I guess you and Jonathan are making similar points. You might be right
that's the best thing to do if you have a spherical-earth model (I dont
know what GIS actually does, though, it seems more likely that they make
a correction. I will try to find out more). If you do have a high
resolution model, you are probably using an ellipsoidal earth, as Rich
Signell does. In either case, then, just using the given lat/lon would
be best, as you both suggest
Another question is which ellipsoid is being used. According to Snyder,
when US switched from NAD27 to NAD83, nearly every point in North
America got a new lat/lon, and "the difference can reach 300 m". (p 13).
So perhaps we are talking about 300 m rather than 20 km ? Rich can you
comment more on this?
Karl Taylor wrote:
> Dear all,
>
> I also think we should discuss this further at the GO-ESSP meeting,
> but just one preliminary thought ...
>
> What would be important to know as models go to higher resolutions is
> how the topography is specified in the model. My understanding is
> that modelers simply transfer the latitude/longitude locations of
> topographical features and surface characteristics from the
> non-spherical earth to his idealized spherical model earth without
> correction. Thus, surely one should simply compare modeled and
> observed fields at corresponding longitudes and latitudes (without
> adjustment). I don't think 20 km errors are introduced by this procedure.
>
> Maybe I'm missing something.
>
> cheers,
> Karl
>
> Jonathan Gregory wrote:
>
>> Dear John
>>
>>
>>> 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.
>>
>>
>>
>> It shouldn't alter the lat and lon coords; they are what the model
>> believes
>> it is using on a spherical Earth.
>>
>>
>>> 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.
>>
>>
>>
>> That would be not satisfactory, I don't think.
>>
>>> Perhaps these are issues to be discussed at the GO-ESSP conference?
>>
>>
>>
>> Good idea.
>>
>> Best wishes
>>
>> Jonathan
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>
>
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Received on Thu Apr 07 2005 - 12:38:43 BST
