[Proj] help with clark66 as datum

[Paul Ramsey]

I live for these nuggets! More, more! One of the most powerful pieces  
of knowledge of a discipline is an understanding of its history and  
evolution.

P

On 10-Mar-06, at 7:30 AM, Clifford J Mugnier wrote:

> The DCW - Digital Chart of the World was HAND digitized by the Defense
> Mapping Agency in the early 80s from small scale "ONC" Operational
> Navigation Charts.  That became the defacto FREE dataset of the  
> world's
> coastlines, rivers, etc used by all software companies including  
> ESRI.  (It
> came on a 9-Track tape from DMA as did the GCTP come on a 9-track  
> tape from
> the U.S. Geological Survey.)  The ellipsoid used as a default then  
> was the
> Clarke 1866.  That is how it wound up being used for New Zealand.   
> (So was
> the entire world.)
>
> You have to be old enough to be able to remember this stuff!
>
> -----------------------
> 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
> LSU
>
> --------------------------------------
>
> 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
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