Defines a netCDF dimension. This dimension initially only exists in memory.
The dimension is later added to a netCDF variable using a call to ncvar_def(),
and written to disk using nc_create().
Name of the dimension to be created (character string).
The dimension name can
optionally have forward slashes in it, in which case the dimension will be defined
in the indicated group. For example, a dimension
named model3/run1/Longitude will define a group named model3, with a
subgroup named run1, which will hold a dimension named Longitude. Using
groups forces a netcdf version 4 file to be written. Note that older software might not
be able to read netcdf version 4 files.
units
The dimension's units (character string).
vals
The dimension's values (vector of numeric type). If integers are passed, the associated dimensional variable will be integer type; otherwise, it will be double precision.
unlim
If TRUE, this dimension is unlimited. Unlimited dimensions are convenient for storing, for example, data that extends over time;
the time dimension can be made unlimited, and extended as needed. Or, an unlimited dimension could be the number of stations, and extended
as more stations come on-line. Note that in netCDF version 4, multiple dimensions can be unlimited. In netCDF version 3, there could
only be one unlimited dimension, typically the time dimension.
create_dimvar
If TRUE, a dimensional variable (aka coordinate variable) will be created for this dimension. Note: if this is set to FALSE, then 'units' must be an empty string. It is good practice to always leave this as TRUE.
calendar
If set, the specified string will be added as an attribute named "calendar" to the dimension variable. Used almost exclusively with unlimited time dimensions. Useful values include "standard" (or "gregorian"), "noleap" (or "365_day"), and "360_day").
longname
If set, AND create_dimvar is TRUE,
then the created dimvar will have a long_name attribute with this value.
Details
This routine creates a netCDF dimension in memory. The created dimension can then
later be passed to the routine ncvar_def() when defining a variable.
Note that this interface to the netCDF library by default includes that more than the
minimum required by the netCDF standard. I.e., the netCDF standard allows
dimensions with no units or values. This call encourages creating dimensions
that have units and values,
as it is useful to ensure that all dimensions have units and
values, and considerably easier to include them in this call than it is
to add them later. The units and values are implemented through "dimensional
variables," which are variables with the same name as the dimension. By default, these
dimensional variables are created automatically – there is no need for the
user to create them explicitly. Dimensional variables are standard practice
in netCDF files. To suppress the creation of the dimensional variable for
the dimension, set passed parameter create_dimvar to FALSE. As a
check, if create_dimvar is FALSE, you must ALSO pass an empty string (”) as the unit, and the values
must be simple integers from 1 to the length of the dimension (e.g., 1:10 to
make a dimension of length 10).
This empahsizes that without a dimensional variable, a netCDF file cannot
store a dimension's units or values.
The dimensional variable is usually created as a double precision floating
point. The other possibility is to pass integer values (using as.integer,
for example), in which case the dimensional variable with be integer.
The return value of this function is an object of class ncdim4, which
describes the newly created dimension.
The ncdim object is used for more than just creating a new
dimension, however.
When opening an existing file, function nc_open returns a
ncdf4 class object, which itself has a list of ncdim objects
that describe all the dimensions in that existing file.
The ncdim object has the following fields, which are all read only:
1) name, which is a character string containing the name of the dimension;
2) units, which is a character string containing the units for the dimension,
if there are any (technically speaking, this is the "units" attribute of the
associated coordinate variable); 3) vals, which is a vector containing the
dimension's values (i.e., the values of the associated coordinate variable,
or, if there is none, an integer sequence from 1 to the length of the dimension);
3) len, which is the length of this dimension; 4) unlim, which is a boolean
indicating whether or not this is an unlimited dimension; 5) (optional) calendar,
which is set if and only if the on-disk dimvar had an attribute named
"calendar" (in which case, it is set to the value of that attribute).
Value
An object of class ncdim4 that can later be passed to
ncvar_def().
Note
It is good practice, but not necessary, to pass the dimension's values
to this routine when the dimension is created. It is also possible to
write them later with a call to 'ncvar_put', using as the dimension
name as the 'varid' in the call. This is useful when creating large variables
with long unlimited dimensions; it can take a long time to write out the
unlimited dimension's values. In this case, it can be more efficient to
step through the file, writing one timestep at a time, and write that
timestep's dimensional value at the same time.
# Define some straightforward dimensions
x <- ncdim_def( "Lon", "degreesE", 0.5:359.5)
y <- ncdim_def( "Lat", "degreesN", as.double(-89:89))
t <- ncdim_def( "Time", "days since 1900-01-01", 1:10, unlim=TRUE)
# Make a variable with those dimensions. Note order: time is LAST
salinity <- ncvar_def("Salinity", "ppt", list(x,y,t), 1.e30 )
# Create a netCDF file with this variable
ncnew <- nc_create( "salinity.nc", salinity )
nc_close(ncnew)
# Now, illustrate some manipulations of the ncdim object.
filename <- "salinity.nc"
nc <- nc_open( filename )
print(paste("File",filename,"contains",nc$ndims,"dimensions"))
for( i in 1:nc$ndims ) {
print(paste("Here is information about dimension number",i,":"))
d <- nc$dim[[i]]
print(paste(" Name :",d$name))
print(paste(" Units :",d$units))
print(paste(" Length:",d$len))
print(" Values:")
print(d$vals)
print(paste(" Unlimited:",d$unlim))
}