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

[Melita Kennedy]

>Date: Thu, 18 May 2006 17:42:12 -0700 (PDT)
>From: Anthony Dunk <AnthonyD at rocketmail.com>
>Subject: [Proj] Still trying to get SPCS Alaska Zone 1 working!
>To: Proj at lists.maptools.org
>
>Hi,
>
>Can anyone explain to me why the US Govt document on NAD83 State Plane
>Coordinate System (see link below) says that the origin for Alaska Zone
>1 is lat=57N lon=133'40W or Easting=5,000,000, Northing=-5,000,000, but
>when I run the Corpscon program with the appropriate setup it projects
>this lat/lon to Easting=818676.713, Northing=575097.689 ? I also found
>that MapInfo projects it to exactly the same Easting/Northing as
>Corpscon.
>
>If I use PROJ's cs2cs command (Rel 4.4.7 31 March 2003) to convert
>these lat/lon coordinates using the following command:
>
>cs2cs +proj=latlong +a=6378137.0 +rf=298.257222 +to +proj=omerc
>+lonc=-133.6666666667 +lat_0=57 +alpha=-36.8698976458 +k=0.9999
>+x_0=5000000 +y_0=-5000000  +a=6378137 +rf=298.257222101
>
>Then I get Easting=4999999.80 Northing=-5000000.0.
>
>So, the PROJ command gives results which agree with the NAD83 SPCS
>document, but both Corpscon and MapInfo give a consistent but different
>result. What is going on, and who is correct ??!
>
>Thanks,
>Anthony.
>
>NAD83 SPCS document:
>http://www.ngs.noaa.gov/PUBS_LIB/ManualNOSNGS5.pdf

Hi Anthony,

The version of Oblique Mercator that's used for Alaska zone 1 doesn't have a grid
origin at the defining projection parameters. Instead, the grid origin is approximately 
where the central line crosses the equator (technically the aposphere's equator). 
It's around longitude = -101.51384, latitude = -0.25527276. 

You should believe Corpscon, MapInfo, ArcGIS, and the National Geodetic Survey's
SPC program (http://www.ngs.noaa.gov/cgi-bin/spc_getpc.prl). 

State Plane zones were defined so that positions within the zone have positive 
values. The y0 = -5000000 makes sure that the y values aren't too large. 

Other countries that use oblique Mercator usually do 'recenter' the projection 
so that the 0,0 point is at the defining angular parameters.

Melita 

Melita Kennedy
ESRI Product Specialist