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provide an example of map algebra
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@ -3295,7 +3295,7 @@ WHERE A.rid =2 ) As foo;
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<refentry id="RT_ST_MapAlgebra">
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<refnamediv>
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<refname>ST_MapAlgebra</refname>
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<refpurpose>Creates a new raster formed by applying algebraic operations defined by the expression on the input raster and of pixeeltype provided. band 1 is assumed if no band is specified.</refpurpose>
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<refpurpose>Creates a new one band raster formed by applying a valid PostgreSQL algebraic operations on the input raster band and of pixeeltype provided. band 1 is assumed if no band is specified.</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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@ -3345,8 +3345,11 @@ WHERE A.rid =2 ) As foo;
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<refsection>
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<title>Description</title>
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<para>Creates a new raster formed by applying algebraic operations defined by the expression on the input raster. If no band number is specified
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band 1 is assumed.</para>
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<para>Creates a new one band raster formed by applying algebraic operations defined by the <varname>expression</varname> on the input raster (<varname>rast</varname>). If no <varnum>band</varnum> is specified
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band 1 is assumed. The new raster will have the same georeference, width, and height as the original raster but will only have one band.</para>
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<para>If <varname>pixeltype</varname> is passed in, then the new raster will have a band of that pixeltype. If no pixeltype is passed in, then the new raster band will have the same pixeltype as the input <varname>rast</varname></para>
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<para>Use the term <varname>rast</varname> to refer to the band you are changing.</para>
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<para>Availability: 2.0.0 </para>
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</refsection>
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@ -3354,12 +3357,31 @@ WHERE A.rid =2 ) As foo;
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<refsection>
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<title>Examples</title>
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<!-- TODO -->
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<para>Create a new 1 band raster from our original that is a function of modulo 2 of the original raster band.</para>
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<programlisting>ALTER TABLE dummy_rast ADD COLUMN map_rast raster;
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UPDATE dummy_rast SET map_rast = ST_MapAlgebra(rast,'mod(rast,2)') WHERE rid = 2;
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SELECT ST_Value(rast,1,i,j) As origval, ST_Value(map_rast, 1, i, j) As mapval
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FROM dummy_rast CROSS JOIN generate_series(1, 3) AS i CROSS JOIN generate_series(1,3) AS j
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WHERE rid = 2;
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origval | mapval
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---------+--------
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253 | 1
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254 | 0
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253 | 1
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253 | 1
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254 | 0
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254 | 0
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250 | 0
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254 | 0
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254 | 0
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</programlisting>
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</refsection>
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<refsection>
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<title>See Also</title>
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<para></para>
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<para><xref linkend="RT_ST_GeoReference" />, <xref linkend="RT_ST_BandPixelType" /></para>
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</refsection>
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</refentry>
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