Dear Jonathan, All, Coming from the perspective of a geographer, for CF to be "a convention whereby people can provide accurate and complete metadata for their data." Datum specification would be required.
I understand that the normal practice in the climate science community is to throw out datum information from ingested ground based data as it is being applied to a model or summary product that essentially smears the measurement in space. So in fact, for strict accuracy of geolocation for course resolution low numerical accuracy applications, a datum may not add anything of value. Its clear I'm not going to win this argument by claiming that the status quo should not impose inaccuracy on the edge case. I hope that software developers do throw warnings when data has an unknown piece of information that is required to perform a geographic transformation. This lack of required information for processing is really the crux of the issue and why I have pushed it this far. In Chapter 4 (.1-.2) no recognition is given to the fact that latitude or longitude coordinates are essentially meaningless without the assumption of a datum, this recognition would go a long way to making me more comfortable with the spec. Something like the follows is may be warranted. Data consumers should be aware that latitude and longitude coordinates lacking description of the ellipsoid shape and prime meridian, or datum, must be assumed to lie on any arbitrary datum. It should be understood that it is up to end users of such data to assign this information according to their best judgement. We recommend, when accurate grid geolocation is appropriate, for data producers or publishers to use a latitude_longitude grid mapping as described in chapter 5.6. Cheers, Dave On Jul 28, 2011, at 5:00 AM, Jonathan Gregory wrote: > Dear Dave > > I sympathise with your concerns about the consequences of missing datums but > I don't think CF can help a lot. > >> Are there any arguments against CF recommending a standard datum assumption >> when intersecting data without a datum specified with data that does have a >> datum specified? > > I don't think that's the role of CF. CF is a convention whereby people can > provide accurate and complete metadata for their data. If the data provider > doesn't have a real-world datum that could be specified, it is not appropriate > for CF to suggest what it should be, I would argue. In fact, most parts of CF > are optional. People may choose not to describe their data with standard_names > for example. They are not mandatory, but of course it makes the data less > useful if they are not supplied. I think the best can be done, rather as > Balaji suggests, is for software that reads CF files to emit warnings about > metadata which could have been included but isn't, so that analysts are aware. > > Best wishes > > Jonathan
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