[postgis-users] Retrieving Data from PostGIS Raster using ST_Value

[Pierre Racine]

Please read "In any case you should TILE your raster when loading it " in the previous mail...

> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net [mailto:postgis-users-
> bounces at postgis.refractions.net] On Behalf Of Pierre Racine
> Sent: Friday, October 14, 2011 3:32 PM
> To: PostGIS Users Discussion
> Subject: Re: [postgis-users] Retrieving Data from PostGIS Raster using ST_Value
> 
> If you want to get the pixel value under a point just do:
> 
> SELECT ST_Value(rast, ST_Transform(ST_SetSRID(ST_Point(yourLongitude,
> yourLatitude), 4326),  ST_SRID(rast)) FROM yourTable
> 
> That should be pretty fast.
> 
> In any case you should load your raster when loading it (-k100x100 or -k 20x20)
> and then index it:
> 
> CREATE INDEX srtm_tiled_rast_gist_idx  ON srtm_tiled USING GIST
> (ST_ConvexHull(rast));
> 
> Interpolating the elevation of a point is another story. You could build a grid of
> 8, 15 or 24 points around your lat/long, query the elevation at those points and
> use your algorithm to compute the interpolation.
> 
> Really getting the elevation for 1 or 25 points should be a matter of
> microseconds.
> 
> Pierre
> 
> > -----Original Message-----
> > From: postgis-users-bounces at postgis.refractions.net
> > [mailto:postgis-users- bounces at postgis.refractions.net] On Behalf Of
> > elliott
> > Sent: Thursday, October 13, 2011 9:38 AM
> > To: PostGIS Users Discussion
> > Subject: Re: [postgis-users] Retrieving Data from PostGIS Raster using
> > ST_Value
> >
> > The table contains SRTM3 data.  What I would really like to do is
> > retrieve the altitude for any given lat/lon.  We currently have
> > scientific codes that read the
> > SRTM3 flat file into an array and interpolate the value for the
> > lat/lon pair.  We would like to retrieve the data from a database
> > instead of flat file thinking the performance would be better.
> > However, the times retrieving the array from the database are many
> > orders of magnitude worse than just reading the array in from a flat file.
> >
> > On 10/12/2011 8:34 PM, Pierre Racine wrote:
> > > If you want to get all the values for every pixel do:
> > >
> > > SELECT (ST_DumpAsPolyogns(rast)).val FROM your table
> > >
> > > Can I ask why you want to do that? Normally people want to extract
> > > values
> > intersecting with a polygon or a point.
> > >
> > > You can also get the histogram of the raster using ST_Histogram().
> > >
> > > Pierre
> > > ________________________________________
> > > From: postgis-users-bounces at postgis.refractions.net
> > > [postgis-users-bounces at postgis.refractions.net] On Behalf Of elliott
> > > [elliott at cpi.com]
> > > Sent: Wednesday, October 12, 2011 9:04 AM
> > > To: PostGIS Users Discussion
> > > Subject: [postgis-users] Retrieving Data from PostGIS Raster using
> > > ST_Value
> > >
> > > Hello Everyone,
> > >
> > > I am very new to using PostGIS Raster.  I need to retrieve data
> > > fields from a tile loaded into the database.  Using ST_Height and
> > > ST_Width, I get the number of rows and columns and iterate over
> > > these using the ST_Value to retrieve the data.  This is extremely
> > > slow taking 1/2 hour to get all of the data required.  Is there a faster way to
> accomplish this?
> > >
> > > Thanks!
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