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-boun...@lists.osgeo.org] On Behalf Of
David Haynes
Sent: Wednesday, June 13, 2018 12:56 PM
To: postgis-users@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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