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9.6. GIS Book: Map algebra

The raster layer calculator (or simply raster calculator) is created to run algebraic expressions using various tools and operators through the interface of a simple calculator tool (Raster Calculator tool).

The performance of equations depends on the tools or operators involved in an expression. In general, data processing is faster if each operator or tool is executed individually (ESRI, 2016c). The versatility of the map algebra lies in its simplicity and power to perform geographical analysis.

A raster layer can be created from an algorithm by assigning numerical values to the pixels that compose it or it can be derived from one or more previously existing raster layers. A simple example is shown in Figure 47. On the left-hand side you can see the initial raster map, in which you want to execute a simple operation consisting of multiplying all the pixels of the layer with by factor 5. The result is stored in a new raster layer (Figure 47 right).

Figure 47. Basic map algebra operation

Practice: From the data in Table 3, you can create a raster image by interpolating the temperature values (see Figure 49, left), following the steps in section 9.1. The idea of this exercise is to familiarize yourself with the use of algebraic expressions using the Raster Calculator tool. The temperature raster layer is generated in Celsius Degrees, but if we want to convert the values to Fahrenheit (see Figure 49 right), the following equation must be applied within the raster image calculator:

where:

F = Temperature in degrees Fahrenheit

C = Temperature in degrees Celsius

The Raster Calculator tool for running algebra map expressions is located at:

ArcToolbox > Spatial Analyst Tools > Map Algebra > Raster Calculator

Figure 48. Configuration of expressions in Raster Calculator

The configuration of the Raster Calculator tool (Figure 48) is as follows:

Figure 49. Temperature raster in °C (left). Temperature raster in °F obtained through a map algebra expression (right).

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