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Change the prime meridian of a variable in a netCDF file

Source: R/nctools-extras.R
nc_changePrimeMeridian.Rd

Reorders the longitude dimension of a variable so that longitudes follow either the `[-180, 180]` convention or the `[0, 360]` convention, and writes the result to a netCDF file.

Usage

nc_changePrimeMeridian(
  filename,
  output,
  varid = NA,
  MARGIN = 1,
  primeMeridian = "center",
  verbose = FALSE,
  overwrite = FALSE,
  compression = NA,
  mem.limit = 3072,
  ignore.case = FALSE
)

Arguments

filename

Character string giving the path to the input netCDF file.

output

Optional character string giving the path to the output netCDF file. If omitted, `overwrite = TRUE` must be used and the input file is replaced.

varid

Variable identifier. This can be a character string giving the variable name. If omitted and the file contains a single variable, that variable is used.

MARGIN

Integer or character string of length one specifying the longitude dimension. This can be either the dimension index or its name.

primeMeridian

Character string specifying the target longitude convention. Use `"center"` for longitudes in the `[-180, 180]` range and `"left"` for longitudes in the `[0, 360]` range.

verbose

Logical. If `TRUE`, report progress while processing the variable.

overwrite

Logical. If `TRUE`, allow overwriting an existing output file, including the input file itself.

compression

Optional numeric compression level for the output variable. Supplying this typically requires netCDF4 output.

mem.limit

Numeric. Approximate memory limit, in MiB, used when processing large variables. If the variable exceeds this limit, it is reordered iteratively in chunks.

ignore.case

Logical. If `TRUE`, ignore case when matching `varid` or a character `MARGIN`.

Value

Invisibly returns `output`.

Details

The function updates the longitude coordinate values of the selected variable, reorders the data accordingly, and writes the result to a new netCDF file. If the variable is small enough, the full array is processed in memory. Otherwise, the variable is processed iteratively in chunks to reduce memory use.

If the longitude values already follow the requested convention, the function returns with a warning. When `output` differs from `filename`, the input file is copied to `output` unchanged.

See also

[nc_subset()], [nc_apply()]

Examples

if (FALSE) { # \dontrun{
nc_changePrimeMeridian(
  filename = "input.nc",
  output = "output.nc",
  varid = "temp",
  MARGIN = "lon",
  primeMeridian = "center"
)

nc_changePrimeMeridian(
  filename = "input.nc",
  output = "output_pacific.nc",
  varid = "temp",
  MARGIN = 1,
  primeMeridian = "left",
  compression = 4
)
} # }