[Proj] tmerc "folding back" far from the central meridian

Gerald I. Evenden geraldi.evenden at gmail.com
Tue Jun 17 18:20:52 EDT 2008

On Tuesday 17 June 2008 12:45 pm, Frank Warmerdam wrote:
> Folks (mostly Gerald),
> The tmerc projection has a tendency to fold back on itself when you get
> really far from the central meridian.  This causes problems when
> applications naively try to reproject locations from all around the world
> into transverse mercator without regard to appropriateness.
> What I would like is if we could make the tmerc projection fail (ie return
> HUGE_VAL) for points so far from the central meridian that they are likely
> to start folding back on themselves.

etmerc has one limit level which is only a few degrees from del-lambda=90.  
The original code had several "levels" of error of with different return 
codes but because there was not anyway to do this multilevel return in 
libproj4 I just used the one at the furthest extent.

But the method of establishing a limit would appear to be done on the basis of 
Easting rather than a cutoff due to a lat-lon limit check.  Perhaps do the 
calculation regarless of geographic coordinates and then reject based upon 
easting greater than a percentage of a or R or, of course, equation failure.

On inverse projection just reject if |easting| too large.

Allow an override option for those who want to play around.  ;-)

The whole religious complexion of the modern world is due
to the absence from Jerusalem of a lunatic asylum.
-- Havelock Ellis (1859-1939)  British psychologist

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