[Proj] help with clark66 as datum

[Richard Greenwood]

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
>

That's pretty lame. Clarke 1866 is an ellipsoid, not a datum. But
that's ESRI. The provided definition is not enough that you could put
it to any other software, or any standard algorithm, and verify it.

If I were in your shoes, I would assume towgs84=0,0,0, but it seems
like you would need to run it thru ESRI software to confirm that. If
you don't have access to ArcGIS or something, send me a few
coordinates in the new projection and I'll try to convert them w/
ArcGIS to WGS84 or another system.

Having the origin somewhere way the the southwest is pretty common.
That way you don't have any negative coordinates in the area actually
covered by the projection. And using clarke 1866 is probably a
perfectly legitimate ellipsoid, but it does not specify a datum.

Rich

--
Richard Greenwood
richard.greenwood at gmail.com
www.greenwoodmap.com