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How to use R scripts in QGIS Processing

QGIS is a popular and open source desktop geographic information system (GIS) program that supports various spatial analysis tools and formats. R is a powerful statistical programming language that can be used for geospatial data processing and visualization. In this article, we will show you how to use R scripts in QGIS Processing, which is a framework that allows you to run algorithms from different providers, such as GDAL, SAGA, GRASS, or R.

Prerequisites

To use R scripts in QGIS Processing, you need to have the following installed on your computer:

qgis r

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QGIS >= 3.14.16 (download instructions for all platforms are available at https://download.qgis.org/)

R and the required packages (such as sp, rgdal, and raster)

The Processing R Provider plugin for QGIS (you can install it from the Plugins menu in QGIS)

Creating and running R scripts

The easiest way to create an R script is to open the Processing toolbox and choose Create new R scriptâ from the R menu (labelled with an R icon) at the top of the Processing Toolbox. You can also create the script in a text editor and save it in your R scripts folder (processing/rscripts). When it has been saved there, it will be available for editing by right-clicking on the script name in the processing toolbox and then choosing Edit Scriptâ).

In the script editor window, you have to specify some parameters before you can add the script body. For example:

##plots=group # The name of the group in which you want to put your script

##showplots # You want to display a plot

##Layer=vector # The input is a vector layer

##X=Field Layer # The input field of the vector layer

##My box plot script=name # The name of your script

After defining the parameters, you can write your R code using the variables that correspond to the parameters. For example:

boxplot(Layer[[X]]) # Create a box plot of the input field

To run your script, you can double-click on it in the Processing toolbox or right-click on it and choose Executeâ . You will then see a dialog window where you can select the input layer and field, and optionally change the output file name and location. After clicking Run, you will see a link to the plot in the Result Viewer panel (can be turned on / off in View âº Panels and with Processing âº Results Viewer).

Conclusion

In this article, we have shown you how to use R scripts in QGIS Processing, which is a convenient way to integrate your own R algorithms in QGIS. You can also use QGIS Processing algorithms from R using the qgisprocess package, which provides an R interface to QGIS. For more information and examples on using R scripts in QGIS Processing, you can check out the QGIS documentation or this blog post.

Advanced tips

If you want to create more complex R scripts in QGIS Processing, here are some advanced tips that you may find useful:

You can use the @qgis_output variable to write the output of your script to a file. For example, if you want to save your plot as a PNG image, you can use something like this:

png(@qgis_output) # Set the output file name

boxplot(Layer[[X]]) # Create a box plot of the input field

dev.off() # Close the device

You can use the @alg decorator to define the parameters and outputs of your script in a more concise and flexible way. For example, you can use something like this:

@alg(name="my_box_plot_script", label="My box plot script", group="plots", group_label="Plots")

# Define the name, label, group, and group_label of your script

@alg.input(type=alg.VECTOR_LAYER, name="INPUT", label="Input layer")

# Define an input parameter of type vector layer

@alg.input(type=alg.FIELD, name="FIELD", label="Input field", parentLayerParameterName="INPUT")

# Define an input parameter of type field, linked to the input layer

@alg.output(type=alg.FILE, name="OUTPUT", label="Output file", defaultFileExtension="png")

# Define an output parameter of type file, with a default extension of png

png(alg.getOutputValue("OUTPUT")) # Set the output file name using the output parameter

boxplot(INPUT[[FIELD]]) # Create a box plot of the input field using the input parameters

dev.off() # Close the device

You can use QGIS expressions in your R scripts to access various QGIS features and functions. For example, you can use something like this:

##showplots

##Layer=vector

##X=Field Layer

##Y=Field Layer

# Create a scatter plot of two fields with QGIS expressions for labels and title

plot(Layer[[X]], Layer[[Y]],

xlab = qgis_expression("@layer_name ' (' @field_name ')'"),

ylab = qgis_expression("@layer_name ' (' @field_name ')'"),

main = qgis_expression("concat('Scatter plot of ', @layer_name)"))

For more details and examples on using the @alg decorator and QGIS expressions in R scripts, you can check out this vignette from the qgisprocess package. 06063cd7f5

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