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Book Overview & Buying
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Table Of Contents
PostGIS Cookbook
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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() method is easy to use but limited to operating on at most two raster bands. This restricts the ST_MapAlgebra() function's usefulness for processes that require more than two input raster bands, such as
Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI).
There is a variant of ST_MapAlgebra() designed to support an unlimited number of input raster bands. Instead of taking an expression, 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. Additionally, this variant of ST_MapAlgebra() permits operations on neighborhoods (sets of pixels around a center pixel).
PostGIS comes with a set of ready-to-use ST_MapAlgebra...
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