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).

**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*

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

**Layers and variables:**Displays a list of layers available for use in the map algebra expression.**Operators:**Allows you to enter mathematical and logical operations in the expression. Click on the calculator button to enter the number or operator into the expression.**Tools:**Displays a list of tools (sine, cosine, tangent) to use anywhere in the expression.**Expression:**This allows you to create a set of rules to execute a valid statement. It is not necessary to place the sign “*equal*” (=) at the beginning of the expression. In the equation for converting degrees Celsius to Fahrenheit, replace**“C”**with the raster image name:**“T_°C” * 9 / 5 + 32**. Use parentheses to group algebraic expressions, if needed.**Output raster:**Select a directory or geodatabase to store the raster file generated by the resulting expression.

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