[postgis-users] How do you use PostGIS Raster?

[Pierre Racine]

Two things:

1) For ST_Intersection to be fast the total area covered by nbr_polygons must be relatively smaller than the modis area. If the intersecting surface is similar to your  modis area, ST_Intersection will try to vectorize all the raster and this takes generally too much time and produce too much vector data. 

2) For ST_Intersection to be fast with ST_Intersects your raster must be tiled when you import it with raster2pgsql using the -k option

If the polygon area is relatively small comparing with the raster area AND your raster is tiled, this kind of query should be (relatively) fast.

Pierre

> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net [mailto:postgis-users-
> bounces at postgis.refractions.net] On Behalf Of Bryce L Nordgren
> Sent: Tuesday, June 07, 2011 5:05 PM
> To: PostGIS Users Discussion
> Subject: Re: [postgis-users] How do you use PostGIS Raster?
> 
> 
> 
> postgis-users-bounces at postgis.refractions.net wrote on 06/07/2011 08:15:43
> PM:
> 
> > The first step is a
> > >"global" (image-wide) analysis. It was very slow when I tried it.
> > What if you tiled your landsat in smaller tiles (100x100)?
> >
> 
> I didn't get past the global analysis. On my little tutorial thing, the step that took
> 8 hours before I killed it was:
> 
> DATABASE=> CREATE TABLE tmp_burned AS
> DATABASE->  SELECT gid, ST_Collect(ST_Intersection(polygon, utm_geom)) as
> tmp_burned
> DATABASE->   FROM nbr_polygons, modis
> DATABASE->   WHERE ST_Intersects(polygon, utm_geom)
> DATABASE->   GROUP BY gid ;
> 
> 
> This is under "Calculate burned area in polygon coverage" on:
> https://collab.firelab.org/software/projects/modisfire/wiki/Calculating_histogr
> ams_with_PostGIS_Raster
> <https://collab.firelab.org/software/projects/modisfire/wiki/Calculating_histog
> rams_with_PostGIS_Raster>
> 
> Bryce