State Plane Scale Factor in WKT

Question

I need to convert an old-style projection file (created with Global Mapper) to a WKT format for use in my GIS program (ArcMap or GDAL or whatever). The problem is that my old-style projection file has an additional parameter called "State plane scale factor" which does not seem to be recognized. My old-style projection file is in the following format:

Projection     STATE_PLANE
Datum          NAD83_NEW_MEXICO_HPGN
Zunits         NO
Units          FEET
Zone           3002
Xshift         0.000000
Yshift         0.000000
Parameters
1.000210000 /* state plane scale factor

I can get GDAL (and probably ArcMap) to recognize that this file contains a New Mexico Central NAD83 projection, but neither does anything with the state plane scale factor. I found this link that describes multiplying the false easting, northing, and scale factor by the local scale factor but this did not seem to fix the problem (though I think it's on the right track). GlobalMapper will only save to the above format and will not convert this to a WKT format that can be used by other software programs. The New Mexico Central projection is based on the Transverse Mercator projection, I'm trying to figure out a way to use the base parameters for this projection with Transverse Mercator and change them using the state plane scale factor. Does anybody have any idea of the correct way to convert this old-style projection to the correct WKT?

Answer 1

Here's the unmodified Esri version of the WKT:

PROJCS["NAD_1983_HARN_StatePlane_New_Mexico_Central_FIPS_3002_Feet",GEOGCS["GCS_North_American_1983_HARN",DATUM["D_North_American_1983_HARN",SPHEROID["GRS_1980",6378137.0,298.257222101]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",1640416.666666667],PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",-106.25],PARAMETER["Scale_Factor",0.9999],PARAMETER["Latitude_Of_Origin",31.0],UNIT["Foot_US",0.3048006096012192],AUTHORITY["EPSG",2903]]

The important parameters are the false easting, false northing, and scale factor as you've already found. The "state plane scale factor" is probably a grid-to-ground adjustment to minimize the distortion due to the map projection. It should also "move" the projection plane up to the average elevation of area from something closer to the ellipsoid surface.

new false easting = 1640416.666666667 * 1.00021 = 1640761.154166667 new false northing = 0.0 * 1.00021 = 0.0 new scale factor = 0.9999 * 1.00021 = 1.000109979

So new WKT becomes:

PROJCS["NAD_1983_HARN_StatePlane_NM_Central_USFt_Modified",GEOGCS["GCS_North_American_1983_HARN",DATUM["D_North_American_1983_HARN",SPHEROID["GRS_1980",6378137.0,298.257222101]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",1640761.154166667],PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",-106.25],PARAMETER["Scale_Factor",1.000109979],PARAMETER["Latitude_Of_Origin",31.0],UNIT["Foot_US",0.3048006096012192]]

It's possible the inverse of the scale factor should be applied instead; only way is to try it.

The Global Mapper format looks like a variation what is used in ArcInfo Workstation or PC Arc/Info, but doesn't exactly match. Esri never supported a grid-to-ground value, for instance.