[Proj] Insignificant difference between Swiss and Hungarian Oblique Mercator?

Mikael Rittri Mikael.Rittri at carmenta.se
Tue Dec 5 07:53:22 EST 2006

I think I have implemented the Hungary EOV projection, which is a
of the Swiss Oblique Mercator (+proj=somerc).  In the Hungarian version,
can specify a normal parallel, distinct from the central one.  The
parallel gives the center of the first step of the double projection
(ellipsoid -> sphere), while the _central_ parallel gives the center 
of the second step (sphere -> oblique cylinder).  

I've modified the code of the PROJ.4 somerc, to cover the Hungarian
usage as well, but I am rather surprised: the effect of having 
normal parallel != central parallel is about 0.01 millimeters.  

Yet, to quote Molnar and Timar,
http://www.fomi.hu/honlap/magyar/szaklap/2002/03/mar3.pdf ,
"its normal parallel slightly but intentionally differs ..." [from the
central one].

But I cannot understand the intention, if the difference in projected
is so small.  (And I don't read Hungarian; only the abstract is in

I've read on this list that PROJ.4 users do use +proj=somerc for
but I thought the approximation would be worse.  Just to rule out the 
possibility that I have made a silly mistake: could anyone confirm that
the approximation error is about 0.01 millimeters?

Best regards,
  --  Mikael Rittri 

Hungary EOV specification: http://lazarus.elte.hu/gb/geodez/geodind.htm
(chapter 2 and 9).

My test points used the projected coordinates roughly in the four
corners of 
Hungary, which I unprojected with and without the distinct normal
The differences were:
Easting Northing   Difference 
400000, 0        -> 0.01350 mm
400000, 400000   -> 0.01399 mm
900000, 0        -> 0.01350 mm 
900000, 400000   -> 0.01399 mm

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