[Proj] help with clark66 as datum

Clifford J Mugnier cjmce at lsu.edu
Fri Mar 10 10:07:17 EST 2006

Very early versions of ESRI software were on Unix machines that were
running Arc/Info, long before personal computers were invented.  The
initial implementations of projection math were based on GCTP, a Fortran
translation of John P. Snyer's first book (GCTP was written by Dr. Atef
Elassal).  All examples used the Clarke 1866 ellipsoid, because that was
the legal ellipsoid in use (for the NAD27) by the U.S. Geological Survey in
the U.S. before 1983.

You merely have an ancient dataset, and you may change it to any ellipsoid
that you wish without degrading any of the data.  The Normal Mercator
projection is rarely used for large-scale mapping where an actual datum is
of importance.  The exceptions are for all of Indonesia and for the city of
Guyaquil, Ecuador.

It's not a mistake, it was correct at the time.  John P. Snyder NEVER
concerned himself with datum transformations.  He considered that datums
were geodesy and not cartography, so he deferred to me on that stuff.
(John had a Master's Degree in Chemical Engineering.)

Cliff Mugnier


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


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
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