[Proj] Geref Projection Definition Fixed!

[Frank Warmerdam]

Clever, Max wrote:
  > Rectified Skew Orthomorphic Oblique Mercators with Natural Origins can
> be handled by proj.  The flag that determines whether it is a “natural 
> origin” (origin at the equator) or a projection center origin is 
> determined by the inclusion of the “+no_uoff” flag in the projection 
> parameters.  The Michigan Georef coordinate system requires that the 
> “+no_uoff” parameter is inserted.  I’m not sure if that exact parameter 
> exists in epsg definitions.  It might need to be changed to no_off or 
> something like that.  There really was no fix needed in the proj engine, 
> just a correct definition for the projection. 

Melita Kennedy wrote:
 > EPSG already has codes for 2 versions of Michigan GeoRef, although they call 
them "Michigan Oblique Mercator".
 >
 > 3078 is based upon NAD83 and uses meters.
 > 3079 is based upon NAD83(HARN) and also uses meters.
 >
 > The method listed is "Hotine Oblique Mercator" (9812) with these parameters 
(note the azimuth):
 >
 > latitude of projection center: 45 18 33.0 N
 > longitude of projection center: 86 0 0.0 W
 > Azimuth of initial line: 337 15 20.016
 > Angle of rectified to skew grid: 337 15 20.016
 > scale factor on initial line: 0.9996
 > false easting: 2546731.496
 > false northing: -4354009.816
 >
 > EPSG Note: If using Oblique Mercator (9815), Ec = 499840.252 m and Nc = 
528600.303 m

Max / Melita,

It it true that there are two projections methods associated with the EPSG
projection method 9815, those being (in ESRI projection engine WKT terms) as
follows?

   Hotine_Oblique_Mercator_Azimuth_Natural_Origin
   Hotine_Oblique_Mercator_Azimuth_Center

Is it also fair to say that Hotine_Oblique_Mercator_Azimuth_Natural_Origin
is what is also sometimes known as Rectified Skew Orthomorphic Oblique
Mercator?

Is there a way to distinguish the two above projection methods by examining
a 9815 record in EPSG?  Perhaps if the angle of rectified to skew grid is
non zero or something like that?

I would add I already have code in my translation to PROJ.4 for distinguishing
swiss oblique mercator (proj=somerc) from plain oblique mercator (proj=omerc).

There are just too many things going by the name oblique mercator!

PS. I still don't see a bug report summarizing this issue.   without one there
is a great danger of my losing track of the issue and it being left unresolved
in the core for another few years.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org