[CF-metadata] What do models assume for the shape of the Earth?

[Cj Beegle-Krause]

Hi All,

Certainly many ocean and atmospheric modelers don't pay attention to the 
shape of the earth beneath their models.  My view of the importance of 
the CF standards is allowing people to use other people's work to make 
"value added" products.  To that end, the user needs to be able to 
figure out where on the planet a point is from someone's model.  We 
could certainly express a "user beware" policy with respect to geoid 
given (lat, long), and have GIS users that cannot convert on the fly to 
do their own conversion at their own risk. 

We should have the metadata standards set up so that caring modelers can 
express their geoids correctly, and maybe the others could identify 
their models as "not GIS compatible" or we'll assume the "user beware".  
The providers most interested in getting their products used will be 
most interested in small details.

I work with the NOAA Office of Response and Restoration.  We do response 
in navigable waters for oil and chemical spills, airline disasters and 
things like that.  One of our goals is to be able to use more than just 
our own circulation models within our trajectory model, GNOME. So we are 
working to develop GNOME as a grid independent application. 
(http://response.restoration.noaa.gov/software/gnome/gnome.html though 
the capabilities I'm talking about are in Diagnostic Mode)
To that end, the CF standards are critically important.  Developing our 
model to understand any circulation model on its native grids using the 
CF metadata standards means that we could meet our response timelines 
with outside data.  Our office works 24 x 7, providing a verbal 
description of how the incident is expected to unfold within 30 min of 
notification, and producing any needed model derived forecasts within 2 
hours of notification.  Mostly we respond to incidents within the US or 
her territories, but we do at times get involved internationally. So, we 
can't be messing around with format conversions at all. 

Getting the coordinates correct is important:
-- if you are going to send someone out to look for something
-- if you care where something contacts the shoreline (calculating 
beaching correctly is a big issue for us.)
-- if you are going to overlay the trajectory with the location of 
resources as risk (most often represented in GIS systems).

As I said above, even if most modelers aren't going to pay attention to 
their geoid, the wind and water modelers that want their 
nowcasts/forecasts used for emergency response will pay attention to 
details like geoid.  So we should have the metadata standards thought 
out and ready for them.  NOAA/ORR is increasingly working with the IOOS 
Regional Associations on how to set up model connectivity between our 
office and their models.  Being able to point to the CF standards may be 
more important in saving the world a piece at a time than you think.

-CJ

-- 
CJ Beegle-Krause, Ph.D.
NOAA/NOS/ORR/Hazmat
7600 Sand Point Way NE
Seattle, WA  98115
     voice:  (206) 526-6961
     fax:  (206) 526-6329

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John Caron wrote:

> Bryan Lawrence wrote:
>
>> On Wednesday 06 April 2005 20:19, Jonathan Gregory wrote:
>>  
>>
>>> The data provider probably doesn't care very much. This information 
>>> is not
>>> seen as critical for interpretation or even formulation of the 
>>> model. Hence
>>> you can probably legitimately serve it using any reasonable assumption.
>>>   
>>
>>
>> I would go even further than this. While it is possible that the 
>> coordinate transformation error becomes more significant at very high 
>> resolution global models and mesoscale model scales, the actual 
>> significance of the model results at those scales is poor (typically, 
>> for a grid point model, one should be looking at an effective 
>> resolution about four times the grid resolution).  What that means is 
>> the error in the grid projection transformation (if there is one) is 
>> vastly smaller than the effective grid registration (ie. how 
>> accurately the model coordinates reflect the real world). This is 
>> probably true even in a data assimilation model, where the model 
>> knows a bit more about the real world ...
>> Nowcasting models at the very finest scales may be a different story, 
>> but in that case the grid formulation should probably be rather 
>> different than a global spherical formulation ...  ("should" I say, 
>> don't know that they are :-)
>>  
>>
>> I don't think that CF can cope with everything ...
>>  
>>
> Well, Mr Toyoda has shown that the difference between geodetic 
> (ellipsoidal earth) and geocentric (spherical earth) latitude is 
> around 20 km at mid-latitude.  My understanding of the conversation is 
> that we should assume spherical earth for global models, unless 
> otherwise stated. Now when I send that info to a GIS package that uses 
> an ellipsoidal earth, presumably they will correct the coordinates to 
> take this into account.
>
> Now for models that do have accuracy in which 20 km matters, they will 
> need to record information as to earth shape in order to get correct 
> geolocation. Perhaps there are no such models yet (??) but someday 
> there may be.
>
> If CF doesnt want to cope with this, it would be helpful if CF stated 
> that the scope of its mission is to deal with model data with 
> resolution greater than such and such.
>
> Another possibility would be to create an extension of CF for model 
> data with resolution less than such and such.
>
> Perhaps these are issues to be discussed at the GO-ESSP conference?
>
> In any case, I really appreciate everyone's help in responding to this 
> question, and in everyone's thoughts about what CF should do.
>
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