[CF-metadata] proposed UTM projection

[John Caron]

Hi Jonathan:

Yes, a lot is left unspecified, im never sure how much to put in. I will 
try to answer your questions although im not particularly an expert, so 
you (and others) tell me when its clear enough.


Jonathan Gregory wrote:

>Dear John
>
>Thanks. This looks useful. I've had a look at the geotiff notes and I don't
>find them very clear, so I think we need a bit more explanation.
>
>  
>
>>   * UTM_Zone_Number - in range [1,60] (north) or [-1, -60] (south)
>>    
>>
>I suggest lower case (the software should be case-insensitive anyway, as in
>CF standard 2.6). 
>
I chose this name because its what FGDC uses. i will change to lower case.


>What does this parameter mean? In particular,
>  
>
>>The central meridian and 
>>false_northing parameters are specified by the UTM zone.
>>    
>>
>how is this done? Can we give the formulae? Otherwise the specification may be
>incomplete.
>  
>
central meridian = ( abs(zone) * 6 - 183) in degrees_east

>  
>
>>false easting parameters (50000 m)
>>    
>>
>That should be 500000 m i.e. 500 km.
>  
>
thanks

>  
>
>>The ellipsoid parameters /semimajor_axis/ and /inverse_flattening /are 
>>optional. If not present, the standard ellipsoid for the given UTM zone 
>>should be used.
>>    
>>
>What are these default values? I think we need to say how they are chosen.
>  
>
Im actually not sure, does anyone else know or have a reference? Snyder 
says that that the Clarke 1866 sphere is used for all US territory 
except Hawaii. Perhaps the simplest thing is to require this, although 
my fear is that it may not be easily known.

>Since it has these extra parameters, it appears that the UTM projection is not
>just a special case of TM. 
>
It is a special case of  TM using an ellipsoidal (instead of spherical)  
earth. FGDC puts the ellipsoidal parameters in a seperate metadata 
section (section 4.1.1 "Geodeic Model"). I think we've been resisting 
making things too damn complicated since most model data is fine on a 
spherical earth, but as we get to regional and local models and try to 
connect  to GIS, we are going to see more complexity. UTM in my 
experience so far is the most important of these projections that 
require specifying the ellipsoidal earth, but all of them could in 
principle use an ellipsoid.

>I am not sure that the formulae in the geotiff page
>actually say what to do with these numbers, do they? They are hard to follow.
>Do you think there is enough information there for an implementation of UTM?
>  
>
the geotiff pages arent quite good enough. The standard reference is a 
USGS publication by Snyder, which would be useful to reference.

Does anyone know of a good online reference for UTM?