On Thu, Jun 21, 2012 at 8:17 PM, Martin Davis <mda...@opengeo.org> wrote:
> The reason for the exception is that the JTS intersection operation only
> works on valid geometries. Unfortunately the output of the
> gs:CollectGeometries process may not be valid in the case where it is
> collecting a set of Polygons. This is because polygons which are adjacent
> to each other or which overlap do not form valid MultiPolygons. Both of
> these situations occur in your process (the input states are adjacent, and
> the buffers will overlap).
The current version of collectGeometries now does a union before returning
polygon collections, I guess you haven't used it in a few months, the
change has been made in March (http://jira.codehaus.org/browse/GEOT-4077)
However this might explain why Cesare's case is not working, I believe he
has a 2.1.3 installed.
>
> Actually the buffer operation should work with any kind of input, and
> produce a valid polygonal output. So the problem is due to collecting the
> STATES polygons together and trying to use that in JTS:intersection.
>
> In fact, the JTS:intersection operation is not designed to work on
> collections of geometries anyway.
>
The gs:CollectGeometries process made sure to build a type specific
collection for that very reason, you will get back
a GeometryCollection only if the collected geometries are etherogeneous:
http://svn.osgeo.org/geotools/trunk/modules/library/main/src/main/java/org/geotools/geometry/jts/GeometryCollector.java
Or do you mean that intersecting a Multipolygon against another
Multipolygon is less robust than an intersection
between simple polygons?
> If you want to intersect a single polygon against a collection of
> features, you should look at using gs:Clip instead.
>
Two different use cases. gs:Clip generates features, it's a good tool for a
"clip and ship" scenario where you want
to download a layer. The process setup they way it has been it's good if
all you want is the resulting geometry,
though it would have been more scalable if the gs:Clip was used as the
first step, and a collectGeometry
called later, that is, something like:
collectGeometries
gs:Clip
layerB
collectGemetries(layerA)
Cheers
Andrea
--
Ing. Andrea Aime
GeoSolutions S.A.S.
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