[Proj] help with clark66 as datum

[Clifford J Mugnier]

I stand corrected.  My nits have been picked!

:-)

Cliff Mugnier
LSU
------------------------------
Paul -

Well, it's off-topic and nit-picking, but  (for the historical footnote
crowd) ESRI's Arc/Info software was first  released on a Prime minicomputer
rather than UNIX, and there were plenty of  personal computers around at
the time (1982).  Arc/Info was shortly ported  to UNIX, and the first PC
Arc/Info product was released in 1986.

     - Ed (old enough  to remember <g>)

Ed McNierney
TopoZone.com


From: proj-bounces at lists.maptools.org on behalf  of Paul Ramsey
Sent: Fri 3/10/2006 10:58 AM
To: PROJ.4 and  general Projections Discussions
Subject: Re: [Proj] help with clark66  as datum


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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