[Esip-documentation] on documenting instrument and platform metadata

[John Graybeal]

What great questions!  The following thoughts are my own, and are unrelated to my previous ACDD activities.

Clearly some data files will have data from multiple platforms and instruments; whereas others will get all their variables from a single platform/instrument; and some will have data so heavily processed as to be barely traceable to the source platforms/instruments. Each of the specifications is oriented toward somewhat different scenarios, and none of them are yet great at representing all the scenarios.

Let's consider ACDD first. Note that this specification provides _recommendations_ for attributes, not _requirements_; and the platform and instrument attributes are only 'Suggested'. These give a broad-brush categorization of the platform(s) or instrument(s) that collected a data set; if it seems reasonable to specify that information for the whole data set, use these attributes to do so. (If it isn't useful/applicable, then don't worry about these attributes.)  The most precise way to give that info is to use a controlled vocabulary (see https://marinemetadata.org/references?filter0=**ALL**&filter1%5B%5D=129&filter1%5B%5D=141 for a partial list of vocabularies), and typically I'd expect a type vocabulary here (with e.g. "CTD", "Mooring") not a specific instance.

If you *are* following ACDD, I don't recommend putting the names of variables containing information on platforms and instruments in the global attributes 'platform' and 'instrument'. Without ACDD describing that as a possibility, it's highly unlikely any software will know how to treat that, and some people will get confused too. The existence of the variables will be pretty self-explanatory, so just leave out those ACDD attributes. (I see this conflicts with NODC and OceanSITES recommendations, to my surprise; perhaps others will offer another view here.)

Whether to put the information in a global instrument variable, or attributes of each data variable, or both, seems to me a matter of circumstances and taste. Let's stipulate the best solution is human-recognizable, computer-parseable, extensible, and unambiguous. Thus, global attributes are inadequate. Assuming a sophisticated data system 5 years from now would look in both variables and data attributes, then the model of 'one variable per instrument or platform', and association from the data variables to those variables or platforms seems best.  

But given that the examples in each of the different profiles still seems inconsistent, likely you should follow the guidance most close to your work. If you aren't particularly close to any of them, I'd do it a bit differently still. But I'll hold that thought unless you want to extend the discussion in that direction.

John


On Mar 16, 2015, at 11:56, Aaron Sweeney via Esip-documentation <esip-documentation at lists.esipfed.org> wrote:

> Hi, folks,
> 
>      I’m struggling with how to capture platform and instrument information in the appropriate places in netCDF files.  The NODC templates suggest creating separate variables for platforms and instruments, with multiple variable attributes (make, model, serial number, precision, accuracy, etc.), and associating these platform and instrument variables with physical variables via an “ancillary_variable” variable attribute attached to the physical variable. Along these lines, SeaDataNet suggests including variable attributes for instrument (sdn_instrument_urn and sdn_instrument_name), and OceanSites suggests either the use of a separate instrument variable (with make, model, serial number attributes) or capturing make, model and serial number as variable attributes contained directly within the relevant physical     variable.  But ACDD-1.3 only has single global attributes for platform and instrument (and vocabularies).  The NODC templates suggest placing the names of the variables containing information on platforms and instruments in the global attributes: instrument and platform.  I like NODC’s and OceanSites’ “associative” approach (i.e. “this instrument goes with this physical variable”), but the use of variable names in global attributes conflicts with ACDD-1.3.
> 
>      It seems that in order to comply with ACDD-1.3, I need to capture instruments and platforms at a global level (dropping their association with physical measurement variables), but in order to comply with OceanSites and others, I need to capture instrument and platform information in separate variables and associate these with physical measurement variables via an ancillary_variable attribute.
> 
>      In order to comply with both, in the interest of enabling interoperability, I seem to need to repeat instrument and platform metadata in two different places.
>  
>      Any thoughts or guidance?
> 
> References:
> NODC netCDF timeseries template: http://www.nodc.noaa.gov/data/formats/netcdf/v1.1/timeSeriesOrthogonal.cdl
> OceanSITES Data Format Reference Manual (See Appendix 2 for sensor variables): http://www.oceansites.org/docs/oceansites_data_format_reference_manual.pdf
> SeaDataNet Data Transports Manual (See Section 4.2.2 Co-ordinate Variables): http://www.seadatanet.org/content/download/16251/106283/file/SDN2_D85_WP8_Datafile_formats.pdf
> 
> Thanks,
> Aaron
> 
> -- 
> Aaron D. Sweeney
> Water Level Data Manager
> 
> Cooperative Institute for Research in Environmental Sciences (CIRES)
> University of Colorado at Boulder
> and
> NOAA National Geophysical Data Center
> Marine Geology and Geophysics Division
> 325 Broadway, E/GC3
> Boulder, CO 80305-3328
> 
> Phone: 303-497-4797, Fax: 303-497-6513
> 
> DISCLAIMER: The contents of this message are mine personally and do not necessarily reflect any position of NOAA.
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