Mark et al.-
We at NCDC are working on a similar issue. Specifically, we're looking
at how best to store satellite data by writing a new convention that
extends CF for some satellite specific issues. The metadata
content/attributes is an excellent example of what needs to decided.
IMO, I recommend a naming convention similar to that used by Unidata
for
http://www.unidata.ucar.edu/software/netcdf-java/formats/DataDiscoveryAttConvention.html
be used. Unidata made an attempt to show how the metadata can be mapped
to other standards at:
http://www.unidata.ucar.edu/software/netcdf-java/formats/ncACDD-metadataMappings.html
In short, what could be done is
a) Data producers provide data in netCDF files following some
convention (CF, ...)
b) Producers provide metadata following a "generic" Metadata Convention
(e.g., specified using global attribute "Metadata_Conventions") .
c) Metadata harvesters can then build algorithms to search for and map
the generic metadata to their preferred convention (e.g., FGDC)
Perhaps a standard metadata naming convention could be developed
(similar to the standard_names in CF) which is a "lowest common
denominator" of the existing conventions? Of course the
downside to this is that it produces another metadata convention...
Or
perhaps each standard (FGDC, ISO19115, ...) could have its own list of
netCDF attributes.
Also, NCML could also be used to provide mapping and attribute renaming
from one convention to another.
Additionally, with respect FGDC information, it would good to provide
only dynamic metadata in the netCDF file and provide static metadata
through a link/URL.
These aren't answers, just more ideas.
-Ken
p.s. As a post-script/appendix, I am attaching a portion of our
strawman (which is still being developed mind you ;-) CF-Satellite
convention [an extension of the CF convention to archive satellite data]
The CF convention
specifies a small
number of attribute names (at least compared to metadata conventions
like FGDC,
etc.). For CF-SAT, we categorize
metadata as ‘use’ or ‘discovery’ metadata. ‘Use’ metadata are
those
which need to be applied to the data to make use of the variables (this
describes the bulk of the CF attributes). On the other hand,
‘discovery’
attributes are those which summarize or provide more detail of the
data. In
netCDF data, discovery attributes tend to be global attributes.
There is, of course,
overlap
between use and discovery metadata, however, it provides a conceptual
means for
identifying the purpose of the attributes: attributes are either used
to help
the data be used or understood.
Furthermore,
conventions such as
FGDC require metadata that is both dynamic and static. For instance, in
satellite data, orbital parameters change periodically so they are
dynamic. However,
the launch of the satellite is constant and not changing, thus it is
static. It
is proposed that some static metadata not be recorded in the CFSAT
convention
(i.e., in the netCDF file), rather data users can be pointed to a
static URL using
a global attribute from which the remaining static metadata can be
determined.
It is convenient
that the efforts
of dataset conventions (like CF) have primarily been concerned with Use
metadata while other conventions (e.g., FGDC) provide requirements on
discovery
metadata. The two are largely orthogonal in that using the complete CF
attributes and a metadata convention like FGDC will provide little
conflict.
Mark Ohrenschall said the following on 7/9/2007 5:09 PM:
Hi folks,
We are developing CF compliant netCDF files with additional (or
"optional") metadata in the netCDF header from our own sources, e.g., a
subset of the FGDC metadata content standard, our Rich Inventory
database, etc... Our sources are potentially new, additional netCDF
conventions, and so our essential question is if there is a best
practices guide for netCDF convention developers, especially so that
various conventions do not clash or conflict with each other.
For example, if we are free to name our new, homemade netCDF attributes
however we like, could those attribute names conflict with existing or
future netCDF conventions? Is there any mechanism for indicating which
convention(s) a netCDF attribute complies with? Is there a namespace
mechanism so that if two conventions use the same name for an
attribute, that attribute can be disambiguated?
We're also wondering what to do for those attributes which serve both
the CF convention and our own FGDC (or others) convention. For the sake
of netCDF software that is looking for CF attributes we'd have to name
such a common attribute using its CF name, but what if we also wanted
to ensure that that common attribute remained associated with its FGDC
siblings? (E.g., suppose we wanted to extract all FGDC attributes from
the netCDF header?) Do we then need to define that attribute twice,
once with its CF name and once with its FGDC name?
So we are wondering what the netCDF conventions community thinks about
these considerations, and any advice or experience they may offer us on
how to add our own attributes to a netCDF file in a coherent way that
is harmonious with other, existing conventions.
Thank you -- Mark
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Ken Knapp
Remote Sensing and Applications Division
National Climatic Data Center
151 Patton Ave
Asheville, NC 28801
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