PostGIS
Name
ST_MapAlgebraExpr — 1 raster band version: Creates a new one band raster formed by applying a valid PostgreSQL algebraic operation on the input raster band and of pixeltype provided. Band 1 is assumed if no band is specified.
Synopsis
raster +`*`+ST_MapAlgebraExpr*(`raster `rast, integer
band, text pixeltype, text expression, double precision
nodataval=NULL`)`;
raster +`*`+ST_MapAlgebraExpr*(`raster `rast, text
pixeltype, text expression, double precision
nodataval=NULL`)`;
Description
|
Creates a new one band raster formed by applying a valid PostgreSQL
algebraic operation defined by the expression on the input raster
(rast). If no band is specified band 1 is assumed. The new
raster will have the same georeference, width, and height as the
original raster but will only have one band.
If pixeltype is passed in, then the new raster will have a band of
that pixeltype. If pixeltype is passed NULL, then the new raster band
will have the same pixeltype as the input rast band.
In the expression you can use the term [rast] to refer to the pixel
value of the original band, [rast.x] to refer to the 1-based pixel
column index, [rast.y] to refer to the 1-based pixel row index.
Availability: 2.0.0
Examples
Create a new 1 band raster from our original that is a function of modulo 2 of the original raster band.
ALTER TABLE dummy_rast ADD COLUMN map_rast raster;
UPDATE dummy_rast SET map_rast = ST_MapAlgebraExpr(rast,NULL,'mod([rast]::numeric,2)') WHERE rid = 2;
SELECT
ST_Value(rast,1,i,j) As origval,
ST_Value(map_rast, 1, i, j) As mapval
FROM dummy_rast
CROSS JOIN generate_series(1, 3) AS i
CROSS JOIN generate_series(1,3) AS j
WHERE rid = 2;
origval | mapval
---------+--------
253 | 1
254 | 0
253 | 1
253 | 1
254 | 0
254 | 0
250 | 0
254 | 0
254 | 0Create a new 1 band raster of pixel-type 2BUI from our original that is reclassified and set the nodata value to be 0.
ALTER TABLE dummy_rast ADD COLUMN map_rast2 raster;
UPDATE dummy_rast SET
map_rast2 = ST_MapAlgebraExpr(rast,'2BUI'::text,'CASE WHEN [rast] BETWEEN 100 and 250 THEN 1 WHEN [rast] = 252 THEN 2 WHEN [rast] BETWEEN 253 and 254 THEN 3 ELSE 0 END'::text, '0')
WHERE rid = 2;
SELECT DISTINCT
ST_Value(rast,1,i,j) As origval,
ST_Value(map_rast2, 1, i, j) As mapval
FROM dummy_rast
CROSS JOIN generate_series(1, 5) AS i
CROSS JOIN generate_series(1,5) AS j
WHERE rid = 2;
origval | mapval
---------+--------
249 | 1
250 | 1
251 |
252 | 2
253 | 3
254 | 3
SELECT
ST_BandPixelType(map_rast2) As b1pixtyp
FROM dummy_rast
WHERE rid = 2;
b1pixtyp
----------
2BUI
original (column rast_view) |
rast_view_ma |
Create a new 3 band raster same pixel type from our original 3 band raster with first band altered by map algebra and remaining 2 bands unaltered.
SELECT
ST_AddBand(
ST_AddBand(
ST_AddBand(
ST_MakeEmptyRaster(rast_view),
ST_MapAlgebraExpr(rast_view,1,NULL,'tan([rast])*[rast]')
),
ST_Band(rast_view,2)
),
ST_Band(rast_view, 3)
) As rast_view_ma
FROM wind
WHERE rid=167;