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ncdump - Convert netCDF files to ASCII form (CDL)
ncdump generates an ASCII representation of a specified
netCDF file on standard output. The ASCII representation is in a form called
CDL (``network Common Data form Language'') that can be viewed, edited, or
serve as input to ncgen. ncgen is a companion program that can generate
a binary netCDF file from a CDL file. Hence ncgen and ncdump can be used
as inverses to transform the data representation between binary and ASCII
representations. See ncgen for a description of CDL and netCDF representations.
[-h] [-v var1,...] [-b lang] [-f lang] [-l len] [-n name] [-d f_digits[,d_digits]]
ncdump defines a default format used for each type of netCDF data, but
this can be changed if a `C_format' attribute is defined for a netCDF variable.
In this case, ncdump will use the `C_format' attribute to format each value.
For example, if floating-point data for the netCDF variable `Z' is known
to be accurate to only three significant digits, it would be appropriate
to use the variable attribute
- Z:C_format = "%.3g"
ncdump may also be used
as a simple browser for netCDF data files, to display the dimension names
and sizes; variable names, types, and shapes; attribute names and values;
and optionally, the values of data for all variables or selected variables
in a netCDF file.
- Show the values of coordinate variables (variables
that are also dimensions) as well as the declarations of all dimensions,
variables, and attribute values. Data values of non-coordinate variables
are not included in the output. This is the most suitable option to use
for a brief look at the structure and contents of a netCDF file.
- Show only
the header information in the output, that is the declarations of dimensions,
variables, and attributes but no data values for any variables. The output
is identical to using the -c option except that the values of coordinate
variables are not included. (At most one of -c or -h options may be present.)
- -v var1,...,varn
- The output will include data values for the specified variables,
in addition to the declarations of all dimensions, variables, and attributes.
One or more variables must be specified by name in the comma-delimited
list following this option. The list must be a single argument to the command,
hence cannot contain blanks or other white space characters. The named
variables must be valid netCDF variables in the input-file. The default,
without this option and in the absence of the -c or -h options, is to include
data values for all variables in the output.
- -b lang
- A brief annotation in
the form of a CDL comment (text beginning with the characters ``//'') will
be included in the data section of the output for each `row' of data, to
help identify data values for multidimensional variables. If lang begins
with `C' or `c', then C language conventions will be used (zero-based indices,
last dimension varying fastest). If lang begins with `F' or `f', then Fortran
language conventions will be used (one-based indices, first dimension varying
fastest). In either case, the data will be presented in the same order;
only the annotations will differ. This option is useful for browsing through
large volumes of multidimensional data.
- -f lang
- Full annotations in the form
of trailing CDL comments (text beginning with the characters ``//'') for every
data value (except individual characters in character arrays) will be included
in the data section. If lang begins with `C' or `c', then C language conventions
will be used (zero-based indices, last dimension varying fastest). If lang
begins with `F' or `f', then Fortran language conventions will be used (one-based
indices, first dimension varying fastest). In either case, the data will
be presented in the same order; only the annotations will differ. This
option may be useful for piping data into other filters, since each data
value appears on a separate line, fully identified.
- -l len
- Changes the default
maximum line length (80) used in formatting lists of non-character data
- -n name
- CDL requires a name for a netCDF data set, for use by ncgen
-b in generating a default netCDF file name. By default, ncdump constructs
this name from the last component of the pathname of the input netCDF file
by stripping off any extension it has. Use the -n option to specify a different
name. Although the output file name used by ncgen -b can be specified, it
may be wise to have ncdump change the default name to avoid inadvertantly
overwriting a valuable netCDF file when using ncdump, editing the resulting
CDL file, and using ncgen -b to generate a new netCDF file from the edited
- -d float_digits[,double_digits]
- Specifies default number of significant
digits to use in displaying floating-point or double precision data values
for variables that don't have a `C_format' attribute. Floating-point data will
be displayed with float_digits significant digits. If double_digits is
also specified, double-precision values will be displayed with that many
significant digits. If a variable has a `C_format' attribute, that overrides
any specified floating-point default. In the absence of any -d specifications,
floating-point and double-precision data are displayed with 7 and 15 significant
digits respectively. CDL files can be made smaller if less precision is
required. If both floating-point and double-presision precisions are specified,
the two values must appear separated by a comma (no blanks) as a single
argument to the command. If you really want every last bit of precision
from the netCDF file represented in the CDL file for all possible floating-point
values, you will have to specify this with -d 9,17 (according to Theorem
15 of the paper listed under REFERENCES).
Look at the structure
of the data in the netCDF file `foo.nc':
- ncdump -c foo.nc
Produce an annotated
CDL version of the structure and data in the netCDF file `foo.nc', using C-style
indexing for the annotations:
- ncdump -b c foo.nc > foo.cdl
Output data for
only the variables `uwind' and `vwind' from the netCDF file `foo.nc', and show
the floating-point data with only three significant digits of precision:
- ncdump -v uwind,vwind -d 3 foo.nc
Produce a fully-annotated (one data value
per line) listing of the data for the variable `omega', using Fortran conventions
for indices, and changing the netCDF dataset name in the resulting CDL
file to `omega':
- ncdump -v omega -f fortran -n omega foo.nc > Z.cdl
Every Computer Scientist should Know About Floating-Point Arithmetic, D.
Goldberg, ACM Computing Surveys, Vol. 23, No. 1, March 1991, pp. 5-48.
Character arrays that contain a null-byte are
treated like C strings, so no characters after the null byte appear in
Multidimensional character string arrays are not handled well,
since the CDL syntax for breaking a long character string into several
shorter lines is weak.
There should be a way to specify that the data should
be displayed in `record' order, that is with the all the values for `record'
variables together that have the same value of the record dimension.
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