[Proj] Re: Global Gauss-Kruger and libproj4---the final story
Gerald I. Evenden
geraldi.evenden at gmail.com
Thu Aug 28 20:04:53 EDT 2008
On Thursday 28 August 2008 5:30:30 pm strebe at aol.com wrote:
> >The only context that I use the term "interruption" in discussing
> >cartographic projections are in cases like Goode's world maps or
> >"orange peal" charts often using the sinusoidal projection. Please
> >define what *you* mean by interruptions.
> An interruption is any location where two points of infinitesimal
> separation on the globe are mapped to two points having finite separation
> on the plane. This happens somewhere on all projections. All along an
> interruption, the projection formulæ are no longer a function; they are
Let's see. tan(x) has two values at x=90 degrees: +inf and -inf. So it is no
longer a function, eh?
I can't find any such distinction for the definition of a "function." Please
Even if your questionable definition were true, it merely verifies that
proj_fwd() is a function because it only returns one value. It is defined
that way in the documentation.
In terms of interruptions applying to all projections I presume you are
referring to the del-lon=180 condition. In many projection software this is
not a break and the map merely repeats on in the x direction. As far as I am
aware it is always the graphic programmers job to handle this condition and
not the projection routine: when at the map edge recall the function with the
sign of the lon term reversed.
BTW: you are the first person I have ever heard that refers to the wrap-around
condition of longitude as an interruption if, indeed, that is what you are
If there are other examples, other than projections commonly recognized as
interrupted (for example Goode) *please* name them.
> -- daan Strebe
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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