[Proj] Geref Projection Definition Fixed!

Frank Warmerdam warmerdam at pobox.com
Tue Jan 30 23:08:21 EST 2007

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


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

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,
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

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