NetCDF Software Requirements for the NetCDF CF Library
The NetCDF CF Library supports the creation of scientific data files conforming to the CF conventions, using the netCDF API. This page lists the software requirements for version 1.0 of the library.
Important Note
These requirements represent a snapshot of our understanding of the NetCDF CF library. They are subject to change without notice. This document does not represent a promise that the NetCDF CF Library will meet these requirements, though that is our ultimate goal.

Requirements for libcf are determined by the libcf Requirements Committee.

Comments are welcome, and should be sent to the libcf mailing list: libcf@unidata.ucar.edu

General Requirements
• libcf allows manipulation of higher order data objects, defined using the CF conventions (plus suggested extensions). The intent is to allow Earth science researchers to write files in conformance with the CF metadata standards, to promote inter-operability between scientific groups and tools.

• libcf layer is implemented on top of the netCDF-3 API.

• libcf reads and writes attributes from and to the data file. If users change these attributes externally without care, they can expect problems.

• libcf handles simple rectilinear, nested, structured and unstructured grids. Where current CF standards do not support these features, proposed standards are used. Close contact with the CF governance body will ensure that libcf continues to represent the CF standard as it evolves.

• When recognizing attribute names, libcf always does so in a case-insensitive manner (except _FillValue), but always creates such attributes in lower-case, following the lead of the CF/Unidata documentation.

• In the requirements below, when an attribute is said to be "supported" we mean that it may be set and read though the libcf API. Attributes which are mentioned in the same requirement will be set and read as a group.
Description of File Contents
• libcf supports the appending of "CF-1.0" to the Conventions global attribute, and can scan for "CF-1.0" in an existing file.

• libcf supports the title and history global attributes. Setting the history causes a string to be appended, including generation of CF-recommended time stamp.

• libcf supports the institution, source, references, and comment attributes, as global or variable attributes.
Description of Variables
• libcf supports the following attributes for variables: _FillValue, valid_min, valid_max, valid_range, units, long_name, standard_name.

• libcf supports looking up the standard name in a copy of the standard name table distributed with libcf, and checking the compatibility of the units.

• libcf supports reading or appending the ancillary_variables attribute.

• libcf supports the flag_values and flag_meanings attributes.
Coordinate Variables
• libcf uses COARDS conventions to identify the following special coordinate types: latitude, longitude, vertical, and time.

• The latitude, longitude, and time dimensions always have a units attribute.

• libcf supports the standard_name and formula_terms attributes on coordinate variables. However, libcf does not attempt to construct the formula indicated by the formula_terms attribute.

• libcf supports the axis attribute on coordinate variables, which, when present, will have one of these values: "X", "Y", "Z", or "T". In the event that the units and the axis don't agree, libcf will return an error and throw up its hands in disgust.

• libcf identifies the latitude with a units value of degrees_north (but also accepts degree_north, degree_N, degrees_N, degreeN and degreesN. When writing, libcf adds the standard_name (set to "latitude") and axis (set to "Y").

• libcf identifies the longitude with a units value of degrees_east (but also accepts degree_east, degree_E, degrees_E, degreeE and degreesE, all case-insensitive). When writing, libcf adds the standard_name (set to "longitude") and axis (set to "X").

• All coordinate variables must have a units attribute.

• libcf supports the positive attribute to indicate the direction of the vertical coordinate.

• libcf recognizes the following values for the units attribute as indicating a vertical coordinate variable: bar, millibar, decibar, atmosphere, atm, pascal, Pa, hPa, meter, metre, m, kilometer, km. Other units listed in the file udunits.dat that may under certain circumstances reference vertical position such as units of density or temperature. (Which ones?) (Plural forms are also acceptable.)

• libcf recognizes a time coordinate variable by a units attribute with the value since , where is one of day, d, hour, hr, h, minute, min, second, sec, and s, and is of the form YYYY-MM-DD [HH:MM:SS.S [+|- HH[:MM]]].

• libcf supports the calendar attribute, which may be set to gregorian, standard, proleptic_gregorian, noleap, 365_day, all_leap, 366_day, 360_day, julian, or none.

• libcf supports the months_lengths, leap_year, leap_month attributes. If these attributes are used the calendar can be any text value.

• libcf supports scalar coordinate variables, either as a dimension of length one and accompanying coordinate variable, or as a scalar variable (and no dimension) which is listed in the variable's coordinates attribute.
Coordinate Systems
• libcf supports the coordinates attribute, which is an unordered, space-separated list of names of auxiliary coordinate variables.

• libcf an NC_CHAR label variable which contains labels that apply to a coordinate variable. In this case the name of the label variable will be listed in the coordinates attribute of the variable that uses the labeled axis as one of it's dimensions.

• libcf also supports the region attribute.

• libcf also supports Common Data Model Coordinate Systems.
Grid Mapping and Projections
• libcf supports the grid_mapping attribute. Setting this attribute causes an NC_CHAR variable to be created in order to hold the grid mapping attributes.

• All grid_mapping variables have a grid_mapping_name attribute. The presence of other attributes is determined by the grid_mapping_name, in accordance with appendix F of the CF 1.0 conventions.

• libcf supports the following grids: albers_conical_equal_area, azimuthal_equidistant, lambert_azimuthal_equal_area, lambert_conformal_conic, polar_stereographic, rotated_latitude_longitude, stereographic, and transverse_mercator.
Nested Grids
• libcf supports nested grids.
Unstructured Grids
• libcf supports unstructured grids.
Observational Data
• libcf supports observational data.
Distribution, Build, and Installation
• Libcf is distributed in source and binary form via Unidata's FTP and web sites.

• Libcf can be installed with the usual configure/make invocations, with the usual options supported, including "make check", "make install", "make distcheck", "make dist", "make clean", and "make distclean". Configure options include --disable-shared, --with-netcdf=, --prefix=. Shared libraries are built on platforms that support them.

• Libcf is a C library and one or more sets of fortran wrapper libraries. (Each fortran compiler requires its own set of wrapper functions). Programs which use libcf must link to a netCDF 3.x library, plus the libcf library, plus a fortran wrapper library, if they are using fortran.
Documentation
•A libcf website serves as the central repository for libcf documentation.

•Libcf C and Fortran APIs are fully documentated, and the documentation is available from the libcf website in the following formats: pdf, HTML, ASCII, postscript.

•The documentation is included with the libcf distribution in the following formats: pdf, HTML, ASCII, postscript.
Testing
• libcf can be tested with "make check", which will run automatic tests covering all API functions.

• libcf is tested on Linux, SunOS, AIX, HP-UX, Irix, and Macintosh OS/X.

• libcf is tested with the following fortran compilers: gfortran, g95, SunOS f90, Portland Group, Lahey.