[Proj] Alaska State Plane Coordinates (NAD83) - Hotine Oblique Mercator ??

[Oscar van Vlijmen]

> From: Anthony Dunk <AnthonyD-rocketmail.com>

> Has anyone successfully implemented support for the Hotine Oblique
> Mercator for the Alaska SPCS Zone 1 (5001) using PROJ4 ?

....

> Are these parameters correct ??

Don't think so.

One 'official' test point from the GCTP test suite, including the header
information:

Alaska, Zone 1 AK_1 6101 5001
STATE PLANE PROJECTION PARAMETERS:
   Zone:      5001
   Datum:     NAD 27
OBLIQUE MERCATOR (HOTINE) PROJECTION PARAMETERS:
   Semi-Major Axis of Ellipsoid:     6378206.400000 meters
   Semi-Minor Axis of Ellipsoid:     6356583.800000 meters
   Scale Factor at C. Meridian:     0.999900
   False Easting:      818585.567227 meters
   False Northing:     575219.245107 meters
   Azimuth of Central Line:     -36.869898
   Longitude of Center:     -101.514031 degrees
   Latitude of Center:     57.000000 degrees
 INPUT LONG LAT        X              Y           OUTPUT LON      OUTPUT LAT
-135.00  58.50   740839.52660   743026.80896    -135.00000     58.50000

The "Longitude of Center" is not the correct lonc, it is indeed -133d40m
With my version of omerc (derived from Libproj4) and the following
parameters I got:

// first set Clarke 1866, NAD27 ellipsoid
lat = 58.5; lon = -135;
lat0 = 57; lonc = -133d40m;
alfac = -36.869898; gamma = "";
k0 = 0.999900; x0 = 818585.567227; y0 = 575219.245107;
Result: x = 740839.526599844; y = 743026.808961898

The alfac is officially defined as the arcsine of -0.6, so an even better
value of alfac would be -36.869897645844 degrees.