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.
>>
>> _______________________________________________
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>> postgis-users@postgis.refractions.net
>> http://postgis.refractions.net/mailman/listinfo/postgis-users
<<attachment: dummy_rast.png>>
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