[OSRS-PROJ] Rectified Skew Orthomorphic...EPSG

Gerald Evenden gerald.evenden at verizon.net
Fri Feb 6 21:17:04 EST 2004

Working on this projection is a lot like that other solo activity that 
one go blind.

It FINALLY sank in that the so called gamma_c in the skewed versions is
really just gamma_0 computed from alpha_c.  Similarly, alpha_c can be
obtained if only gamma_c is given.

To summarize: in the Alaska zone only alpha_c is given and the u,v grid
is rotated by alpha at lonc,lat_0 with x,y aligned with N,S at the 
point and in the RSO versions the grid is rotated at lonc,lat_0 by
gamma_0 with x,y aligned with N,S at the aposphere equator.

The nice thing about this is that you *do not* need to know values for 
alpha_c *and* gamma_c.  One only need to tell the projection
which angle to use in the rotation.

NOW the question I am going to drop in y'all laps is how do you
want proj and PJ_omerc to handle these options???

Right now PJ_omerc handles Alaska #1 the old fashion way
with merely specifying +alpha=.

I am not sure whether alpha_c or gamma_o is the defining
specification in RSO.  EPSG does not recognize the relationship
with the two so is no help.  The barely readable documentation on
Malaya RSO seems to indicate that gamma_0 is the primary

Should I set it up that +alpha= exclusive or +gamma_0= be the
options?  (Drop gamma_c all together)

Should I allow for arbitrary rotation with +gamma_c?

Note: +no_rot flag option is still available which omits any
rotation--offset and thus the user can roll his own.

BTW:    sin(gamma_0) = sin(alpha_c) / D
where D is a function of only latitude_c and eccentricity.  Thus you
can see how they are tied together.
What kept me going around in circles was that EPSG report kept
using another variable gamma_c that was user specified.(?!)
Jerry and the low riders: Daisy Mae and Joshua

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