selpoint.multi: Extract data at multiple points.

Description

This function extracts all data at given points. The points are given by a pair of vectors with longitude and latitude coordinates. The function will find the closest grid points to the given coordinates and extracts the data for these points. For each point a separate output file is written. The output-files can be optional in NetCDF or csv. Input can be a single NetCDF file (given by the infile attribute) or a bunch of NetCDF files (given by the path and pattern attributes).

Usage

selpoint.multi(
  var,
  infile,
  path,
  pattern,
  outpath,
  lon1,
  lat1,
  station_names = NULL,
  format = "nc",
  nc34 = 4,
  verbose = FALSE,
  nc = NULL
)

Value

For each pair of longitude and latitude coordinates one separate NetCDF or csv file including the selected data is written. The csv files are tested for use in Excel and include four columns (Time ; Data ; Longitude ; Latitude), which are separated by ';'. If station_names are defined, the output files will be named according to this vector. Otherwise, the output files will be named as selpoint_longitude_latitude.format. Already existing files will be overwritten in case that station_names are given or renamed (e.g., selpoint1_longitude_latitude.nc) in case that no station_names are given.

Arguments

var: Name of NetCDF variable (character).
infile: Filename of input NetCDF file. This may include the directory (character). Infile is not needed if path and pattern are given.
path: Directory of input files (character). Will not be used if infile is given.
pattern: Pattern that all desired files in the 'path' directory have in common (character).
outpath: Directory where output files will be stored (character).
lon1: Longitude vector of desired points (numeric vector). Must have the same length as lat1.
lat1: Latitude vector of desired points (numeric vector). Must have the same length as lon1.
station_names: Optional vector of names, which will be used for the output files (character vector). Must have the same length as lon1 and lat1.
format: Intended output format. Options are nc or csv. Default is nc (character).
nc34: NetCDF version of output file. If nc34 = 3 the output file will be in NetCDFv3 format (numeric). Default output is NetCDFv4.
verbose: logical; if TRUE, progress messages are shown
nc: Alternatively to infile you can specify the input as an object of class ncdf4 (as returned from ncdf4::nc_open).

Examples

Run this code

## Create an example NetCDF file with a similar structure as used by CM
## SAF. The file is created with the ncdf4 package.  Alternatively
## example data can be freely downloaded here: 

library(ncdf4)

## create some (non-realistic) example data

lon <- seq(5, 15, 0.5)
lat <- seq(45, 55, 0.5)
time <- seq(as.Date("2000-01-01"), as.Date("2010-12-31"), "month")
origin <- as.Date("1983-01-01 00:00:00")
time <- as.numeric(difftime(time, origin, units = "hour"))
data <- array(250:350, dim = c(21, 21, 132))

## create example NetCDF

x <- ncdim_def(name = "lon", units = "degrees_east", vals = lon)
y <- ncdim_def(name = "lat", units = "degrees_north", vals = lat)
t <- ncdim_def(name = "time", units = "hours since 1983-01-01 00:00:00",
 vals = time, unlim = TRUE)
var1 <- ncvar_def("SIS", "W m-2", list(x, y, t), -1, prec = "short")
vars <- list(var1)
ncnew <- nc_create(file.path(tempdir(),"CMSAF_example_file.nc"), vars)
ncvar_put(ncnew, var1, data)
ncatt_put(ncnew, "lon", "standard_name", "longitude", prec = "text")
ncatt_put(ncnew, "lat", "standard_name", "latitude", prec = "text")
nc_close(ncnew)

## Select two points of the example CM SAF NetCDF file and write the
## output to a csv-file.
selpoint.multi(var = "SIS", infile = file.path(tempdir(),"CMSAF_example_file.nc"),
 outpath = tempdir(), lon1 = c(8, 9), lat1 = c(48, 49),
 station_names = c("A", "B"), format = "csv")

unlink(c(file.path(tempdir(),"CMSAF_example_file.nc"), file.path(tempdir(),"A.csv"), 
 file.path(tempdir(),"B.csv")))

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