[postgis-users] Area calculation
Markus Schaber
schabios at logi-track.com
Tue Jan 27 14:43:37 PST 2004
Hi, Dirk,
On Tue, 27 Jan 2004 17:25:21 +0100
Dirk Geschwandtner <dg at mapmedia.de> wrote:
> I nee to calculate the areas of swiss-cheese-polygons without the
> holes in postgis.
>
> I tried convexhull, but this not always returns the correct results,
> the area is slighty bigger than the area of the original.
This is because of the definition of a convex hull, it builds a hull
that is convex around the object. That means that it removes all concave
corners, means all corners that point "into" the object.
If you e. G. have a shape like
+-+
| |
+-* *-+ (I marked all "guilty" corners with a * )
| |
+-* *-+
| |
+-+
it gets converted to
+-+
/ \
+ +
| |
+ +
\ /
+-+
or if you have a shape like
+-+ +-+
| | | |
| *-* |
+-----+
it has the following convex hull:
+-----+
| |
| |
+-----+
So, as it was stated by some other folks, you have to use the area of
the exterior rings of the polygon.
Markus
--
markus schaber | dipl. informatiker
logi-track ag | rennweg 14-16 | ch 8001 zürich
phone +41-43-888 62 52 | fax +41-43-888 62 53
mailto:schaber at logi-track.com | www.logi-track.com
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