# [postgis-users] Finding Empty Space

Bob Pawley rjpawley at shaw.ca
Sun Mar 29 12:55:07 PDT 2009

```Hi Regine

Yes I want to be able to puit identifiers like #1, #2 etc. in the clear space. In my world there is always a clear space (think line drawings of containment tanks, compressors etc), but this space is not always in the same area of the drawing. The trick is to have the computer identify such space.

Can you elaborate on "taking the exterior of the covering area" ?

Thanks

Bob

----- Original Message -----
From: Paragon Corporation
To: 'PostGIS Users Discussion'
Sent: Saturday, March 28, 2009 11:48 AM
Subject: RE: [postgis-users] Finding Empty Space

Bob,
Not quite clear what you are looking for here.

So are you trying to put your text identifier on a hole in a geometry?  What if there is no hole?

Also the centroid of a geometry is not guaranteed to lie on the geometry so even if you turned empty space into non-empty so you can take the centroid of it, the centroid may not be in the hole.

To get the centroid of a hole, I think you need to turn it into non-hole (like turning anti-matter into matter).

you can do this by say taking the exterior of the covering area, and then using ST_Difference.  The result would be the holes turned into geometries

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From: postgis-users-bounces at postgis.refractions.net [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Bob Pawley
Sent: Saturday, March 28, 2009 11:24 PM
To: PostGIS Users Discussion
Subject: [postgis-users] Finding Empty Space

Hi

The expression st_isempty returns a boolean set.

Is there any method of returning the centroid coordinates of an empty space wihtin a given geometry?

I want to add a text identifier to a geometry without it being lost in the clutter.

Bob

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