Using variable attributes for mapping - WOCE Exchange

[Rob Thomas]

Hi all,

I'm planning to add a variable attribute to some of our datasets that would provide a mapping to the WOCE Exchange format metadata/variable/parameter column names. This is to facilitate the download and ingestion of our data by a third party without us needing to reformat and email files etc. They would then look at the reformatting on ingestion by their system from a netCDF file download. I'm reluctant to rename the existing columns because they are named that way for other users.

Is there an already defined attribute used by the Erddap community to hold this information? Is anyone else using attributes in this way and if so, for WOCE Exchange what have you used? If possible, I don't want to introduce variation within Erddap dataset metadata across the community.

If nothing has been adopted to date, could I propose a variable attribute name of <woce_exchange>?

Cheers

Rob

[Roy Mendelssohn - NOAA Federal]

Hi Rob:

Thanks for the inquiry, this sounds interesting.  Sometimes LLMs can your friend - I havent’ looked into this but here is my conversation with Claude.

Are there any mappings between the WOCE Exchange format and the CF conventions

Yes, there are well-documented mappings, primarily maintained by CCHDO (CLIVAR and Carbon Hydrographic Data Office) as part of the GO-SHIP program. Here's the landscape:

The main reference: GO-SHIP CF/netCDF Format Spec

The GO-SHIP CF/netCDF format specification at hydro.readthedocs.io is the canonical mapping between WHP Exchange (WOCE) parameters and CF conventions. It defines two non-CF custom attributes that bridge the two worlds: whp_name(the name the variable would have in a WHP Exchange or WOCE sea/ctd file) and whp_unit (the value that would appear in the units line of the WHP Exchange file).

How CF standard names are applied

The spec follows CF usage strictly: if a CF standard name exists for the physical quantity represented by a variable, the most specific name must be used and appear in the standard_name attribute. The standard names table is updated frequently, and new names are evaluated for inclusion as they're added. It's important to understand that standard names represent the physical quantity, not how the data was collected.

Units translation nuances

Not all WHP parameter/unit pairs have CF standard names yet. Some WHP units are not valid in CF units attributes — notably the temperature reference scales ITS-90 and IPTS-68, which are both mapped to degC in CF files. These instead appear in a reference_scale attribute of the variable rather than in the units attribute.

The machine-readable parameter database

The Exchange Format 1.2.1 documentation provides a machine-readable JSON parameter list (CCHDO Parameter database version 2025.10.0) and a validation schema. For each parameter it documents the WHP name, units, data type, quality flag definitions, and CF/netCDF attributes.

Where to look

The most useful resources for doing the actual mapping:

• exchange-format.readthedocs.io/en/latest/parameters.html — per-parameter tables with CF standard_name and units for each WHP Exchange parameter (CTDPRS, CTDTMP, CTDSAL, OXYGEN, NITRAT, etc.)
• hydro.readthedocs.io/en/latest/format_spec.html — the GO-SHIP CF/netCDF spec with the full variable list linking WHP names to their CF representations
• hydro.readthedocs.io/en/latest/variables/ — individual variable pages (e.g. /variables/pressure.html) showing the complete attribute mapping for each parameter

The cchdo.hydro Python package also implements these mappings programmatically if you need to do format conversion.

Hope this helps.  

-Roy

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[Rob Thomas]

Hi Roy,

Thanks for the direction.

Cheers

Rob