[postgis-users] raster operations

[Regina Obe]

It doesn't grab adjacent tiles.  To consider adjacent tiles you'd need to use ST_Union to stitch together multiple tiles.

 

From: postgis-users [mailto:postgis-users-bounces at lists.osgeo.org] On Behalf Of David Haynes
Sent: Wednesday, June 13, 2018 12:56 PM
To: postgis-users at lists.osgeo.org
Subject: [postgis-users] raster operations

 

I have a question about how to focal operations for raster statistics are working. I started off comparing the old depreciated function st_mapalgebrafctngb with the current version st_mapalgebra. The difference I have determined is that the st_mapalgebrafctngb will pass the call back function to the raster dataset similar to the st_mapalgebra, but as the function approaches the edge of the raster tile things get wonky. What are values that are assigned to pixels that do not have sufficient neighbors? When I run this bit of code. The ST_Value function reports that no values have been assigned. I assume this can be remedied by adding information through userargs.

 

with smoothed_raster as

(

SELECT st_mapalgebrafctngb(rast, 1, NULL, 1, 1, 'st_mean4ma(double precision[][][],text,text[])'::regprocedure, 'ignore', NULL) as rast

FROM glc2000_clipped_250 r

WHERE r.rid = 25

), original_raster as

(

SELECT r.rast

FROM glc2000_clipped_250 r

WHERE r.rid = 25

)

SELECT x, y, ST_Value(s.rast, x,y) as smoothed, ST_Value(o.rast, x,y) as original

FROM smoothed_raster s, original_raster o

CROSS JOIN generate_series(1,10) as x

CROSS JOIN generate_series(1,10) as y

 

This leads me to a larger question regarding focal analysis functions for raster datasets. Do these functions cross the tiles? Assume you have 9 tiles and each tile is composed of 3x3 pixels. Tiles 1-3 in row 1, 4-6 row 2 etc. If you have a neighborhood analysis function performing on tile 5. Will it grab adjacent pixels in tile 6? Unfortunately the examples I created returned with the same result.

 

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