You could try variants 9 or 10 (the last two) described in the docs... http://postgis.refractions.net/documentation/manual-svn/RT_ST_AsRaster.html
You'll probably need to call ST_Rescale afterwards to change the scale of the resulting raster to match that of your reference raster. By default, ST_AsRaster returns a raster having the extent of the input geometry's bounding box. -bborie On 06/25/2012 06:48 PM, Mark Wynter wrote: > Hi Bborie > > As part of a pl/pgsql update process, my requirement is to create a "new" > raster tile with same alignment (AND upperleftx, upperlefty, block > dimensions) as the reference raster tile. For the new tile, I then want to > set the pixel values = 100 for those pixels which intersect the "updated" > vector polygon. I'm not interested in parts of the vector polygon beyond > the extent of the reference tile. > > The reference raster is: > CREATE TABLE dummy_rast (rid integer, rast raster) WITH (OIDS=FALSE); > INSERT INTO dummy_rast VALUES(1, ST_MakeEmptyRaster(30, 30, 576000, -3780375, > 50, -30, 0, 0, 3577)); > UPDATE dummy_rast SET rast = ST_AddBand(rast, 1, '8BUI', NULL); > SELECT AddRasterConstraints('dummy_rast'::name, 'rast'::name); > > As an approach, my initial thoughts were to rasterise the vector layer using > ST_AsRaster. > > From scratch, the vector layer comprises two overlapping polygons which > protrude beyond the extent of the reference tile... > CREATE TABLE viewshed_vectors (gid integer, geometry geometry(polygon, 3577)) > WITH (OIDS=FALSE); > INSERT INTO viewshed_vectors VALUES(1, > ST_Polygon(ST_LineFromWKB(ST_AsBinary(ST_GeomFromText('LINESTRING(576500 > -3780500, 577000 -3780500, 577000 -3781000, 576500 -3781000, 576500 > -3780500)')),3577),3577)); > INSERT INTO viewshed_vectors VALUES(2, > ST_Polygon(ST_LineFromWKB(ST_AsBinary(ST_GeomFromText('LINESTRING(576750 > -3780250, 577250 -3780250, 577250 -3780750, 576750 -3780750, 576750 > -3780250)')),3577),3577)); > > To rasterise the viewshed_vectors... > CREATE TABLE viewshed_rast AS > WITH vt as (SELECT ST_Union(geometry) as geometry FROM viewshed_vectors) > SELECT rt.rid, ST_AsRaster(vt.geometry, rt.rast, '8BUI', 100, NULL) as rast > FROM vt, dummy_rast as rt; > SELECT AddRasterConstraints('viewshed_rast'::name, 'rast'::name); > > SELECT ST_SameAlignment(rt.rast, vt.rast) as sm FROM dummy_rast as rt, > viewshed_rast as vt; > sm > ---- > t > > The result when mapped in QGIS is as per this updated screenshot. > > Is there another step I must now do using ST_MapAlgebra to burn the > (intersecting) viewshed_rast values to the underlying dummy_rast tile? What > might that expression look like? > > Should I be approaching this a different way? > > I appreciate your thoughts and help. > > Best regards > > > > >> >> Message: 18 >> Date: Mon, 25 Jun 2012 10:39:52 -0700 >> From: Bborie Park <bkp...@ucdavis.edu> >> Subject: Re: [postgis-users] Problem using ST_AsRaster >> To: postgis-users@postgis.refractions.net >> Message-ID: <4fe8a268.6050...@ucdavis.edu> >> Content-Type: text/plain; charset=ISO-8859-1 >> >> Hi Mark, >> >> Can you elaborate on what you mean by "resultant raster does not map to >> the reference raster"? >> >> The output from ST_AsRaster should result in a raster with the same >> SRID, scale and skew as the reference raster. The output raster should >> also be aligned with the reference raster as tested by ST_SameAlignment. >> >> -bborie >> >> On 06/23/2012 11:04 PM, Mark Wynter wrote: >>> I can rasterise a vector layer, but I'm having trouble mapping it to a >>> reference raster. >>> >>> The reference raster, called dummy_rast is a 1x1 raster tile with a height >>> and width of 500pixels, each of 250m in size. I created using a pl/pgsql >>> function: >>> SELECT make_tiled_raster('public', 'dummy_rast', 576000, -3780000, 1, 1, >>> 500, 500, 250, -250); >>> The result is >>> >>> srid | scale_x | scale_y | blocksize_x | blocksize_y | num_bands | >>> pixel_types | nodata_values >>> ------+---------+---------+-------------+-------------+-----------+-------------+--------------- >>> 3577 | 250 | -250 | 500 | 500 | 1 | {8BUI} >>> | {NULL} >>> >>> >>> I now wish to burn a vector layer onto this raster: >>> >>> CREATE TABLE viewshed_rast AS >>> WITH vt as (SELECT ST_Union(geometry) as geometry FROM viewshed_vectors) >>> SELECT rt.rid, ST_AsRaster(vt.geometry, rt.rast, '8BUI', 120, 100) as rast >>> FROM dummy_rast as rt, vt; >>> >>> The result is >>> srid | scale_x | scale_y | blocksize_x | blocksize_y | num_bands | >>> pixel_types | nodata_values >>> ------+---------+---------+-------------+-------------+-----------+-------------+--------------- >>> 3577 | 250 | -250 | 67 | 38 | 1 | {8BUI} >>> | {100} >>> (1 row) >>> >>> I do not understand why the resultant raster does not map to the reference >>> raster? Refer screenshot attached showing the resultant layers in QGIS. >>> The upperleftx and upperlefty, and the block size of the resultant raster >>> are defined by the extent of the vector layer and not the reference raster. >>> >>> >>> Is there something obvious I'm doing wrong? Thanks. >>> >>> _______________________________________________ >>> postgis-users mailing list >>> postgis-users@postgis.refractions.net >>> http://postgis.refractions.net/mailman/listinfo/postgis-users >>> >>> >>> _______________________________________________ >>> postgis-users mailing list >>> postgis-users@postgis.refractions.net >>> http://postgis.refractions.net/mailman/listinfo/postgis-users -- Bborie Park Programmer Center for Vectorborne Diseases UC Davis 530-752-8380 bkp...@ucdavis.edu _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users