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

[David M. Kaplan]

Hi,

Yes, this does seem to be the solution. For completeness, the following
did the same thing as my aggregate function:

SELECT ST_Union(rast,1,'SUM') FROM blah;
 
This includes replacing 0's with NULL's - ST_Union had the same
behavior at my aggregate function. 

Can you explain this? For my sum, this isn't a big deal, but
ST_Union(... 'MEAN') seems to be treating 0's at NULL values, thereby
removing them from the mean calculation:

db=# SELECT (ST_SummaryStats(ST_Union(rast,1,'SUM'))).sum FROM blah; sum  
------
 2792
(1 row)

db=# SELECT (ST_SummaryStats(ST_Union(rast,1,'MEAN'))).sum FROM blah;
 sum  
------
 1346
(1 row)

The table "blah" has 3 rasters with no null values, so I would expect
the sum of the mean to be 1/3 the sum of the sum, but clearly this is
not the case.

I tried playing around with the no data value for my rasters to see if
this would change things. It does change things, but not in any way I
can explain:

fads=# SELECT (ST_SummaryStats(ST_Union(ST_SetBandNoDataValue(rast,1,0),1,'MEAN'))).sum FROM blah;
 sum  
------
 2025
(1 row)

fads=# SELECT (ST_SummaryStats(ST_Union(ST_SetBandNoDataValue(rast,1,-999),1,'MEAN'))).sum FROM blah;
 sum  
------
 2025
(1 row)

Setting the no data value has no effect on the sum of the sum.

Does any of this make sense?

Thanks,
David


On Mon, 2018-03-05 at 12:00 -0800, postgis-users-
request at lists.osgeo.org wrote:
> Date: Mon, 5 Mar 2018 12:05:40 -0500
> From: "Regina Obe" <lr at pcorp.us>
> To: "'PostGIS Users Discussion'" <postgis-users at lists.osgeo.org>
> Subject: Re: [postgis-users] Aggregating rasters by adding and other
>         confusions
> Message-ID: <001f01d3b4a4$30d1fa20$9275ee60$@pcorp.us>
> Content-Type: text/plain; charset="utf-8"
> 
> I think what you are looking for is ST_Union (.. SUM)  note this has union types – FIRST, MIN, MAX, COUNT, SUM, MEAN, RANGE
> 
>  
> 
> http://postgis.net/docs/manual-2.4/RT_ST_Union.html
> 
>  
> 
>  
> 
>  
> 
> From: postgis-users [mailto:postgis-users-bounces at lists.osgeo.org] On Behalf Of David M. Kaplan
> Sent: Monday, March 05, 2018 10:03 AM
> To: postgis-users at lists.osgeo.org
> Subject: [postgis-users] Aggregating rasters by adding and other confusions
> 
>  
> 
> 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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