[postgis-users] Summarystats (Raster, Polygon)

[Duncan Golicher]

Hello Andreas,

Comparisons with Arc are always interesting to have.

Can I ask how the speed of running the queries compares? We have been
implementing some similar queries but found them to run rather more
slowly than would be ideal. Pierre has suggested that this is due to
using the pgsql version of st_clip rather than the new C version, but
I have not yet managed to update to that as it does not seem to be
part of the nightly build yet.

I have also found these sort of queries to be VERY sensitive to tile
size, with speed optimising at a mid range size making it quite tricky
to find the best tile size, especially as it will also depend on the
size of the polygons that are being used.  It would be interesting to
know what your experience has been.

Duncan

On Fri, Nov 16, 2012 at 5:41 AM, Andreas Forø Tollefsen
<andreasft at gmail.com> wrote:
> Hi Pierre and thanks for your reply.
>
> My nodata values are well defined and are excluded in the summarystats
> operation.
> I also tried inverting the order of (geom, rast) without this changing the
> results.
>
> Could this be explained by the possibility that Postgis and ArcGIS handels
> pixels at the border of the polygon differently? i.e. that one includes
> pixels intersecting, while the other only includes pixels if the centroid of
> the pixel intersects?
>
> Andreas
>
>
> 2012/11/15 Pierre Racine <Pierre.Racine at sbf.ulaval.ca>
>>
>> > Any suggestions why the results are not the same?
>>
>> Are your nodata values well defined?
>>
>> Might also be, depending on the version you have, the order in which you
>> pass the geometry and the raster to the ST_Intersects() function. Try
>> inverting them.
>>
>> > Any improvements to the query maybe?
>>
>> Not really.
>>
>> Pierre
>>
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-- 
Dr Duncan Golicher
Investigador Titular,
El Colegio de la Frontera Sur, Chiapas,Mexico
Mexico tel +52 1 967 137 94 20
Skype name duncangolicher

Publications: http://www.mendeley.com/profiles/duncan-golicher

Senior lecturer, Bournemouth University, UK
Centre for Conservation Ecology & Environmental Change
School of Applied Sciences
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Bournemouth University
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For list of publications see Researcher ID:
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dgolicher at bournemouth.ac.uk
dgoliche at ecosur.mx

Researcher ID:
http://www.researcherid.com/rid/B-4240-2009