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PROGRAM:

NAME

ncdump - Convert netCDF files to ASCII form (CDL)

SYNOPSIS

ncdump [-c] [-h] [-v var1,...] [-b lang] [-f lang] [-l len] [-n name] [-d
f_digits[,d_digits]] file

DESCRIPTION

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.

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.

OPTIONS

-c 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.

-h 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 values.

-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 CDL file.

-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).

EXAMPLES

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

REFERENCES

What Every Computer Scientist should Know About Floating-Point Arithmetic, D. Goldberg,
ACM Computing Surveys, Vol. 23, No. 1, March 1991, pp. 5-48.

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