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List: postgis-users
Subject: Re: [postgis-users] PostGIS Raster Area from a Clip
From: Marcello Benigno <benigno.marcello () gmail ! com>
Date: 2013-10-23 0:35:09
Message-ID: CAKPVhY+O6CH6AbW3EBxy32vMLDzgJxPVASCgjEdH4mBGQKRz0w () mail ! gmail ! com
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Just complementing ...
In my case there is no overlap in any of the layers, there really was no
need ST_Union.
Thanks again for the help and the discussion.
Regards,
2013/10/22 Pierre Racine <Pierre.Racine@sbf.ulaval.ca>
> > Won't that only yield the same answer if you can guarantee no overlapping
> > geometries in the fazendas table? My understanding was that the other
> > table was the raster table (which it is reasonable to assume has no
> > overlaps), but we don't know if that's the case with this one.
>
> Right. But you generally have to have a minimal knowledge about the
> topology of a layer when you query it. Querying a layer with overlaps is
> never the same as querying a layer without.
>
> So if you know your fazendas layer has overlaps it is a good idea to use
> ST_Union but that means you are getting stats about the global farming area
> not about each farming area. What about areas overlapping? Should they
> count twice? Do these overlaps make sense in reality? Maybe you have to fix
> then first...
>
> Pierre
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> postgis-users@lists.osgeo.org
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>
--
*Marcello Benigno B. de Barros Filho*
Prof. do Curso Superior de Tecnologia em Geoprocessamento - IFPB
Mestre em Ciências Geodésicas e Tecnologias da Geoinformação - UFPE
Doutorando em Tecnologia Ambiental e Recursos Hídricos - UFPE
http://profmarcello.blogspot.com
http://about.me/marcello.benigno
[Attachment #5 (text/html)]
<div dir="ltr"><div>Just complementing ...</div><div><br></div><div>In my case there \
is no overlap in any of the layers, there really was no need ST_Union. \
</div><div><br></div><div>Thanks again for the help and the discussion.</div> \
<div><br></div><div style>Regards,</div></div><div class="gmail_extra"><br><br><div \
class="gmail_quote">2013/10/22 Pierre Racine <span dir="ltr"><<a \
href="mailto:Pierre.Racine@sbf.ulaval.ca" \
target="_blank">Pierre.Racine@sbf.ulaval.ca</a>></span><br> <blockquote \
class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc \
solid;padding-left:1ex"><div class="im">> Won't that only yield the same \
answer if you can guarantee no overlapping<br> > geometries in the fazendas table? \
My understanding was that the other<br> > table was the raster table (which it is \
reasonable to assume has no<br> > overlaps), but we don't know if that's \
the case with this one.<br> <br>
</div>Right. But you generally have to have a minimal knowledge about the topology of \
a layer when you query it. Querying a layer with overlaps is never the same as \
querying a layer without.<br> <br>
So if you know your fazendas layer has overlaps it is a good idea to use ST_Union but \
that means you are getting stats about the global farming area not about each farming \
area. What about areas overlapping? Should they count twice? Do these overlaps make \
sense in reality? Maybe you have to fix then first...<br>
<span class="HOEnZb"><font color="#888888"><br>
Pierre<br>
</font></span><div class="HOEnZb"><div \
class="h5">_______________________________________________<br> postgis-users mailing \
list<br> <a href="mailto:postgis-users@lists.osgeo.org">postgis-users@lists.osgeo.org</a><br>
<a href="http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users" \
target="_blank">http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users</a><br> \
</div></div></blockquote></div><br><br clear="all"><div><br></div>-- <br><b>Marcello \
Benigno B. de Barros Filho</b><br><font size="1">Prof. do Curso Superior de \
Tecnologia em Geoprocessamento - IFPB<br>Mestre em Ciências Geodésicas e Tecnologias \
da Geoinformação - UFPE</font><div> <font size="1">Doutorando em Tecnologia Ambiental \
e Recursos Hídricos - UFPE</font><font size="1"><br></font><font size="1"><a \
href="http://profmarcello.blogspot.com" \
target="_blank">http://profmarcello.blogspot.com</a></font><div> <a \
href="http://about.me/marcello.benigno" target="_blank"><font \
size="1">http://about.me/marcello.benigno</font></a></div></div> </div>
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