Database Reference
In-Depth Information
Performing
advanced
map-algebra
operations
In a prior recipe, we used the expression-based map-algebra function
ST_MapAlgebra()
to convert the PRISM pixel values to their true values. The
expression-based
ST_MapAlgebra()
methodiseasytousebutlimitedtooperating
onatmosttworasterbands.Thisrestrictsthe
ST_MapAlgebra()
function'suseful-
ness for processes that require more than two input raster bands, such as
Normal-
ized Difference Vegetation Index
(
NDVI
) and
Enhanced Vegetation Index
(
EVI
).
Thereisavariantof
ST_MapAlgebra()
designedtosupportanunlimitednumberof
inputrasterbands.Insteadoftakinganexpression,this
ST_MapAlgebra()
variant
requires a callback function. This callback function is run for each set of input pixel
values and returns either a new pixel value or
NULL
for the output pixel. Addition-
ally,thisvariantof
ST_MapAlgebra()
permitsoperationsonneighborhoods(setsof
pixels around a center pixel).
PostGIScomeswithasetofready-to-use
ST_MapAlgebra()
callbackfunctions.All
ofthesefunctionsareintendedforneighborhoodcalculations,suchascomputingthe
average value of a neighborhood or interpolating empty pixel values.
Getting ready
WewillusetheMODISrastertocomputetheEVI.
EVI
isathree-bandoperationcon-
sistingofthered,blue,andnear-infraredbands.Todoa
ST_MapAlgebra()
opera-
tiononthreebands,PostGIS2.1orahigherversionisrequired.IfyouhavePostGIS
2.0, a two-band EVI version is provided after we discuss the three-band variant.
How to do it...
Touse
ST_MapAlgebra()
onmorethantwobands,wemustusethecallbackfunc-
tionvariant.Thismeansweneedtocreateacallbackfunction.Callbackfunctionscan
bewritteninanyPostgreSQLPLlanguage,suchasPL/pgSQLorPL/R.Ourcallback
functionsareallwritteninPL/pgSQLasthislanguageisalwaysincludedwithabase
PostgreSQL installation.
