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Map Algebra

Map algebra is a way to perform raster calculations using mathematical expressions. The expression can be a simple arithmetic operation or a complex combination of multiple operations. The expression can be applied to a single raster band or multiple raster bands. The result of the expression is a new raster. Apache Sedona provides two ways to perform map algebra operations:
  1. Using the RS_MapAlgebra function.
  2. Using RS_BandAsArray and array based map algebra functions, such as RS_Add, RS_Multiply, etc.
Generally, the RS_MapAlgebra function is more flexible and can be used to perform more complex operations. We’ll demonstrate both approaches to implementing commonly used map algebra operations below.

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NDVI

The Normalized Difference Vegetation Index (NDVI) is a simple graphical indicator that can be used to analyze remote sensing measurements, typically, but not necessarily, from a space platform, and assess whether the target being observed contains live green vegetation or not. NDVI has become a de facto standard index used to determine whether a given area contains live green vegetation or not. The NDVI is calculated from these individual measurements as follows: 
NDVI = (NIR - Red) / (NIR + Red)
where NIR is the near-infrared band and Red is the red band. Assume that we have a bunch of rasters with 4 bands: red, green, blue, and near-infrared. We want to calculate the NDVI for each raster. We can use the RS_MapAlgebra function to do this: 
SELECT RS_MapAlgebra(rast, 'D', 'out = (rast[3] - rast[0]) / (rast[3] + rast[0]);') as ndvi FROM raster_table
The Jiffle script is out = (rast[3] - rast[0]) / (rast[3] + rast[0]);. The rast variable is always bound to the input raster, and the out variable is bound to the output raster. Jiffle iterates over all the pixels in the input raster and executes the script for each pixel. the rast[3] and rast[0] refers to the current pixel values of the near-infrared and red bands, respectively. The out variable is the current output pixel value. The result of the RS_MapAlgebra function is a raster with a single band. The band is of type double, since we specified D as the pixelType argument. We can implement the same NDVI calculation using the array based map algebra functions: 
SELECT RS_Divide(
        RS_Subtract(RS_BandAsArray(rast, 1), RS_BandAsArray(rast, 4)),
        RS_Add(RS_BandAsArray(rast, 1), RS_BandAsArray(rast, 4))) as ndvi FROM raster_table
The RS_BandAsArray function extracts the specified band of the input raster to an array of double, and the RS_Add, RS_Subtract, and RS_Divide functions perform the arithmetic operations on the arrays. The code using the array based map algebra functions is more verbose. However, there is a RS_NormalizedDifference function that can be used to calculate the NDVI more concisely: 
SELECT RS_NormalizedDifference(RS_BandAsArray(rast, 1), RS_BandAsArray(rast, 4)) as ndvi FROM raster_table
The result of array based map algebra functions is an array of double. User can use RS_AddBandFromArray to add the array to a raster as a new band.

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AWEI

The Automated Water Extraction Index (AWEI) is a spectral index that can be used to extract water bodies from remote sensing imagery. The AWEI is calculated from these individual measurements as follows: 
AWEI = 4 * (Green - SWIR2) - (0.25 * NIR + 2.75 * SWIR1)
AWEI can be implemented easily using RS_MapAlgebra: 
-- Assume that the raster includes all 13 Sentinel-2 bands
SELECT RS_MapAlgebra(rast, 'D', 'out = 4 * (rast[2] - rast[11]) - (0.25 * rast[7] + 2.75 * rast[12]);') as awei FROM raster_table
We can also implement the same AWEI calculation using array based map algebra functions. The code looks more verbose: 
SELECT RS_Subtract(
    RS_Add(RS_MultiplyFactor(band_nir, 0.25), RS_MultiplyFactor(band_swir1, 2.75)),
    RS_MultiplyFactor(RS_Subtract(band_swir2, band_green), 4)) as awei
FROM (
SELECT RS_BandAsArray(rast, 3) AS band_green,
       RS_BandAsArray(rast, 12) AS band_swir2,
       RS_BandAsArray(rast, 13) AS band_swir1,
       RS_BandAsArray(rast, 8) AS band_nir
FROM raster_table) t

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Further Reading