[Proj] help with clark66 as datum

[Frank Warmerdam]

Hamish 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.
> 
> 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.

Hamish,

The projection is straight forward.  But D_Clark_1866 does not relate to
anything I have know of as a datum.  The ESRI .prj files don't any indication
of what mechanism to use to relate the datum name to WGS84, so this is
really not enough information to go on.  Somewhere inn ArcGIS there is some
sort of definition of what parameters to use for datums.  If you can dig
that up you should know what to use.

I would add +towgs84=0,0,0 is a pretty bold thing to do when using
such an ancient ellipsoid that is quite different than WGS84.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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