I'm trying to use RQGIS interface with QGIS to create a polygon grid. One of the required parameters is TYPE.

Where can i know the type of parameters to this algorithm? and to others?




neto  <- readRDS("/path/") #download data from link

# Finding QGIS algoritms 
dir_tmp <- "/my/path/"
my_env <- set_env()
find_algorithms(search_term = "grid")
params <- get_args_man(alg = "qgis:creategrid", qgis_env = my_env)

params$TYPE <- "polygon"
params$EXTENT <- nnetpro
params$HSPACING <- "10" does it accept numeric?
params$VSPACING <- "10" does it accept numeric?
params$CRS <- "EPSG:31983" #does it work with CRS("+init=epsg:31983")?
params$OUTPUT <- file.path(dir_tmp, "g10m.shp")

out <- run_qgis(alg = "qgis:creategrid",
                params = params,
                load_output = params$OUTPUT,
                qgis_env = my_env)

Error in run_qgis(alg = "qgis:creategrid", params = params, load_output = params$OUTPUT,  : 
Error: Wrong parameter value: polygon

Obviously, you want to use another projection, therefore we reproject your data:

neto  <- readRDS("neto.rds")
neto_projected <- spTransform(neto,  CRS("+init=epsg:31983"))

Next, we access the function arguments of "qgis:creategrid".

qgis_env <- set_env(root = "C:/OSGeo4W64/")
params <- get_args_man(alg = "qgis:creategrid", qgis_env = qgis_env)

To access the options for a specific QGIS geoalgorithm, use get_options:

get_options(alg = "qgis:creategrid", qgis_env = qgis_env)

You can also access its online help:

open_help(alg = "qgis:creategrid", qgis_env = qgis_env)

Though I have to admit that the online help in this case is a bit confusing...

Let's specify the arguments accordingly:

params$TYPE <- 1
# extent has to be given in the form of: xmin, xmax, ymin, ymax
params$EXTENT <-  paste(as.vector(raster::extent(neto_projected)), 
                        collapse = ", ")

Obviously, you would like to use a 10 m spacing. That's why we have reprojected your data in the first place. However, be careful if you use another projection, e.g., when using WGS84 10 would correspond to 10 decimal degrees.

params$HSPACING <- 10
params$VSPACING <- 10
params$CRS <- "EPSG:31983" 
params$OUTPUT <- file.path(tempdir(), "g10m.shp")

Now run QGIS. Due to your fine resolution, this takes a little while.

out <- run_qgis(alg = "qgis:creategrid",
                params = params,
                load_output = params$OUTPUT,
                qgis_env = qgis_env)
  • I forgot to say that everything was in EPSG:31983, in that case, is it necessary to write the CRS? anyways, the get_options and open_help are the key. Thanks. – Sergio Mar 9 '17 at 13:01
  • 1
    Well, your neto-shapefile used the WGS84 projection. Anyways, you need to specify the CRS-argument. Not doing so would result in an QGIS error message saying "Wrong parameter value: None". – Jannes Mar 9 '17 at 13:32

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