[Proj] Re: Global Gauss-Kruger and libproj4---the final story

[Gerald I. Evenden]

On Wednesday 27 August 2008 1:40:43 pm strebe wrote:
> Not knowing anything about libproj4, I am surprised by this note. All
> projections of the complete globe are "multivariate". What does libproj4 do
> for other projections? Why would it not do something similar for
> ellipsoidal transverse Mercator?

Of course there is what might be called longitude "wrap around" of 
unrestrained longitude.  libproj4 clips longitude at pi to prevent such 
problems (can be overridden). Granted there is an ambiguity at lon=pi(180) 
but that is the problem that is well understood and to be resolved by the 
calling applications.  But libproj4 and any predecessor gives only *one* 
answer at lambda=pi. 

In the case of the split of the equator by TM, the longitude where the split 
occurs is determined by the eccentricity of the ellipsoid and thus there is 
no "standard" longitude of occurrence.  Regardless of this detail it creates 
a problem totally unique to TM and would require specialized modification of 
application  programs merely to handle this single projection.  Given the 
*very* questionable application of TM in this region there is no practical 
excuse to pursue this problem.  Also, unless I missed something in Lee's and 
Thompson's math, the mechanism for computing TM by their math would create 
execution times orders of magnitude slower than any of the current 
approximations.

There are certainly legit debate over extended TM (beyond UTM zones) but I do 
not see any reason to continue with global TM.

> Regards,
> -- daan Strebe
	...
-- 
The whole religious complexion of the modern world is due
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