Conventions for the standardization of NetCDF files 

Sponsored by the "Cooperative Climate Research Virtual Data Center", a
NOAA/university cooperative for the sharing and distribution of global
atmospheric and oceanographic research data sets

10 February 1995 - version 1.0
13 March 1995 - minor editorial repairs

University participants:

   CIRES (University of Colorado Joint Institute, Boulder CO)
   JISAO (University of Washington Joint Institute, Seattle WA)

NOAA participants:

   CDC, Asheville NC
   NCDC, Boulder CO
   PMEL, Seattle WA



Relationship to other Standards:

This standard is a set of conventions adopted in order to promote the interchange and
sharing of files created with the netCDF Application Programmer Interface (API).
This standard is based upon version 2.3 of netCDF. Documentation of the netCDF
API may be found in the "NetCDF Users' Guide, Version 2.3, April 1993 available
from URL http://www.unidata.ucar.edu/packages/netcdf/ or via anonymous ftp at
ftp.unidata.ucar.edu. All conventions named in that document will be adhered to in
this standard unless noted to the contrary.

This standard also refers to version 1.7.1 of the udunits standard supported by
Unidata. The udunits package is available via anonymous ftp at ftp.unidata.ucar.edu.
Included in the udunits package is a file, udunits.dat, which lists collections of unit
names. The names given therein and their plural forms will be regarded as acceptable
unit names for this standard with the following additions and deletions:

   "degrees" - deleted 
   "level", "layer", "sigma_level" - added 

The unit "degrees" creates ambiguities when attempting to differentiate longitude
and latitude coordinate variables; files must use "degrees_east" for units of longitude
and "degrees_north" for units of latitude or the alternative "spellings" of those names
listed in the sections on longitude and latitude coordinates below. The dimensionless
units "level", "layer", and "sigma_level" are sometimes needed when representing
numerical model outputs.

The udunits package also supports linear transformation of all units through the
syntax "scale_factor unit_name@offset", for example, "0.0005 degC@40000". This
syntax, however, is not supported by this standard.



Conventions:

File Name:
NetCDF files should have the file name extension ".nc". 

Coordinate Variables: 
1-dimensional netCDF variables whose dimension names are identical to their
variable names are regarded as "coordinate variables" (axes of the underlying grid
structure of other variables defined on this dimension).

Global attributes: 
Although not mandatory the attribute "history" is recommended to record the
evolution of the data contained within a netCDF file. Applications which process
netCDF data can append their information to the history attribute.

Data Variable attributes:

   long_name - a long descriptive name (title). This could be used for labelling
   plots, for example. If a variable has no long_name attribute assigned, the
   variable name will be used as a default. 
   scale_factor - If present for a variable, the data are to be multiplied by this
   factor after the data are read by the application that accesses the data. (see
   further discussion under the add_offset attribute) 
   add_offset - If present for a variable, this number is to be added to the data
   after it is read by the application that accesses the data. If both scale_factor
   and add_offset attributes are present, the data are first scaled before the offset
   is added. The attributes scale_factor and add_offset can be used together to
   provide simple data compression to store low-resolution floating-point data
   as small integers in a netCDF file. When scaled data are written, the
   application should first subtract the offset and then divide by the scale factor.
   The NOAA cooperative standard is more restrictive than the netCDF Users
   Guide with respect to the use of the scale_factor and add_offset attributes;
   ambiguities and precision problems related to data type conversions are
   resolved by these restrictions. If the scale_factor and add_offset attributes are
   of the same data type as the associated variable no restrictions apply; the
   unpacked data is assumed to be of the same data type as the packed data.
   However, if the scale_factor and add_offset attributes are of a different data
   type than the associated variable (containing the packed data) then in files
   adhering to this standard the associated variable may only be of type byte,
   short, or long. The attributes scale_factor and add_offset (which must match
   in data type) must be of type float or double. The data type of the attributes
   should match the intended type of the unpacked data. (It is not advised to
   unpack a long into a float as there is a potential precision loss.) 
   _FillValue - If a scalar attribute with this name is defined for a variable and
   is of the same type as the variable, it will be subsequently used as the fill
   value for that variable. The purpose of this attribute is to save the applications
   programmer the work of prefilling the data and also to eliminate the duplicate
   writes that result from netCDF filling in missing data with its default fill
   value, only to be immediately overwritten by the programmer's preferred
   value. This value is considered to be a special value that indicates missing
   data, and is returned when reading values that were not written. The missing
   value should be outside the range specified by valid_range (if used) for a
   variable. It is not necessary to define your own _FillValue attribute for a
   variable if the default fill value for the type of the variable is adequate. 
   missing_value - missing_value is a conventional name for a missing value
   that will not be treated in any special way by the library, as the _FillValue
   attribute is. It is also useful when it is necessary to distinguish between two
   kinds of missing values. The netCDF data type of the missing_value attribute
   should match the netCDF data type of the data variable that it describes. In
   cases where the data variable is packed via the scale_value attribute this
   implies that the missing_value flag is likewise packed. The same holds for the
   _FillValue attribute. The NOAA cooperative standard does not endorse any
   particular interpretation of the distinction between missing_value and
   _FillValue.

