[postgis-devel] Is errorIfGeometryCollection necessary forrelate ops?

[Brian M Hamlin]

Hi Martin -

  Is it always better to have a single MultiPoly?  my naive thoughts 
are a) single polys are computationally faster, and b) the raison'detre 
of Multipolys is the "hole" , which could never be in a product of 
otherwise independent single polys.. 

  I admit this email is off-the-cuff so there could be something I am 
not thinking about at all.. there it is though.. option to combine ? 
 never combine ? 

  very best regards from Berkeley, California
    -Brian

On Mon, 13 Apr 2015 09:42:40 -0700, Martin Davis  wrote:

On Mon, Apr 13, 2015 at 12:59 AM, Sandro Santilli  wrote:

       On Fri, Apr 10, 2015 at 10:40:32AM -0700, Martin Davis wrote:
> re what is happening in Shapely with relate on GeometryCollections:
>
> In fact, homogeneous GeometryCollections of Points and LineStrings can be
> trivially converted to MultiPoints and MultiLineStrings.  Is is possible
> that Shapely is doing this automatically?
>
> Handling homogeneous GCs would be a nice enhancement for JTS & GEOS.... 

There's actually a ST_CollectionHomogenize function in PostGIS that
can be used to do just that. 

I'm starting to think about adding a "flatten" method to JTS that would 
do all the heavy lifting to transform an arbitrary GeometryCollection 
into something suitable for using with relate.  In addition to 
converting homogenous collections into the appropriate Multi-type, it 
would union all polygons to make them into a valid MultiPolygon. 

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--
Brian M Hamlin
OSGeo California Chapter
blog.light42.com


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