# Raster Map Algebra

## 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:

- Using the
`RS_MapAlgebra`

function. - 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. The `RS_MapAlgebra(rast, pixelType, script, [noDataValue])`

function takes three to four arguments:

`rast`

: The raster to apply the map algebra expression to.`pixelType`

: The data type of the output raster. This can be 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.`script`

: The map algebra script.`noDataValue`

: (Optional) The nodata value of the output raster.

`RS_MapAlgebra`

also has good performance, since it is backed by Jiffle and can be compiled to Java bytecode for
execution. We'll demonstrate both approaches to implementing commonly used map algebra operations.

### 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.

### 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
```