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RQGIS establishes an interface between R and QGIS, i.e. it allows the user to access QGIS functionalities from within R. It achieves this by establishing a tunnel to the Python QGIS API via the reticulate-package. This provides the user with an extensive suite of GIS functions, since QGIS allows you to call native as well as third-party algorithms via its processing framework (see also https://docs.qgis.org/2.14/en/docs/user_manual/processing/index.html). Third-party providers include among others GDAL, GRASS GIS, SAGA GIS, the Orfeo Toolbox, TauDEM and tools for LiDAR data. RQGIS brings you this incredibly powerful geoprocessing environment to the R console.
Please check also out our paper presenting RQGIS in detail:
The main advantages of RQGIS are:
In order to run RQGIS properly, you need to download various third-party software packages. Our vignette should help you with the download and installation procedures on various platforms (Windows, Linux, Mac OSX). To access it, use
vignette("install_guide", package = "RQGIS"). Overall, we recommend to use the current LTR of QGIS (2.14) with RQGIS.
You can install:
Subsequently, we will show you a typical workflow of how to use RQGIS. Basically, we will follow the steps also described in the QGIS documentation. In our first and very simple example we simply would like to retrieve the centroid coordinates of a spatial polygon object. First off, we will download the administrative areas of Germany using the raster package.
# attach packages library("raster") library("rgdal") # download German administrative areas ger <- getData(name = "GADM", country = "DEU", level = 1) # ger is of class "SpatialPolygonsDataFrame"
Now that we have a spatial object, we can move on to using RQGIS. First of all, we need to specify all the paths necessary to run the QGIS-API. Fortunately,
set_env does this for us (assuming that QGIS and all necessary dependencies were installed correctly). The only thing we need to do is: specify the root path to the QGIS-installation. If you do not specify a path,
set_env tries to find the OSGeo4W-installation first in the ‘C:/OSGeo4W’-folders. If this is unsuccessful, it will search your C: drive though this might take a while. If you are running RQGIS under Linux or on a Mac,
set_env assumes that your root path is “/usr” and “/applications/QGIS.app/Contents”, respectively. Please note, that most of the RQGIS functions, you are likely to work with (such as
run_qgis), require the output list (as returned by
set_env) containing the paths to the various installations necessary to run QGIS from within R. This is why,
set_env caches its result in a temporary folder, and loads it back into R when called again (to overwrite an existing cache, set parameter
# attach RQGIS library("RQGIS") # set the environment, i.e. specify all the paths necessary to run QGIS from # within R set_env() # under Windows set_env would be much faster if you specify the root path: # set_env("C:/OSGeo4W~1") ## $root ##  "C:\\OSGeo4W64" ## ## $qgis_prefix_path ##  "C:\\OSGeo4W64\\apps\\qgis-ltr" ## ## $python_plugins ##  "C:\\OSGeo4W64\\apps\\qgis-ltr\\python\\plugins"
Secondly, we would like to find out how the function in QGIS is called which gives us the centroids of a polygon shapefile. To do so, we use
find_algorithms. Here, we look for a geoalgorithm that contains the words “polygon” and “centroid”. Note that you can use regular expressions.
library("RQGIS") find_algorithms(search_term = "([Pp]olygon)(centroid)") ##  "Polygon centroids------------------------------------>qgis:polygoncentroids" ##  "Polygon centroids------------------------------------>saga:polygoncentroids"
This gives us two functions we could use. Here, we’ll choose the QGIS function named
qgis:polygoncentroids. Subsequently, we would like to know how we can use it, i.e. which function parameters we need to specify.
