[Proj] help with clark66 as datum

[Hamish]

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.

see GRASS bug #4160  https://intevation.de/rt/webrt?serial_num=4160 
for a link to the data and the error it causes in GRASS.

Resoution of the data is 1km, so I'm not too worried about 5m shifts, but
100m gets into the unacceptable levels.


cheers and thanks for any insights,

Hamish