[Proj] Re: Still trying to get SPCS Alaska Zone 1 working!

[Oscar van Vlijmen]

I found another amusing tidbit about the Alaska Zone 1 SPCS projection.

A test point not on the central line:
lat = 58.5; lon = -135; (decimal degrees, longitude is west)
Datum: NAD83.

The 'official' NGS calculator
http://www.ngs.noaa.gov/cgi-bin/spc_getpc.prl
produced an easting, northing of:
740933.648, 742902.522 meters

The formulae and data mentioned in:
NOAA Manual NOS NGS 5, State Plane coordinate system of 1983
gave me an easting, northing of:
740933.6477, 742902.5217

The NGS 5 manual specifies a false easting, northing of:
x0 = 5e6; y0 = -5e6; (meters)

My omerc version at first gave the wrong answer, but the correct answer
emerged when I set the no_ofs flag, as was mentioned earlier:
740933.6477, 742902.5217

Now a surprise.
The PROJ.4 file "nad83" has for the false easting, northing of this region:
x_0=818676.7344011233 y_0=575097.6888751927
With these I can use omerc in the regular manner, without specifying a
no_ofs.
The resulting easting, northing I got were:
740933.6485, 742902.5220

These 'offset' values of x0, y0 can also be found in the file "nad1983." in
the USGS GCTP Fortran distribution. How they are derived, I don't know.
The numbers have something to do with the 1e7 m shifted easting, northing of
the lat0, lon0 on the aposphere. I can reproduce the numbers, but don't ask
me about the theory...

It's nonetheless interesting to know that in this case (Alaska 5001) still
the regular omerc can be used!