Units attribute:
A character array that specifies the units used for the variable's data. Where possible
the units attribute should be formatted as per the recommendations in the Unidata
udunits package. 

Other attributes: 
A file will normally contain many attributes that are not standardized in this profile.
Those attributes do not represent a violation of this standard in any way. Application
programs should ignore attributes that they do not recognize.

Variable names:
Variable names should begin with a letter and be composed of letters, digits, and
underscores. It is recommended that variable names be case-insensitive implying that
the same case-insensitive name should not be used for multiple variables within a
single file.

Rectilinear coordinate systems, only:
The space/time locations of points within the netCDF variables should be the simple
ordered tuples formed by associating values from their coordinate axes. Thus, for
example, curvilinear coordinate systems in which the coordinate locations must be
inferred from other non-coordinate variables or from an equation are not
standardized by this netCDF profile.

Number of dimensions:
All netCDF variables will be defined on either one, two ,three, or four dimensions
(the nature of the data will dictate the natural encoding). Where it makes sense single
point locations should be encoded as coordinate variables, for example, the latitude
and longitude positions of a vertical profile are natural candidates for single point
latitude and longitude coordinate variables.
If it is necessary to create a netCDF file with more than 4 dimensions it is
recommended that the additional dimension(s) be added "to the left" of the space and
time dimensions as represented in CDL. For example 

        float my_variable(param_value,time,height,lat,lon);  

would be the recommended representation of a fifth, parameter value, coordinate. 

Coordinate variable names:
The names of coordinate variables are not standardized by these conventions (since
data sets may in general contain multiple coordinate variables of the same
orientation). Coordinate variable names should follow the same general naming rules
(above) as other netCDF variables.

Order of dimensions:
If any or all of the dimensions of a variable have the interpretations of "date or time"
(a.k.a. "T"), "height or depth" (a.k.a. "Z"), "latitude" (a.k.a. "Y"), or "longitude"
(a.k.a. "X") then those dimensions should appear in the relative order T, then Z, then
Y, then X in the CDL definition corresponding to the file.

Data type:
The data type of coordinate and non-coordinate variables is unrestricted (byte, short,
long, float, and double are all acceptable data types). Although not forbidden by this
standard the data type "char", which is functionally identical to "byte", is not
recommended as netCDF has reserved the option to modify its behavior in future
versions. 

Coordinate value ordering:
The coordinate values of a coordinate variable must be either monotonically
increasing or monotonically decreasing. However, the coordinate values need not be
evenly spaced. Missing values are not permitted in coordinate variables.

Coordinate Variable Attributes:
If a coordinate variable contains longitude, latitude, depth, elevation, date, or time
values then the units attribute is mandatory; it is used to determine the orientation of
the coordinate variable. The long_name attribute is optional but may be used to
enhance clarity and the self-describing nature of the netCDF file. Since coordinate
variables may not contain missing values the attributes _FillValue and missing_value
may not be used with coordinate variables.

Time or date dimension:
Coordinate variables representing time must always explicitly include the units
attribute; there is no default value. A time coordinate variable will be identifiable by
its units, alone. The units attribute will be of character type with the string formatted
as per the recommendations in the Unidata udunits package version 1.7.1. The
following excerpt from the udunits documentation explains the time unit encoding by
example:

The specification:

seconds since 1992-10-8 15:15:42.5 -6:00

indicates seconds since October 8th, 1992 at 3 hours, 15
minutes and 42.5 seconds in the afternoon in the time
zone which is six hours to the west of Coordinated
Universal Time (i.e. Mountain Daylight Time). The time
zone specification can also be written without a colon
using one or two-digits (indicating hours) or three or
four digits (indicating hours and minutes). 

