[postgis-users] Aggregating rasters by adding and other confusions

[David M. Kaplan]

Hi,

I have recently started working with the postgis raster functionality.
In general, I have found this really useful and have been able to do
some neat things fairly simply with this raster functionality.
Nevertheless, there are a few basic things that I am confused about and
I was hoping someone could give me a hand.

(1) First of all, I have a table with a bunch of rasters that have the
same extent, alignment, scale, etc. and I want to aggregate them
together into a single raster using pixel-by-pixel addition. It seems
like there should be a function to do this, but I can't find one. Is
there an aggregate "ST_MapAlgebra" function? 

Given that I couldn't find one, I defined an aggregate function as
follows:

CREATE OR REPLACE FUNCTION AddRasters(r1 raster, r2
raster)       RETURNS raster AS$BODY$SELECT
ST_MapAlgebra($1,$2,'[rast1]+[rast2]');$BODY$ LANGUAGE 'sql' IMMUTABLE
STRICT;
CREATE AGGREGATE sum (raster)(    sfunc = AddRasters,    stype =
raster);
(2) This seems to work, but it has the unexpected behavior that it
replaces 0 values with NULL. In my case, this is fine, but I am
wondering why it does this? I can't find anything that indicates that
it should be replacing zeros with NULL. Here is the metadata associated
with one of my rasters (the others are similar):

# SELECT ST_BandMetadata(rast), ST_Metadata(rast),
ST_SummaryStats(rast) FROM blah;-[ RECORD 1 ]---+--------------------
-----------------------------------st_bandmetadata |
(16BUI,,f,)st_metadata     | (-180,90,360,180,1,-
1,0,0,4326,1)st_summarystats |
(64800,417,0.00643518518518519,0.223617719977485,0,46)
I have not defined a nodataval for these layers and the original layers
have no NULL values.

(3) Is there a postgis command to turn all the NULL values back into
zeros?

(4) I was also considering just defining the '+' operator for raster +
raster to be pixel-by-pixel addition. Is there any reason that I
wouldn't want to do this?

(5) Finally, I have been visualizing results with QGIS using the DB
Manager. However, I don't see how to select a row from a raster table
and incorporate just that row into the canvas. Is there a way to do
this?

Thanks for the assistance,
David




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