[postgis-users] Problem using ST_AsRaster

[Bborie Park]

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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-- 
Bborie Park
Programmer
Center for Vectorborne Diseases
UC Davis
530-752-8380
bkpark at ucdavis.edu