[Proj] help with clark66 as datum

[Paul Ramsey]

I would assume that the "D_Clarke" stuff is just a mistake, and that  
all they mean is a Clarke 1866 spheroid.  Does this work?

+proj=merc +lon_0=100 +lat_ts=-46 +ellps=clrk66

Paul

On 3/9/06, Hamish <hamish_nospam at yahoo.com> wrote:
> Hi,
>
> I'm trying to figure out PROJ.4 parameters for a newly published
> dataset that has been widely distributed down here in New Zealand.
>
>
> ERSI Shapefile .prj file that came with it:
>
> PROJCS["Clarke_1866_Mercator",GEOGCS["GCS_Clarke_1866",
> DATUM["D_Clarke_1866",SPHEROID["Clarke_1866",6378206.4,294.9786982]],
> PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],
> PROJECTION["Mercator"],PARAMETER["False_Easting",0.0],
> PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",100.0],
> PARAMETER["Standard_Parallel_1",-46.0],UNIT["Meter",1.0]]
>
>
> Documentation that came with it:
>
> The projection used [...] is:
> Mercator Projection
> Central Meridian = 100
> Standard Parallel = -46
> False Easting = 0
> False Northing = 0
> Spheroid/Datum = Clarke 1866
>
>
> This confuses both me & the GRASS GIS projection auto-import tool.
> Does clark66 define a datum??
> Is this meaningful:  DATUM["D_Clarke_1866",  ??
> Should I give up and just assume +towgs84=0,0,0 ?
>
> I have no idea why they used clark66 or a point in the ocean 1500km SW
> of Perth Australia as the center of projection for a modern New  
> Zealand
> dataset. But so it is.
>
> Hamish