Information on Entire Data Set (global attributes)

The global attributes store descriptive information pertaining to the entire dataset.

A distinction is made between mandatory attributes and optional attributes (Chapter 3.2). The optional attributes are necessary to accommodate the different data sources/disciplines.

Attribute names can generally be freely chosen, but there are attribute names that are already reserved and can only be used for specified purposes. These are as follows:

add_offset, ancillary_variables, axis, bounds, calendar, cell_measures,
cell_methods, climatology, comment, compress, Conventions, coordinates,
_FillValue, flag_masks, flag_meanings, flag_values, formula_terms,
grid_mapping, history, institution, leap_month, leap_year, long_name,
missing_value, month_lengths, positive, references, scale_factor,
source, standard_error_multiplier, standard_name, title, units,
valid_max, valid_min, valid_range.

For the meaning of the attribute names listed above, please see Appendix A in netCDF Climate and Forecast (CF) Metadata Conventions 7.

Mandatory Global Attributes

Global attributes are the netCDF file’s metadata. Only information that applies to the file as a whole is to be included here.

Attribute	Description/Format	Specification/Example
Conventions	-fixed-	CF-1.8
institution	-fixed-	Helmholtz-Zentrum Hereon, Institute of Coastal Systems, Germany
title	Any text
source	Represents the source and method used to produce the original data. For data generated by models, the model name and version should be listed here.	cosmo_100614_4.14_clm2 int2lm_120824_1.20_clm1 remote sensing in-situ
creation_date	Information that pertains to the creation date of the netCDF file	2017-06-06T15:00:29Z 2017-06-06T15:00:29+02:00
originator	Creator of the data set	John Smith
contact	Person, email or URL	http://coastmod.hereon.de
crs	Coordinate reference system. WGS84 is the default	EPSG:4326

7: http://cfconventions.org/cf-conventions/cf-conventions.html#attribute-appendix

Optional Global Attributes

Optional global attributes are not relevant at the Hereon for all netCDF files, but agreements can be made as necessary in individual groups. If the described attribute is relevant, then this attribute should be used consistently.

The comment attribute should be used in particular to describe the file contents.

The carrier attribute describes the carrier on which a platform is installed temporarily—for example, RV Prandtl.

In COSYNA the platform attribute is mandatory and is usually specified. An example of platform would be “Ferrybox on RV Polarstern”.

The origin of a netCDF file is described in lineage. This is usually an original file in another format.

The individual sequence of processing should be described in processingSteps.

StartTime and StopTime denote the first and last time values and facilitate quick and easy access to these values—for example via OPeNDAP.

Should the file be disseminated externally or published, the user rights and obligations are outlined in the distribution_statement. The content should be coordinated within the respective institute division.

In history one can directly record what was done to the file to reach its current state. Tools like the CDOs or NCOs document the workflow in the history automatically. One can also refer to an external file or URL here.

PI should designate the scientist responsible for the entire project to which the file belongs.

The following attributes are available for describing geographical location: geospatial_lon_min/_max, geospatial_lat_min/_max or Bbox. They are, for example, necessary for metadata catalogue inquiries.

Attribute	Description/Format	Specification/Example
comment	A short description of the file contents; essentially an abstract.
campaign	Campaign ID (for observations)	“belawatt071”
experiment_id	Experiment ID (for models)	“cD3_011_ERAi”
carrier	Any text.	“RV Prandtl”
platform	Any text. The specified name is required for COSYNA platforms	“Scanfish01”
profileType		“DownCast”
lineage	Origin of the file can be written as any text (i.e., no formatting rules)
processingSteps	Any text; example format: `"{original data}{Plausibility check using defaults}{5-point median filter}{0.1-dbar block median}"`
processingSoftware	Name und version. If using several programs, names should be separated by “;”	“GALATON V0.97”
StartTime		“2007-03-13T07:35:10Z”
StopTime		“2007-03-13T07:40:28Z”
Distribution_statement	Coordinated text, Example A3
history	This should be a sort of logbook. This logbook can be an external file. According to CF conventions, it is a list of the applications that have modified the original data.
references	E.g., DOI
contact_email	Email address (Hereon)
StartLatitude	Float Relevant for transects	54.7457933333333
StopLatitude	Float Relevant for transects	54.7451966666667
StartLongitude	Float Relevant for transects	8.30397666666667
StopLongitude	Float Relevant for transects	8.30919166666667
geographicName		“hoernumtief”
station		“KormoranHoernum”
castID		“CTD001DW”
PI	Standard first and last name	“Person”
operator	Standard first and last name	“Volker Schmidt”
contributor	Standard first and last name	“Anna Mayr, Tim Schmalbach”
forcing		“COSMO CLM 5.0”
forcingResolution		“0.11 degrees”
boundaryCondition	Any text
modelNest		“CD16”
geospatial_lon_min	Data in decimal degrees (float) Longitude units = degrees east	5.3457
geospatial_lon_max	Data in decimal degrees (float) Longitude units = degrees east	10.1234
geospatial_lat_min	Data in decimal degrees (float) Latitude units = degrees north	53.1234
geospatial_lat_max	Data in decimal degrees (float) Latitude units = degrees north	56.1234
Bbox	alternative to geospatial_l*. Sequence: lonMin, latMin, lonMax, latMax	5.3,53.1,10.1,56.1
imo_platform_code	International Maritime Organisation platform code	“9144263”
ices_platform_code	ICES platform code	“58LY”
licence	Licence of the submitted data
institution-ID	ROR (Research Organization Registry)	ROR: 03qjp1d79

Dimensions

The data is stored as multi-dimensional fields in netCDF files. Decisive structural elements are the dimensions. They are used to define the internal coordinate structure of the data sets (variables). They contain information on the spatial dimensions (nx,ny,nz) as well as the time dimension (time). These are explained under variables (see Section 4).

A variable can have any number of dimensions.
All dimensions must have different names.
A restriction to four dimensions is recommended.
Dimensions of a variable should be arranged in the relative order of date/time, height/depth, latitude, longitude (T, Z, Y, X). If this is not possible, (e.g., for spectra), other reasonable sequences should be created.

Additional dimensions must be to the left of (T,Z,Y,X).

If time series are stored in netCDF files, then one must keep in mind that the series can be continued by adding new times. The time variable dimension should be “UNLIMITED” to ensure expansion of the time series without difficulties. It is then referred to as the record dimension.