Hi,
I have been following the discussion and think that Jon Blower summarised this
nicely. According to my experience the importance of specifying datum is not
well known to many scientists and a set of conventions that increase the
awareness of this issue is useful, especially in the long term. Working at
coarse resolution (e.g. some NWP, GCM), errors created by ignoring datum
are usually acceptable, but at higher resolution datums are required,
especially since we frequently combine sources of data (e.g. high resolution
NWP, weather radar, satellite and in situ observations).
Among Jon's 3 options I would certainly go for option 2 forcing data providers
to either specify the correct datum or to provide a hopefully useful
explanation for why it is unknown.
All the best
?ystein
> Hi Jonathan, all,
>
> > There is lots of important metadata which is optional, such as standard
> > names
>
> I think the case of a missing datum is different from the case of a missing
> standard name. In order to plot data in a GIS (or similar system that
> allows different datasets to be compared or overlain spatially) you always
> need a datum. If one isn't supplied by the data provider, the user or
> client software has to invent or guess one. That's not true for a
> standard name (although arguably it is true for the coordinate bounds).
>
> Hence Heiko's comment about what ncWMS does - in fact ncWMS
assumes WGS84
> for lat-lon coordinates, but perhaps it should assume something
> different. It does make a difference in lots of cases - both model and
> obs data.
>
>
> It seems we're all agreeing that CF should at least have the right tags to
> ensure that a datum can be well specified, for 1D grid coordinates,
> projected grid coordinates, 2D curvilinear grid coordinates and
> geolocation of observations. Would you agree?
>
> What seems to be under debate is to what happens when the datum is
> genuinely (and regrettably) not known or incompletely specified (or
> perhaps where the dataset has been derived from mixed datums). The
> proposals seem to be:
>
> 1. Rule this out of scope for CF and don't provide any datum information at
> all. 2. Invent CF tags that explicitly say the datum is unknown, and
> perhaps provide a reason (to give the user a bit more information for
> his/her interpretation). 3. Specify a default datum (noting that client
> software or the user will commonly have to invent one anyway).
>
> Is this a fair summary of the situation?
>
>
> Best wishes,
> Jon
--
Dr. Oystein Godoy
Norwegian Meteorological Institute
P.O.BOX 43, Blindern, N-0313 OSLO, Norway
Ph: (+47) 2296 3000 (switchb) 2296 3334 (direct line)
Fax:(+47) 2296 3050 Institute home page: http://met.no/
Received on Fri Aug 05 2011 - 05:29:13 BST
