selpoint {cmsafops}

Extract data at a given point.

Description

This function extracts all data at a given point. A point is given by a pair of longitude and latitude coordinates. The function will find the closest grid point to the given coordinates and extracts the data for this point. The output-file can be optional in NetCDF or csv. The outfile is checked for the correct file extension.

Usage

selpoint(
  var,
  infile,
  outfile,
  lon1 = 0,
  lat1 = 0,
  format = "nc",
  nc34 = 4,
  overwrite = FALSE,
  verbose = FALSE,
  nc = NULL
)

Arguments

var

Name of NetCDF variable (character).

infile

Filename of input NetCDF file. This may include the directory (character).

outfile

Filename of output NetCDF file. This may include the directory (character).

lon1

Longitude of desired point (numeric).

lat1

Latitude of desired point (numeric).

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.

overwrite

logical; should existing output file be overwritten?

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).

Value

A NetCDF or csv file including the selected point is written. The csv file is tested for use in Excel and includes two columns (Time and Data), which are separated by ';'.

See Also

Other selection and removal functions: extract.level(), extract.period(), sellonlatbox(), selmon(), selperiod(), selpoint.multi(), seltime(), selyear()

Examples

## 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: <https://wui.cmsaf.eu/>

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 a point of the example CM SAF NetCDF file and write the output
## to a csv-file.
selpoint(var = "SIS", infile = file.path(tempdir(),"CMSAF_example_file.nc"), 
 outfile = file.path(tempdir(),"CMSAF_example_file_selpoint.nc"),
 lon1 = 8, lat1 = 48, format = "csv")

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