[postgis-users] Problem using ST_AsRaster

[Mark Wynter]

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 <bkpark at ucdavis.edu>
> Subject: Re: [postgis-users] Problem using ST_AsRaster
> To: postgis-users at postgis.refractions.net
> Message-ID: <4FE8A268.6050802 at 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.
>> 
>> _______________________________________________
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>> postgis-users at postgis.refractions.net
>> http://postgis.refractions.net/mailman/listinfo/postgis-users
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