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Apply a map algebra script on a raster. 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. RS_MapAlgebra is backed by Jiffle and can be compiled to Java bytecode for execution. For guidance on scripting for map algebra, refer to the Jiffle language summary. RS_MapAlgebra
The RS_MapAlgebra function can cast the output raster to a different data type specified by pixelType:
  • If pixelType is smaller than the input raster data type, narrowing casts will be performed, which may result in loss of data.
  • If pixelType is larger, widening casts will retain data accuracy.
  • If pixelType matches the input raster data type, no casting occurs.
This allows controlling the output pixel data type. Users should consider potential precision impacts when coercing to a smaller type.

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Signatures

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Single raster input

RS_MapAlgebra(rast: Raster, pixelType: String, script: String)

RS_MapAlgebra(rast: Raster, pixelType: String, script: String, noDataValue: Double)

RS_MapAlgebra(rast: Raster, pixelType: String, script: String, noDataValue: Double, numBands: Int)

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Two raster inputs

RS_MapAlgebra(rast0: Raster, rast1: Raster, pixelType: String, script: String, noDataValue: Double)

RS_MapAlgebra(rast0: Raster, rast1: Raster, pixelType: String, script: String, noDataValue: Double, numBands: Int)

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Parameters

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rast
Raster
required
The input raster to apply the map algebra expression to. In the script, the rast variable is bound to this raster.
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rast0
Raster
required
The first raster to apply the map algebra expression to (for two raster input signatures).
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rast1
Raster
required
The second raster to apply the map algebra expression to (for two raster input signatures).
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pixelType
String
required
The data type of the output raster. Accepts one of: D (double), F (float), I (integer), S (short), US (unsigned short) or B (byte). If specified NULL, the output raster will have the same data type as the input raster.
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script
String
required
The map algebra script written in Jiffle. The rast variable (or rast0/rast1 for two raster inputs) is bound to the input raster(s), and the out variable is bound to the output raster. Use bracket notation to access bands, e.g. rast[0] for the first band.
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noDataValue
Double
The nodata value of the output raster. null is allowed.
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numBands
Int
The number of bands in the output raster. When specified, the output bands can be referenced in the script using out[0], out[1], etc.

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Return type

The resulting raster.

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Examples

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NDVI calculation

Calculate the NDVI of a raster with 4 bands (R, G, B, NIR): 
SELECT RS_MapAlgebra(rast, 'D', 'out = (rast[3] - rast[0]) / (rast[3] + rast[0]);') AS ndvi FROM raster_table

+--------------------+
|              raster|
+--------------------+
|GridCoverage2D["g...|
+--------------------+

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Two raster inputs

SELECT RS_MapAlgebra(rast0, rast1, 'D', 'out = rast0[0] * 0.5 + rast1[0] * 0.5;', null) FROM raster_table

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Multi-band output

SELECT RS_MapAlgebra(rast, 'D', 'out[0] = rast[0] - rast[1]; out[1] = rast[0] + rast[1];', null, 2) FROM raster_table