Armin,<br><br>Cascaded Union works pretty much like Union except that it is optimized to work on more than two geometries.<br>Both OGR and PostGIS uses the GEOS library to perform this.<br><br>Read this blog entry by Paul Ramsey: <a href="http://blog.cleverelephant.ca/2009/01/must-faster-unions-in-postgis-14.html">http://blog.cleverelephant.ca/2009/01/must-faster-unions-in-postgis-14.html</a><br>
<br><div class="gmail_quote">On Wed, Mar 16, 2011 at 3:41 PM, Armin Burger <span dir="ltr"><<a href="mailto:armin.burger@gmx.net">armin.burger@gmx.net</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
Chaitanya<br>
<br>
thanks for the hints. So a sort of Union operation seems to be necessary. I need to check how fast that is with OGR on larger shapefiles (I want to store in PostGIS just the footprint, not the full geometry collection). The ConvexHull could be a good alternative to a simplified boundary.<br>
<br>
I understand how the normal Union() works, it merges 2 geometries into a new one. But I don't get it what UnionCascade() is doing. It applies a Union on a single geometry and returns the new geometry, what is it actually unioning/merging then?<br>
<br>
Regards,<br>
Armin<br>
<br>
<br>
-------- Original-Nachricht --------<br>
> Datum: Wed, 16 Mar 2011 10:06:57 +0530<br>
> Von: Chaitanya kumar CH <<a href="http://chaitanya.ch" target="_blank">chaitanya.ch</a>@<a href="http://gmail.com" target="_blank">gmail.com</a>><br>
> An: <a href="mailto:armin.burger@gmx.net">armin.burger@gmx.net</a><br>
> CC: Marius Jigmond <<a href="mailto:mariusjigmond@hotmail.com">mariusjigmond@hotmail.com</a>>, <a href="mailto:gdal-dev@lists.osgeo.org">gdal-dev@lists.osgeo.org</a><br>
> Betreff: Re: [gdal-dev] Calculate footprints of shapefiles<br>
<div><div></div><div class="h5"><br>
> Armin,<br>
><br>
> This can be done in either OGR or PostGIS.<br>
><br>
> For OGR, refer to the OGRGeometry class reference, especially the<br>
> ConvexHull(), Union() and UnionCascaded() functions.<br>
> For PostGIS, ST_Boundary() and ST_Union().<br>
><br>
<br>
<br>
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