Can't get ST_MapAlgebra to return fractional values

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I have a 4-band raster dataset that I have imported into a Postgres/PostGIS table using raster2pgsql. I am Using ST_MapAlgebra() with a callback function to perform some band math on the dataset. I have set the return type as double precision. However, I am finding that anytime I return a decimal value (e.g., .25 or -.25) it only returns zero. However, if I return a whole number (e.g., 25 or -15), I can receive the values back from the call. I played around with this, but can’t figure out why this is occurring. Any help would be appreciated. Here is the syntax of my callback function:



CREATE OR REPLACE FUNCTION urban_veg.ndvi_callback (
 value double precision, 
 pos integer, 
 VARIADIC userargs text
) 
RETURNS double precision AS 
$$
 DECLARE
 r1_val double precision;
 r4_val double precision;
 result double precision;
 BEGIN
 r1_val := value[1][1][1]::double precision;
 r4_val := value[2][1][1]::double precision;
 result := (r4_val-r1_val)/(r4_val+r1_val);
 RETURN result;
 END;
$$ 
LANGUAGE 'plpgsql' IMMUTABLE COST 1000;


The call to the function is:



ST_MapAlgebra(
 ARRAY[
 ROW(rast, 1), 
 ROW(rast, 4) ]::rastbandarg, 
 'urban_veg.ndvi_callback(
 double precision,integer,text
 )'::regprocedure, '16BSI'::text)





postgis raster postgresql







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    up vote
    3
    down vote

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    I have a 4-band raster dataset that I have imported into a Postgres/PostGIS table using raster2pgsql. I am Using ST_MapAlgebra() with a callback function to perform some band math on the dataset. I have set the return type as double precision. However, I am finding that anytime I return a decimal value (e.g., .25 or -.25) it only returns zero. However, if I return a whole number (e.g., 25 or -15), I can receive the values back from the call. I played around with this, but can’t figure out why this is occurring. Any help would be appreciated. Here is the syntax of my callback function:



    CREATE OR REPLACE FUNCTION urban_veg.ndvi_callback (
     value double precision, 
     pos integer, 
     VARIADIC userargs text
    ) 
    RETURNS double precision AS 
    $$
     DECLARE
     r1_val double precision;
     r4_val double precision;
     result double precision;
     BEGIN
     r1_val := value[1][1][1]::double precision;
     r4_val := value[2][1][1]::double precision;
     result := (r4_val-r1_val)/(r4_val+r1_val);
     RETURN result;
     END;
    $$ 
    LANGUAGE 'plpgsql' IMMUTABLE COST 1000;


    The call to the function is:



    ST_MapAlgebra(
     ARRAY[
     ROW(rast, 1), 
     ROW(rast, 4) ]::rastbandarg, 
     'urban_veg.ndvi_callback(
     double precision,integer,text
     )'::regprocedure, '16BSI'::text)





    postgis raster postgresql







    share|improve this question

























      up vote
      3
      down vote

      favorite









      up vote
      3
      down vote

      favorite











      I have a 4-band raster dataset that I have imported into a Postgres/PostGIS table using raster2pgsql. I am Using ST_MapAlgebra() with a callback function to perform some band math on the dataset. I have set the return type as double precision. However, I am finding that anytime I return a decimal value (e.g., .25 or -.25) it only returns zero. However, if I return a whole number (e.g., 25 or -15), I can receive the values back from the call. I played around with this, but can’t figure out why this is occurring. Any help would be appreciated. Here is the syntax of my callback function:



      CREATE OR REPLACE FUNCTION urban_veg.ndvi_callback (
       value double precision, 
       pos integer, 
       VARIADIC userargs text
      ) 
      RETURNS double precision AS 
      $$
       DECLARE
       r1_val double precision;
       r4_val double precision;
       result double precision;
       BEGIN
       r1_val := value[1][1][1]::double precision;
       r4_val := value[2][1][1]::double precision;
       result := (r4_val-r1_val)/(r4_val+r1_val);
       RETURN result;
       END;
      $$ 
      LANGUAGE 'plpgsql' IMMUTABLE COST 1000;


      The call to the function is:



      ST_MapAlgebra(
       ARRAY[
       ROW(rast, 1), 
       ROW(rast, 4) ]::rastbandarg, 
       'urban_veg.ndvi_callback(
       double precision,integer,text
       )'::regprocedure, '16BSI'::text)





      postgis raster postgresql







      share|improve this question















      I have a 4-band raster dataset that I have imported into a Postgres/PostGIS table using raster2pgsql. I am Using ST_MapAlgebra() with a callback function to perform some band math on the dataset. I have set the return type as double precision. However, I am finding that anytime I return a decimal value (e.g., .25 or -.25) it only returns zero. However, if I return a whole number (e.g., 25 or -15), I can receive the values back from the call. I played around with this, but can’t figure out why this is occurring. Any help would be appreciated. Here is the syntax of my callback function:



      CREATE OR REPLACE FUNCTION urban_veg.ndvi_callback (
       value double precision, 
       pos integer, 
       VARIADIC userargs text
      ) 
      RETURNS double precision AS 
      $$
       DECLARE
       r1_val double precision;
       r4_val double precision;
       result double precision;
       BEGIN
       r1_val := value[1][1][1]::double precision;
       r4_val := value[2][1][1]::double precision;
       result := (r4_val-r1_val)/(r4_val+r1_val);
       RETURN result;
       END;
      $$ 
      LANGUAGE 'plpgsql' IMMUTABLE COST 1000;


      The call to the function is:



      ST_MapAlgebra(
       ARRAY[
       ROW(rast, 1), 
       ROW(rast, 4) ]::rastbandarg, 
       'urban_veg.ndvi_callback(
       double precision,integer,text
       )'::regprocedure, '16BSI'::text)





      postgis raster postgresql




      postgis raster postgresql






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      edited 8 hours ago









      ahmadhanb

      19.4k21746




      19.4k21746










      asked 8 hours ago









      geo derek

      180115




      180115




















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          You specify the return pixeltype to be '16BSI' (16bit signed integer); given this, ST_MapAlgebra won't return a float, not even with a gun to it's head...



          Try with '32BF' (32bit float).






          share|improve this answer




















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            up vote
            4
            down vote













            You specify the return pixeltype to be '16BSI' (16bit signed integer); given this, ST_MapAlgebra won't return a float, not even with a gun to it's head...



            Try with '32BF' (32bit float).






            share|improve this answer
























              up vote
              4
              down vote













              You specify the return pixeltype to be '16BSI' (16bit signed integer); given this, ST_MapAlgebra won't return a float, not even with a gun to it's head...



              Try with '32BF' (32bit float).






              share|improve this answer






















                up vote
                4
                down vote










                up vote
                4
                down vote









                You specify the return pixeltype to be '16BSI' (16bit signed integer); given this, ST_MapAlgebra won't return a float, not even with a gun to it's head...



                Try with '32BF' (32bit float).






                share|improve this answer












                You specify the return pixeltype to be '16BSI' (16bit signed integer); given this, ST_MapAlgebra won't return a float, not even with a gun to it's head...



                Try with '32BF' (32bit float).







                share|improve this answer












                share|improve this answer



                share|improve this answer










                answered 1 hour ago









                ThingumaBob

                4,8431222




                4,8431222



























                     

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