# [Proj] Unknown projection?

Paul B. Anderson pbander at cavtel.net
Sat Sep 13 12:26:58 EDT 2008

```- Janne,

The difficulty comes about because the Tripel is actually composed of 2 map
projections -- the Aitoff (pronounced 'I-toff') projection averaged with the
Equirectangular projection (which provides the Special Parallel input). Yes,
there are inverse formulas out there for the Tripel, but the values produced
are approximations.

One person who has researched the inverse Winkel Tripel formulas issue is
Dr. Robert Schmunk http://www.giss.nasa.gov/staff/rschmunk.html

BTW, a reference for this map projection is Snyder's 'Flattening the Earth'.

Paul B. Anderson FCCS (USN, Retired)
Kingsport, TN native living in Norfolk, VA
My Map Projection web site:
http://www.csiss.org/map-projections/index.html

----- Original Message -----
From: <support.mn at elisanet.fi>
To: "PROJ.4 and general Projections Discussions" <proj at lists.maptools.org>;
<geraldi.evenden at gmail.com>
Sent: Saturday, September 13, 2008 11:42 AM
Subject: Re: [Proj] Unknown projection?

> "Gerald I. Evenden" <geraldi.evenden at gmail.com> kirjoitti:
>> On Saturday 13 September 2008 10:59:24 am Paul B. Anderson wrote:
>> > - Janne,
>> >
>> > Your map graphic appears to be in the Winkel Tripel map projection. I
>> > would
>> > also suggest that it is using Oswald Winkel's original selection of
>> > 50.46667 degrees as the Special Parallel.
>> >
>> > Note that by the 1950s John Bartholomew was using a special parallel of
>> > 40
>> > degrees for his version of the Tripel, giving his graphic a more
>> > flattened
>> > appearance in the Y direction.
>>
>> This is exactly why I include the projection math in the manual.  The
>> factor Anderson is discussing is +lat_1 option for the +proj=wintri
>> projection.  Select your own option value and name a new projection.  ;-)
>>
>
> Ok, now the problem seems to be that "wintri" does not have any inverse
> projection
> available. What is the reason not to have inverse projections for all
> routines
> in the library... math is too hard?
>
> Janne.
>
>
>
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