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ncdump - Convert netCDF files to ASCII form (CDL)
- ncdump
- [-c] [-h] [-v var1,... ] [-b lang ] [-f lang ] [-l len ] [-n name ] [-p f_digits[,d_digits]
] file
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.
ncdump 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 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.
- -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-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 -p 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 -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':
- ncdump -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.
ncgen(1)
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.
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