This standard describes many attributes (some mandatory, others optional), but a file may also contain non-standard attributes. Such attributes do not represent a violation of this standard. Application programs should ignore attributes that they do not recognise or which are irrelevant for their purposes. Conventional attribute names should be used wherever applicable. Non-standard names should be as meaningful as possible. Before introducing an attribute, consideration should be given to whether the information would be better represented as a variable. In general, if a proposed attribute requires ancillary data to describe it, is multidimensional, requires any of the defined netCDF dimensions to index its values, or requires a significant amount of storage, a variable should be used instead. When this standard defines string attributes that may take various prescribed values, the possible values are generally given in lower case. However, applications programs should not be sensitive to case in these attributes. Several string attributes are defined by this standard to contain "blank-separated lists". Consecutive words in such a list are separated by one or more adjacent spaces. The list may begin and end with any number of spaces. See Appendix A, Attributes for a list of attributes described by this standard.
We recommend that netCDF files that follow these conventions
indicate this by setting the NUG defined global attribute
Conventions to the string value "CF-1.3" "CF-1.4" "CF-1.5" "CF-1.6""CF-1.7" . The string is
interpreted as a directory name relative to a directory that is a
repository of documents describing sets of discipline-specific
conventions. The conventions directory name is currently interpreted
relative to the directory pub/netcdf/Conventions/ on the host machine
ftp.unidata.ucar.edu. The web based
versions of this document are linked from the
netCDF Conventions web page . netCDF
Conventions web page .
The following attributes are intended to provide information about where the data came from and what has been done to it. This information is mainly for the benefit of human readers. The attribute values are all character strings. For readability in ncdump outputs it is recommended to embed newline characters into long strings to break them into lines. For backwards compatibility with COARDS none of these global attributes is required.
The NUG defines title and
history to be global attributes. We wish to allow the
newly defined attributes, i.e., institution,
source, references, and
comment, to be either global or assigned to
individual variables. When an attribute appears both globally and as a
variable attribute, the variable's version has precedence.
title A succinct description of what is in the dataset.
institution Specifies where the original data was produced.
source The method of production of the original data. If it was
model-generated, source should name the model
and its version, as specifically as could be useful. If it is
observational, source should characterize it
(e.g., "surface observation" or
"radiosonde").
history Provides an audit trail for modifications to the original data. Well-behaved generic netCDF filters will automatically append their name and the parameters with which they were invoked to the global history attribute of an input netCDF file. We recommend that each line begin with a timestamp indicating the date and time of day that the program was executed.
references Published or web-based references that describe the data or methods used to produce it.
comment Miscellaneous information about the data or methods used to produce it.