've been working on it, and I now realize the problem...not sure if
> it will solve my intersect problem, but I'll get to that later.  There

> is a logical reason for dissolving some of layers, such as lakes at
> certain scales.  If they are multi polys, there is a significant size 
> savings when writing out as SVG.  I just need to see if I can 
> distinguish between a hole, and say another lake.  Examining what you 
> suggested looks like a good start.  Thanks.

Yes, it is easily possible. Maybe you should take a look at the Well
Known Text format specification from the OpenGIS consortium (Part of
their "Simple features for SQL" specification, or use the PostGIS source
or some other program that correctly exports WKT to find out how it
works.

The basic idea for Multipolygon is:

MULTIPOLYGON(((First polygon outer ring),(1st poly first hole),(1st poly
2nd hole)),((2nd Poly outer ring)(2nd Poly hole)),((3rd Poly with no
hole)),((4th Poly outer ring)(4th poly 1st hole)(4th poly 2nd hole)))

All holes (inner rings) have to be completely inside their outer ring,
and may not overlap each other. A polygon may be contained completely
inside the hole of another polygon (like an island), but they may not
overlap.

For the exact rules how the inner and outer rings may touch and
intersect, please see geometry model part of the aforementioned
specification.

HTH,
Markus

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