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.
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Received on Wed Aug 03 2011 - 08:52:24 BST
