Check or convert object to a QGIS-compatible CRS specification.

Transforms various R data and object types so that they can be processed by QGIS' QgsCoordinateReferenceSystem class.

qgisCRS(crs, proj4 = FALSE)

Arguments

crs	a primitive or object that either specifies (or contains a reference to) a coordinate system
proj4	logical: if set to TRUE, stops this function from avoiding proj4 strings in favour of well-known text (WKT) specifications, which is the default behaviour

Value

A numeric or character string.

See also

iface.addVectorLayer()

Examples

# numerics are returned as they are
qgisCRS(4326)
#> [1] 4326

# QGIS understands EPSG, ESRI and PostGIS ids, but this function does not run
# any checks on whether the numerics passed are in fact valid. as a consequence,
# QGIS will still fail to parse the CRS if you pass it nonsense such as this
qgisCRS(12345.6)
#> [1] 12345.6

# numeric ids will automatically be extracted from instances of sp's CRS class (or any
# of the Spatial* dataframes that include a CRS spec) 
spCRS <- sp::CRS('+init=epsg:4326')
qgisCRS(spCRS)
#> [1] 4326
spCRS <- sp::CRS('+init=esri:54009')
qgisCRS(spCRS)
#> [1] 54009


# when a CRS object's EPSG id can't be found, the function returns a well-known text (WKT) spec
# by default
mollweidestring <- '+proj=moll +lon_0=0 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs'
mollweide <- sp::CRS(mollweidestring)
qgisCRS(mollweide)
#> [1] "PROJCS[\"Mollweide\",GEOGCS[\"GCS_WGS_1984\",DATUM[\"D_WGS_1984\",SPHEROID[\"WGS_1984\",6378137,298.257223563]],PRIMEM[\"Greenwich\",0],UNIT[\"Degree\",0.017453292519943295]],PROJECTION[\"Mollweide\"],PARAMETER[\"central_meridian\",0],PARAMETER[\"false_easting\",0],PARAMETER[\"false_northing\",0],UNIT[\"Meter\",1]]"

# automatic conversion from proj4 to WKT can be suppressed using the 'proj4' argument
qgisCRS(mollweide, proj4 = TRUE)
#> [1] "+proj=moll +lon_0=0 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs"


# behaviour is identical when passing proj4 specs as character strings directly
qgisCRS(mollweidestring)
#> [1] "PROJCS[\"Mollweide\",GEOGCS[\"GCS_WGS_1984\",DATUM[\"D_WGS_1984\",SPHEROID[\"WGS_1984\",6378137,298.257223563]],PRIMEM[\"Greenwich\",0],UNIT[\"Degree\",0.017453292519943295]],PROJECTION[\"Mollweide\"],PARAMETER[\"central_meridian\",0],PARAMETER[\"false_easting\",0],PARAMETER[\"false_northing\",0],UNIT[\"Meter\",1]]"

qgisCRS(mollweidestring, proj4 = TRUE)
#> [1] "+proj=moll +lon_0=0 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs"


# other string representations are left untouched for QGIS to deal with
# it is the user's responsibility to ensure their format is understood by QGIS

qgisCRS('EPSG:4326') # QGIS will understand this
#> [1] "EPSG:4326"

qgisCRS('FOO') # but not this
#> [1] "FOO"