In the CDM, we assume a spherical earth when no other info is given.
This reflects the early history of CF and CDM in dealing mostly with
global models.
WGS84 is the default ellipsoid used eg for UTM projection.
It would be well worth CF specifying default assumptions as well as how
to do datums correctly. One of the main goals of TDS/NcML is the ability
to add/change attribute information in legacy datasets, so at least for
the TDS, data providers could quickly correct their datasets if it only
involves attributes.
On 4/19/2010 10:06 AM, Jonathan Blower wrote:
> Thanks Nan, this is a good point. I wonder how such QC information
> could be recorded (I guess this would be a new thread of discussion? ;-)
>
> Jon
>
> -----Original Message-----
> From: Nan Galbraith [mailto:ngalbraith at whoi.edu]
> Sent: 19 April 2010 16:49
> To: Mike Grant
> Cc: Jonathan Blower; cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] Default datum for latitude and longitude?
>
> Just want to point out that many of our older datasets, which are
> still available - and actually still used - predate the routine use of
> GPS on ships. These datasets are from buoys that were set using
> dead reckoning.
>
> And, although we now have pretty nearly exact anchor positions,
> the buoys on our moorings deployed in about 5 km of water
> typically have 5 km watch circle diameters.
>
> So, although the datum specification should be encouraged, there's
> a definite drawback to assuming a default. And, the position's accuracy
> is probably more significant than the datum spec for a lot of in situ
> data-
> so we should really encourage the use of QC terms for all coordinates.
>
> Thanks - Nan
>
> Mike Grant wrote:
>
>> On 19/04/10 15:43, Jonathan Blower wrote:
>>
>>
>>> specification, which datum should be assumed? Spherical Earth?
>>>
> WGS84?
>
>>>
>>>
>> If you're picking one at random, I'd go for WGS84 - that's a pretty
>>
> safe
>
>> bet for a lot of remote sensed and GPS related data.
>>
>> Cheers,
>>
>> Mike.
>>
>>
>>
>>
>>> Hi all,
>>>
>>> The CF conventions define a means to specify the datum used in
>>>
> certain
>
>>> map projections
>>>
>>>
> (http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.4/cf-conventions.ht
>
>>> ml#grid-mappings-and-projections). In the (common) case of a data
>>>
> file
>
>>> containing latitude and longitude axes, but no explicit datum
>>> specification, which datum should be assumed? Spherical Earth?
>>>
> WGS84?
>
>>> The horizontal error resulting from use of an incorrect datum can be
>>> significant for high-resolution and/or local dataset (hundreds of
>>> metres).
>>>
>>> Cheers, Jon
>>>
>
Received on Mon Apr 19 2010 - 12:40:03 BST
