[postgis-users] PostGIS ST_MapAlgebra Assistance

[Paragon Corporation]

The right doc link is

http://postgis.net/docs/manual-dev/RT_ST_MapAlgebra_expr.html

Please note the API changed between 2.0 and 2.1.  If you are working with
2.0 or need your app to support 2.0, you need to use this syntax
http://postgis.net/docs/manual-dev/RT_ST_MapAlgebraExpr2.html

Hope that helps,
Regina
http://www.postgis.us
http://postgis.net
 

-----Original Message-----
From: postgis-users-bounces at lists.osgeo.org
[mailto:postgis-users-bounces at lists.osgeo.org] On Behalf Of Pierre Racine
Sent: Monday, December 16, 2013 2:23 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] PostGIS ST_MapAlgebra Assistance

Assuming your three rasters would be in three different tables and not
tiled, it could look like this: 

WITH
solandcov AS (SELECT ST_MapAlgebra(a.rast, b.rast, '([rast1] >= 1491 and
[rast2] <= 70)::int', "4BUI") FROM sol_gain_clipped a, can_cov_01 b), creek
AS (SELECT ST_MapAlgebra(a.rast, b.rast, '([rast1] and [rast2] <= 10)::int',
"4BUI") FROM solandcov a, imp_sur_10_french_creek b) SELECT ST_AsTiff(rast)
FROM creek;

I'm using the last variant of ST_MapAlgebra from this page:

http://postgis.refractions.net/documentation/manual-svn/RT_ST_MapAlgebra_exp
r.html

If your rasters ares tiled you have to add a WHERE clause to the two first
statements to make sure only aligned tiles get involved in ST_MapAlgebra. I
generally use 

WHERE ST_UpperLeftX(a.rast) = ST_UpperLeftX(b.rast) AND
ST_UpperLeftY(a.rast) = ST_UpperLeftY(b.rast)

for that your tiles have to be well aligned. If not then just use
ST_Intersects(a.rast, b.rast) but that should be slower.

Pierre

> -----Original Message-----
> From: postgis-users-bounces at lists.osgeo.org [mailto:postgis-users- 
> bounces at lists.osgeo.org] On Behalf Of Jason Coombs
> Sent: Monday, December 16, 2013 9:22 AM
> To: postgis-users at lists.osgeo.org
> Subject: [postgis-users] PostGIS ST_MapAlgebra Assistance
> 
> Good Morning,
> I would like to perform the equivalent of the below ArcGIS raster 
> calculator equation in PostGIS using the ST_MapAlgebra function and 
> return it as a tif using ST_AsTIFF.
> 
> Con(("Sol_gain_clipped.tif" >= 1491)  &  ("can_cov_01.tif" <= 70)  & 
> ("imp_sur_10_French_Creek.tif" <= 10),1,0)
> 
> The equation uses three single band rasters with pixel value 
> specifications for each one, and returns a single band raster with 
> values of 1 for pixels meeting all three requirements, and 0 for 
> pixels that do not. Any help in how to write the SQL, including PL/pgSQL,
would be greatly appreciated.
> 
> Best, Jason
> 
> 
> 
> Jason A. Coombs
> 
> Department of Environmental Conservation/US Forest Service
> 
> 201 Holdsworth Hall
> 
> University of Massachusetts
> 
> Amherst, MA 01003
> 
> 
> 
> https://bcrc.bio.umass.edu/pedigreesoftware/
> <https://bcrc.bio.umass.edu/pedigreesoftware/>
> 
> 
> 
> Phone: 413-545-1845
> 
> Fax: 413-545-1860
> 
> Email: jcoombs at cns.umass.edu <mailto:jcoombs at cns.umass.edu>
> 
> 

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