[Proj] Discovery: libproj4 stmerc = French Gauss-Laborde projection

Gerald I. Evenden gerald.evenden at verizon.net
Mon Jun 12 13:42:53 EDT 2006

On Monday 12 June 2006 5:47 am, Oscar van Vlijmen wrote:
> stmerc is a projection in the libproj4 package. There is no documentation.
> Mr. Evenden mentioned it on this discussion list on 2004-03-26.
> Recently I found a connection with a projection in real use, the
> Gauss-Laborde type "sphère de courbure" (probably meaning: the projection
> sphere has a radius equal to the radius of curvature at the point of
> origin).
> What is the connection? They are the same!

Looking at the NT/G 76, the tmerc projection described uses the isometric 
latitude as the intermediate latitude while stmerc in libproj4 uses the Gauss 
conformal latitude which is more complex in its conversion and has side 
effects such as creating a new radius (see the pj_gauss documentation in the
libproj4 manual).

However, other complicating factors later in the 76 development prohibits me 
from saying it is not the same as stmerc.  I will, however, safely say that 
is it not equivalent to Gauss-Kruger that we all know and love. :-)

I think I will code 76 up and see how things do compare.  Thankfully, C99 has 
complex arithmetic and math library.  I must add that NT/G series are quite 
useful for developing code.

BTW, I am having a devil of a time accessing the French site and keep timing 
out.  It took me nearly an hour to just access three of the documents and the 
browser finally completely gave up on the last one.  All other net access 
seems normal.

Jerry and the low-riders: Daisy Mae and Joshua
"Cogito cogito ergo cogito sum"
   Ambrose Bierce, The Devil's Dictionary

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