[postgis-users] Summarystats (Raster, Polygon)

Duncan Golicher dgolicher at gmail.com
Fri Nov 16 08:42:37 PST 2012 

Thanks Pierre, really helpful advice. So, what would be the best way
in your book to calculate summary stats and cell counts over a
graticule? That could also be a useful way to go and more accurate
than nearest neighbour.

Duncan

On Fri, Nov 16, 2012 at 10:35 AM, Pierre Racine
<Pierre.Racine at sbf.ulaval.ca> wrote:
> There are two ways to do that:
>
> 1) Load multiple overviews of the raster with the -l OVERVIEW_FACTOR raster2pgsql option and query the overview instead of the full resolution raster. (You can also create the overviews in SQL with a complex mixture of ST_Union() and ST_Rescale() call. A plpgsql ST_CreateOverview() would be very welcome if you have some spare time. See objective 20 and 21 in http://trac.osgeo.org/postgis/wiki/WKTRaster/SpecificationWorking03)
>
> 2) If you look at rtpostgis.sql you will find an undocumented ST_ApproxSummaryStats() function which accept a sample_percent argument reducing the number of pixels summary stats are computed on. Not sure this one speedup things...  To try!
>
> Pierre
>
>> -----Original Message-----
>> From: postgis-users-bounces at lists.osgeo.org [mailto:postgis-users-
>> bounces at lists.osgeo.org] On Behalf Of Duncan Golicher
>> Sent: Friday, November 16, 2012 11:20 AM
>> To: PostGIS Users Discussion
>> Subject: Re: [postgis-users] Summarystats (Raster, Polygon)
>>
>> Can I ask Pierre his opinion on applying nearest neighbour resampling
>> within a query in order to speed things up when precision is not the
>> top priority, but many polygons or buffers have to be used?
>>
>> Would this work, and if so what would the query look like?
>>
>> Duncan
>>
>>
>> On Fri, Nov 16, 2012 at 10:09 AM, Duncan Golicher <dgolicher at gmail.com>
>> wrote:
>> > 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
>> >>>
>> >>> _______________________________________________
>> >>> postgis-users mailing list
>> >>> postgis-users at lists.osgeo.org
>> >>> http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
>> >>
>> >>
>> >>
>> >> _______________________________________________
>> >> postgis-users mailing list
>> >> postgis-users at lists.osgeo.org
>> >> http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
>> >>
>> >
>> >
>> >
>> > --
>> > 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
>> > Christchurch House rm C218a
>> > Bournemouth University
>> > Fern Barrow
>> > Poole (Dorset) BH12 5BB UK
>> > Tel. +44 (0)1202 961682
>> >
>> > For list of publications see Researcher ID:
>> > http://www.researcherid.com/rid/B-4240-2009
>> >
>> > dgolicher at bournemouth.ac.uk
>> > dgoliche at ecosur.mx
>> >
>> > Researcher ID:
>> > http://www.researcherid.com/rid/B-4240-2009
>>
>>
>>
>> --
>> 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
>> Christchurch House rm C218a
>> Bournemouth University
>> Fern Barrow
>> Poole (Dorset) BH12 5BB UK
>> Tel. +44 (0)1202 961682
>>
>> For list of publications see Researcher ID:
>> http://www.researcherid.com/rid/B-4240-2009
>>
>> dgolicher at bournemouth.ac.uk
>> dgoliche at ecosur.mx
>>
>> Researcher ID:
>> http://www.researcherid.com/rid/B-4240-2009
>> _______________________________________________
>> postgis-users mailing list
>> postgis-users at lists.osgeo.org
>> http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
> _______________________________________________
> postgis-users mailing list
> postgis-users at lists.osgeo.org
> http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users



-- 
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
Christchurch House rm C218a
Bournemouth University
Fern Barrow
Poole (Dorset) BH12 5BB UK
Tel. +44 (0)1202 961682

For list of publications see Researcher ID:
http://www.researcherid.com/rid/B-4240-2009

dgolicher at bournemouth.ac.uk
dgoliche at ecosur.mx

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

More information about the postgis-users mailing list