ncmpidump - Convert netCDF files to ASCII form (CDL)
ncmpidump [-ch] [-v var1,...] [-b lang] [-f
lang] [-l len] [-n name] [-p
f_digits[,d_digits]] file
ncmpidump -k file
ncmpidump 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 ncmpigen. ncmpigen is a companion program
that can generate a binary netCDF file from a CDL file. Hence
ncmpigen and ncmpidump can be used as inverses to transform
the data representation between binary and ASCII representations. See
ncmpigen for a description of CDL and netCDF representations.
ncmpidump may also be used to determine what kind of netCDF
file is used (which variant of the netCDF file format) with the -k
option.
ncmpidump 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, ncmpidump 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
ncmpidump 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.
ncmpidump uses `_' to represent data values that are equal
to the `_FillValue' attribute for a variable, intended to represent data
that has not yet been written. If a variable has no `_FillValue' attribute,
the default fill value for the variable type is used if the variable is not
of byte type.
- -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 ncmpigen -b
in generating a default netCDF file name. By default, ncmpidump
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
ncmpigen -b can be specified, it may be wise to have
ncmpidump change the default name to avoid inadvertantly
overwriting a valuable netCDF file when using ncmpidump, editing
the resulting CDL file, and using ncmpigen -b to generate a new
netCDF file from the edited CDL file.
- -p
float_digits[,double_digits]
- Specifies default precision (number of significant digits) to use in
displaying floating-point or double precision data values for attributes
and variables. If specified, this value overrides the value of the
`C_format' attribute for any variable that has such an 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. In the absence of any
-p 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-precision 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 -p 9,17 (according to Theorem 15 of
the paper listed under REFERENCES).
- -k
- Reports the kind of netCDF file: classic, 64-bit offset, or 64-bit data.
Before netCDF version 3.6, there was only one kind of netCDF file,
designated as `classic' (also know as format variant 1 or CDF-1). Large
file support introduced another variant of the format, designated as
`64-bit offset' (known as format variant 2 or CDF-2). Large data support
introduced another variant of the format, designated as `64-bit data'
(known as format variant 5 or CDF-5).
Look at the structure of the data in the netCDF file
`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:
ncmpidump -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:
ncmpidump -v uwind,vwind -p 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':
ncmpidump -v omega -f fortran -n omega foo.nc > Z.cdl
What 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 the output.
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.