get_usage(alg = "qgis:polygoncentroids") ## ALGORITHM: Polygon centroids ## INPUT_LAYER <ParameterVector> ## OUTPUT_LAYER <OutputVector>
qgis:polygoncentroids only expects a parameter called
INPUT_LAYER, i.e. the path to a polygon shapefile whose centroid coordinates we wish to extract, and a parameter called
OUTPUT_LAYER, i.e. the path to the output shapefile. Since it would be tedious to specify manually each and every function argument, especially if a function has more than two or three arguments, we have written a convenience function, named
get_args_man, that retrieves all function arguments and respective default values for a given GIS function. It returns these values in the form of a list. If a function argument lets you choose between several options (drop-down menu in a GUI), setting
TRUE makes sure that the first option will be selected (QGIS GUI behavior). For example,
qgis:addfieldtoattributestable has three options for the
FIELD_TYPE-parameter, namely integer, float and string. Setting
TRUE means that the field type of your new column will be of type integer.
params <- get_args_man(alg = "qgis:polygoncentroids") params ## $INPUT_LAYER ##  "None" ## ## $OUTPUT_LAYER ##  "None"
In our case,
qgis:polygoncentroids has only two function arguments and no default values. Naturally, we need to specify manually our input and output layer. We can do so in two ways. Either we use directly our parameter-argument list…
params$INPUT_LAYER <- ger params$OUTPUT_LAYER <- file.path(tempdir(), "ger_coords.shp") out <- run_qgis(alg = "qgis:polygoncentroids", params = params, load_output = TRUE) ## $OUTPUT_LAYER ##  "C:\\Users\\pi37pat\\AppData\\Local\\Temp\\Rtmpeil4bS/ger_coords.shp"
… or we can use R named arguments in
out <- run_qgis(alg = "qgis:polygoncentroids", INPUT_LAYER = ger, OUTPUT_LAYER = file.path(tempdir(), "ger_coords.shp"), load_output = TRUE) ## $OUTPUT_LAYER ##  "C:\\Users\\pi37pat\\AppData\\Local\\Temp\\Rtmpeil4bS/ger_coords.shp"
Please note that our
INPUT_LAYER is a spatial object residing in R’s global environment. Of course, you can also use a path to specify
INPUT_LAYER (e.g. “ger.shp”) which is the better option if your data is somewhere stored on your hard drive. Finally,
run_qgis calls the QGIS API to run the specified geoalgorithm with the corresponding function arguments. Since we set
run_qgis automatically loads the QGIS output back into R (an
sf-object in the case of vector data and a
raster-object in the case of raster data). Naturally, we would like to check if the result meets our expectations.
Of course, this is a very simple example. We could have achieved the same using
sp::coordinates(). For a more detailed introduction to RQGIS and more complex examples have a look at our paper:
open_app first sets all necessary paths (among others the path to the QGIS Python binary) to run QGIS, and secondly opens a QGIS application with the help of reticulate. Please note that
open_app establishes a tunnel to the QGIS Pyton API which can only be closed by starting a new R session (see https://github.com/rstudio/reticulate/issues/27). On the one hand, this means that we only have to set up the Python environment once and consequently subsequent processing is faster. Additionally, you can use your own Python commands to customize RQGIS as you wish. On the other hand, it also means that once you have run the first RQGIS command interacting with the QGIS Python API (e.g.,
get_usage, etc.), you have to stay with the chosen QGIS version for this session. For instance, if you are using QGIS 2.14.14 (
qgis_session_info), and you would like to use the developer version QGIS 2.18.7, you have to restart R. Then you can call
set_env(dev = TRUE) and
open_app() to use the QGIS developer version.
If you would like to use QGIS versions 2.16-2.18.1, you need to fix manually a Processing error in order to make RQGIS work. First, add one
import statement (SilentProgress) to
../processing/gui/AlgorithmExecutor.py. Secondly replace
python alg.execute(progress) by
python alg.execute(progress or SilentProgress()):
The QGIS core team fixed this bug, and starting with QGIS 2.18.2 this manual adjustment is no longer necessary (see also this post). However, we would strongly recommend to either use the QGIS LTR (2.14) or QGIS >= 2.18.2.