John & Co.,
Why do we need a new projection for UTM?
UTM is just one type of Transverse Mercator that is completely specified
by the map parameters we already have, right?
From the "Grid Mappings" section of CF 1.0:
grid_mapping_name = transverse_mercator
Map parameters:
* scale_factor_at_central_meridian
* longitude_of_central_meridian
* latitude_of_projection_origin
* false_easting
* false_northing
So people using UTM just specify the appropriate Transvere Mercator map
parameters for their zone, don't they?
-Rich
John Caron wrote:
> 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?
>
>
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--
Richard P. Signell rsignell at usgs.gov
U.S. Geological Survey Phone: (508) 457-2229
384 Woods Hole Road Fax: (508) 457-2310
Woods Hole, MA 02543-1598
Received on Wed Feb 02 2005 - 10:17:57 GMT