The acceptable units for time are listed in the file udunits.dat. The most commonly
used of these strings (and their abbreviations) includes day (d), hour (hr, h), minute
(min), second (sec, s), year (yr). Plural forms are also acceptable. The date string may
include date alone; date and time; or date, time, and time zone. 
It is recommended that the unit "year" not be used as a unit of time. Year is an
ambiguous unit as years are of varying length. Udunits defines a year as exactly 365
days. 

A time coordinate variable is identifiable from its units string, alone. The udunits
routines utScan and utIsTime can be used to make this determination. (*Note that at
the time of this writing the author of this draft profile had not tested these routines
personally.)

Climatological time:
Coordinate variables representing climatological time (an axis of 12 months, 4
seasons, etc. that is located in no particular year) should be encoded like other time
axes but with the added restriction that they be encoded to begin in the year 0000.
(Note - at the time of this writing this encoding has not been tested with the udunits
package.)

Vertical (height or depth) dimension:
Coordinate variables representing height or depth must always explicitly include the
units attribute; there is no default value for the units attribute. The units attribute
will be of character type.

The acceptable units for vertical (depth or height) coordinate variables are 

   units of pressure as listed in the file udunits.dat. For vertical axes the most
   commonly used of these include include bar, millibar, decibar, and
   atmosphere (atm). 
   units of length as listed in the file udunits.dat. For vertical axes the most
   commonly used of these include meter (metre, m), centimeter (cm), decimeter
   (dm), and feet (ft). 
   the (dimensionless) units "level", "layer", or "sigma_level" 
   other units listed in the file udunits.dat that may under certain circumstances
   reference vertical position such as units of density or temperature. 

Plural forms are also acceptable.

The direction of positive, whether up or down, cannot in all cases be inferred from
the units. The direction of positive is useful for applications displaying the data. For
this reason the new attribute positive is defined in this standard. The inclusion of the 
positive attribute is required by this standard if the vertical axis units are not a valid
unit of pressure (a determination which can be made using the udunits routine,
utScan) -- otherwise its inclusion is optional. The positive attribute may have the
value "up" or "down" (case insensitive).

For example, if an oceanographic netCDF file encodes the depth of the surface as 0
and the depth of 1000 meters as 1000 then the axis would use attributes as follows:

axis_name:units="meters"; 
axis_name:positive="down"; 

If, on the other hand, the depth of 1000 meters were represented as -1000 then the
value of the positive attribute would have been "up". If the units attribute value is a
valid pressure unit the default value of the positive attribute is "down".

A vertical coordinate variable will be identifiable by 

   units of pressure; or 
   the presence of the positive attribute with a value of "up" or "down" (case
   insensitive). 

Latitude dimension:
Coordinate variables representing latitudes must always explicitly include the units
attribute; there is no default value for the units attribute. The units attribute will be
of character type with the string formatted as per the recommendations in the
Unidata udunits package.

The recommended unit of latitude is "degrees_north". Also acceptable are
"degree_north", "degree_N", and "degrees_N".

A latitude coordinate variable is identifiable from its units string, alone. The udunits
routines utScan can be used to make this determination. (Note that at the time of this
writing the author of this draft profile had not tested these routines personally.)

Longitude dimension:
Coordinate variables representing longitudes must always explicitly include the units
attribute; there is no default value for the units attribute. The units attribute will be
of character type with the string formatted as per the recommendations in the
Unidata udunits package. 

The recommended unit of longitude is "degrees_east" (eastward positive). Also
acceptable are "degree_east", "degree_E", and "degrees_E". The unit "degrees_west"
(westward positive) is not recommended because it implies a negative conversion
factor from degrees_east.

Longitudes may be represented modulo 360. Thus, for example, -180, 180, and 540
are all valid representations of the International Dateline and 0 and 360 are both valid
representations of the Prime Meridian. Note, however, that the sequence of
numerical longitude values stored in the netCDF file must be monotonic in a
non-modulo sense. 

A longitude coordinate variable is identifiable from its units string, alone. The
udunits routines utScan can be used to make this determination. (Note that at the time
of this writing the author of this draft profile had not tested these routines
personally.)


The example CDL file which follows contains variable and coordinate definitions
(output of "ncdump -h") that illustrate most of the points discussed above. The actual
data values are omitted.

The CDL file contains the followind elements:

   a 1-dimensional (vertical axis) atmospheric variable at a point 
   a 1-dimensional (vertical axis) variable globally averaged in longitude 
   a 2-dimensional packed variable on a latitude/longitude grid 
   a 3-dimensional variable on a lat/long/climatological time grid 
   two 4-dimensional variables on grids that are staggered in lat/long 


hankin@pmel.noaa.gov 
