The raster calculator is a tool that allows for the performance of operations and algebraic expressions using various tools and operators through an interface similar to a basic calculator. This tool is versatile and powerful, making it suitable for conducting complex geographic analyses during map development. However, the performance of these operations may depend on the structure of the expressions. Faster results are generally obtained when operators or tools are executed individually (ESRI, 2016c).
The “Raster Calculator” tool can be found in the following locations:
Geoprocessing > Toolboxes > Spatial Analyst Tools > Map Algebra > Raster Calculator
Geoprocessing > Toolboxes > Image Analyst Tools > Map Algebra > Raster Calculator
Basic Raster Operations
It is possible to create a raster layer from an expression that assigns values to its pixels or derives values from one or more existing raster layers. For instance, a simple example is multiplying all the pixels in a raster by a specific factor. This operation creates a new raster layer that can be used for further analysis or map creation.
Basic operation of map algebra.
In the example shown, the original raster is multiplied by the number 5. The result is saved as a new raster layer, which represents the recalculated pixel values. This function illustrates how map algebra can be used to quickly derive new datasets from existing information.
Temperature Conversion Example: Celsius to Fahrenheit
Another useful application of the raster calculator is converting a raster dataset of temperature values from degrees Celsius (°C) to degrees Fahrenheit (°F). This can be done using a simple mathematical formula:
Where:
F = Temperature in degrees Fahrenheit
C = Temperature in degrees Celsius
In the raster calculator interface, the Celsius raster should be replaced with the actual name of the raster layer. For example, the expression would look like:
("Kriging_T_°C" * 9 / 5) + 32
This expression generates a new raster that contains temperature values in Fahrenheit, based on the original Celsius raster. Parentheses are used to ensure the correct order of operations within the expression.
Raster Calculator Interface
Key components of the raster calculator interface include:
- Rasters: Lists the available raster layers for use in the expression.
- Tools: Offers various mathematical and logical operators, including trigonometric functions such as sine, cosine, and tangent, which can be inserted into the expression.
- Expression: Allows the user to define a set of rules for the equation. An equal sign (=) is not needed at the start of the expression. Expressions must be correctly grouped with parentheses to ensure proper evaluation.
- Output raster: Lets the user specify the destination folder or geodatabase where the resulting raster file will be saved.
Setting expressions in “Raster Calculator”.
Interpreting the Output
The final output of this process is a raster map where the temperature values are expressed in degrees Fahrenheit. This new raster can be used to create thematic maps that illustrate the spatial distribution of temperature across the analyzed area, making it easier to interpret environmental or climatic patterns.
Raster of temperature in °C (left). The raster of temperature in °F obtained through a map algebra expression (right).
