[Proj] Re: Still trying to get SPCS Alaska Zone 1 working!
mkennedy2 at earthlink.net
Fri May 19 19:56:15 EDT 2006
>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
>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
>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 ??!
>NAD83 SPCS document:
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.
ESRI Product Specialist
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