[CF-metadata] axis attribute

[Bob Drach]

CDAT uses the axis attribute to identify the spatio-temporal type of  
axis. (Since the axis attribute is optional, CDAT also checks the  
units). My interpretation of the standard is that, if the axis is a  
longitude, then axis="X" is appropriate, and this applies to  
auxiliary longitude coordinate axes as well as 1-D lon(lon)  
coordinate variables. However, it would not apply to  
projection_x_coordinate, if I understand that attribute correctly.

Bob


On Nov 19, 2006, at 6:05 PM, Simon Wood wrote:

> Karl, Jonathan and others,
>
>> I'll reiterate what I said earlier (but I think somehow the email  
>> didn't get distributed).  I'm in favor of eliminating the  
>> apparently contradictory statements.  Of the two choices, perhaps  
>> least confusion would result from adopting Jonathan's suggestion:  
>> restrict X and Y to genuine longitude and latitude coordinates.
>
> By 'genuine longitude and latitude coordinates' would you intend to  
> exclude auxiliary (lat/lon) coordinates (eg lat(y,x) and lon(y,x)),  
> and allow only for datasets with lat lon dimensions (and hence lat 
> (lat) and lon(lon) coordinate variables)?
>
> I'm a bit confused since I originally interpreted CF 4 to mean that  
> the axis attribute as an extra way of identifying a coordinate  
> variable.  I  assumed it would be good to use it with auxiliary  
> coordinate variables for datasets using x,y dimensions and, say,  
> lambert conformal projection.  But then by analogy to the  
> discussion about rotated pole projections on the earlier part of  
> this thread I got to thinking that maybe the axis attribute should  
> be associated with the x(x) and y(y) 'projection_x/y_coordinate'  
> variables instead (ie so it can be used as a hint to the plot  
> orientation).
>
> However, it seems to me that restricting the attribute to only be  
> used for 'proper' coordinate variables (ie lat(lat), lon(lon)) is a  
> bit drastic (and it doesn't really add much value to these  
> variables does it?).  All thats really needed is to resolve the  
> ambiguity in intended usage (ie is it to hint at plot orientation  
> or to help identify coordinate variables including auxiliary  
> coordinate variables).  Of course if both interpretations are in  
> common use then it gets a bit trickier...
>
> The reason I was intending to use the axis attribute was just in an  
> attempt to include as much metadata as possible so that (hopefully)  
> a wider range of data readers would be able to understand our data;  
> I guess if its just going to add confusion I won't use it.  Does  
> anybody know of data readers that currently rely on the axis  
> attribute in any way?
>
> regards
>
> Simon
>
> -- 
> Simon Wood
> National Institute of Water and Atmospheric Research, Wellington, NZ	
> simon.wood at niwa.co.nz
> http://www.niwa.co.nz
>
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