Correction to the datum/ellipsoid explanation below. The NAD83 datum does use the GRS80 ellipsoid. The NAD27 datum uses the Clarke 1866 ellipsoid. Woops!
Dave On Aug 3, 2011, at 9:07 AM, David Blodgett wrote: > Dear Phil, > >> I would have thought that if a particular piece of >> data analysis is at a resolution that requires a geodetic datum to be >> specified then, > > The problem here is that there is no analysis that does not require this > information. While some choose to assume that all data uses the same datum > and drop those terms of analysis out of their math, that does not mean the > information is not there. > >> giving the user the >> opportunity to select the one s/he believes to be the most appropriate >> for the task in hand. > > > Having personally been in this situation, it took me two years to track down > exactly the right geographic transformation to apply to accurately apply > radar data to the landscape. It is unfair to expect that a terrestrial > modeler understand the handling of geographic data in climate and forecasting > applications to such an extent that they are comfortable making such a > decision. > > Your argument about darwinian evolution of data use would cause a massive set > back in interdisciplinary science. This one minor inclusion of necessary > metadata would allow a broad community of users to more easily leverage data > using the climate and forecasting metadata conventions. Since the CF > community does disregard datum metadata, they could continue to silo > themselves from the rest of the environmental modeling community. Or, they > could extend a bit of an olive branch and recognize that this information is > critical and required for most terrestrial applications that consume > atmospheric data. > > Regarding the difference between GRS 80 and WGS 84 ellipsoids, they are > different by small fractions of a meter. >> From wikipedia: The very small difference in the flattening thus results in >> a—very theoretical—difference of 0.105 mm in the semi polar axis. For most >> purposes, the differing polar axes can be merged to 6 356 752.3 m, with the >> inverse flattening rounded to 298.257. > > > You are referring to the NAD83 Datum which uses the Clarke 1866 ellipsoid > which does fit the continental united states better than the GRS80/WGS84 > ellipsoid. > > Cheers, > > Dave B > > On Aug 3, 2011, at 8:48 AM, Bentley, Philip wrote: > >> Dear Heiko, >> >>> I hope CF could define a default datum, e.g. the GRS1980 >>> Authalic Sphere, since this matches most closely with >>> existing software (netcdf-java). This would make live easier >>> for the software-developers who have to use something if >>> nothing is given. >> >> I'm not sure that defining a default datum for CF is the right way to go >> in this instance. I would have thought that if a particular piece of >> data analysis is at a resolution that requires a geodetic datum to be >> specified then, in absentia the actual one being defined in metadata, >> it's not clear to me that using some semi-arbitrary, and potentially >> invalid, default datum is any better than giving the user the >> opportunity to select the one s/he believes to be the most appropriate >> for the task in hand. >> >> The current CF conventions include a (fairly minimal) set of metadata >> attributes which can be used to describe the basic properties of the >> coordinate reference system associated with a given dataset. The onus >> then is on data producers to utilise those metadata attributes to >> describe their data to the fullest extent possible. Furthermore, other >> non-CF attributes may be used to augment the standard set - over time >> some of these additional attributes would no doubt find their way into >> the CF specification. >> >> Ultimately, if end-users consider that a given dataset has insufficient >> metadata to justify its use within a particular context, then they can >> always choose to ignore that dataset. With the passage of time - and in >> true Darwinian fashion - such datasets (and their producers) will find >> that they are increasingly disregarded/overlooked in analyses. Hopefully >> this would galvanise such data producers into improving the quality of >> their spatial metadata! >> >> Regards, >> Phil >> >> >> PS: if a default datum were to be encoded into the CF conventions, I'd >> imagine that the WGS84 datum would be the way to go rather than GRS80 >> which, if I understand correctly, has somewhat more of a bias towards >> use over the North American continent. That said, I suspect the >> differences between the 2 datums are sufficiently small as to get lost >> in the underflow for many metocean research applications. >> _______________________________________________ >> CF-metadata mailing list >> [email protected] >> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
