[GRASSLIST:3329] Re: no false norting in tmerc projection

[Markus Herrchen]

The problem disappeared. Thanks!
It is really sufficient to add by hand the false northing!

"Eric G. Miller" wrote:

> On Mon, Mar 11, 2002 at 06:55:04PM +0100, Markus Herrchen wrote:
> > Hi,
> > I have a problem in the definition of the coordinate system.
> > I created a new location and mapset with the following PROJ_INFO file:
> >
> > name: Transverse Mercator
> > datum: nad27
> > dx: -22.000000
> > dy: 157.000000
> > dz: 176.000000
> > proj: tmerc
> > ellps: clark66
> > a: 6378206.4000000004
> > es: 0.0067686580
> > f: 294.9786982000
> > lat_0: 42.0000000000
> > lon_0: -114.0000000000
> > k_0: 0.9996000000
> > x_0: 500000.0000000000
> >
> > But i was unable to define a false northing (in my case of 100000.).
> > I added by hand a last line with
> >
> > y_0: 100000.0000000000
>
> I believe this has been fixed for future versions.  The addition of
> y_0 should be sufficient.
>
> > but this didn't help. In fact, when i load an ARC/INFO grid file,
> > by the help of r.in.gdal, the resulting y-coordinate is false:
> > the false northing is ignored!
> > Is this a bug in the tmerc projection?
>
> Are you sure that the imported file didn't have a Y shift?  And are you
> sure r.in.gdal actually imported the data relative to the coordinate
> system, or was the data imported as XY pixels?  The projection
> definition shouldn't have much impact on r.in.gdal, other than it checks
> to see the input data source and location have matching definitions.
>

The data is an ARC/INFO binary grid data which shows land use of Idaho,
(id_g2veg.zip, from http://www.wildlife.uidaho.edu/data.htm)
with an integer classification system of the land use.
The data are relative to the Idaho Transverse Mercator projection.
If i understand right the metadata file below the point 42N and 114W
corresponds to 500'000 Easting and 100'000 Northing and then
everything works fine.

The metadata file reads:

...

  Spatial_Reference_Information:
    Horizontal_Coordinate_System_Definition:
      Planar:
        Map_Projection:
          Map_Projection_Name:            Transverse Mercator
          Transverse_Mercator:
            Scale_Factor_at_Central_Meridian: 0.99960000
            Longitude_of_Central_Meridian: -114
            Latitude_of_Projection_Origin:             42
            False_Easting:  500000.00000
            False_Northing:  100000.00000
        Planar_Coordinate_Information:
          Planar_Coordinate_Encoding_Method: coordinate pair
          Coordinate_Representation:
            Abscissa_Resolution: 30.0
            Ordinate_Resolution: 30.0
          Planar_Distance_Units: Meters
      Geodetic_Model:
        Horizontal_Datum_Name: North American Datum of 1927
        Ellipsoid_Name:  Clarke 1866
        Semi-major_Axis: 6378206.4
        Denominator_of_Flattening_Ratio: 294.98