run_qgis: Interface to QGIS commands

Description Usage Arguments Details Value Note Author(s) Examples

View source: R/processing.R

Description

run_qgis calls QGIS algorithms from within R while passing the corresponding function arguments.

Usage

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run_qgis(alg = NULL, ..., params = NULL, load_output = FALSE,
  show_output_paths = TRUE, NA_flag = -99999, qgis_env = set_env())

Arguments

alg

Name of the GIS function to be used (see find_algorithms()).

...

Triple dots can be used to specify QGIS geoalgorithm arguments as R named arguments. For more details, please refer to pass_args().

params

Parameter-argument list for a specific geoalgorithm. Please note that you can either specify R named arguments directly via the triple dots (see above) or via a parameter-argument list. However, you may not mix the two methods. See the example section, pass_args() and get_args_man() for more details.

load_output

If TRUE, all QGIS output files (sf::sf()-object in the case of vector data and raster::raster()-object in the case of a raster) specified by the user (i.e. the user has to indicate output files) will be loaded into R. A list will be returned if there is more than one output file (e.g., grass7:r.slope.aspect). See the example section for more details.

show_output_paths

Logical. QGIS computes all possible output files for a given geoalgorithm, and saves them to a temporary location in case the user has not specified explicitly another output location. Setting show_output to TRUE (the default) will print all output paths to the console after the successful geoprocessing.

NA_flag

Value used for NAs when exporting raster objects through pass_args() and save_spatial_objects() (default: -99999).

qgis_env

Environment containing all the paths to run the QGIS API. For more information, refer to set_env().

Details

This workhorse function calls the QGIS Python API, and specifically processing.runalg.

Value

The function prints a list (named according to the output parameters) containing the paths to the files created by QGIS. If not otherwise specified, the function saves the QGIS generated output files to a temporary folder (created by QGIS). Optionally, function parameter load_output loads spatial QGIS output (vector and raster data) into R.

Note

Please note that one can also pass spatial R objects as input parameters where suitable (e.g., input layer, input raster). Supported formats are sp::SpatialPointsDataFrame()-, sp::SpatialLinesDataFrame()-, sp::SpatialPolygonsDataFrame()-, sf::sf()- (of class sf, sfc as well as sfg), and raster::raster()-objects. See the example section for more details.

GRASS users do not have to specify manually the GRASS region extent (function argument GRASS_REGION_PARAMETER). If "None" (the QGIS default), run_qgis (see pass_args() for more details) will automatically determine the region extent based on the user-specified input layers. If you do want to specify it yourself, please do it in accordance with the QGIS documentation, i.e., use a character string and separate the coordinates with a comma: "xmin, xmax, ymin, ymax".

Author(s)

Jannes Muenchow, Victor Olaya, QGIS core team

Examples

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## Not run: 
# calculate the slope of a DEM
# load dem - a raster object
data(dem, package = "RQGIS")
# find out the name of a GRASS function with which to calculate the slope
find_algorithms(search_term = "grass7.*slope")
# find out how to use the function
alg <- "grass7:r.slope.aspect"
get_usage(alg)
# 1. run QGIS using R named arguments, and load the QGIS output back into R
slope <- run_qgis(alg, elevation = dem, slope = "slope.asc",
                  load_output = TRUE)
# 2. doing the same with a parameter-argument list
params <- list(elevation = dem, slope = "slope.asc")
slope <- run_qgis(alg, params = params, load_output = TRUE)
# 3. calculate the slope, the aspect and the pcurvature.
terrain <- run_qgis(alg, elevation = dem, slope = "slope.asc",
                    aspect = "aspect.asc", pcurvature = "pcurv.asc",
                    load_output = TRUE)
# the three output rasters are returned in a list of length 3
terrain

## End(Not run)

RQGIS documentation built on Aug. 13, 2018, 5:04 p.m.