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<TITLE>Re: [Proj] help with clark66 as datum</TITLE>
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<DIV dir=ltr><FONT face=Arial color=#000000 size=2>Paul -</FONT></DIV>
<DIV dir=ltr><FONT face=Arial size=2></FONT> </DIV>
<DIV dir=ltr><FONT face=Arial size=2>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.</FONT></DIV>
<DIV dir=ltr><FONT face=Arial size=2></FONT> </DIV>
<DIV dir=ltr><FONT face=Arial size=2> - Ed (old enough
to remember <g>)</FONT></DIV>
<DIV dir=ltr><FONT face=Arial size=2></FONT> </DIV>
<DIV dir=ltr><FONT face=Arial size=2>Ed McNierney</FONT></DIV>
<DIV dir=ltr><FONT face=Arial size=2>TopoZone.com</FONT></DIV></DIV>
<DIV dir=ltr><BR>
<HR tabIndex=-1>
<FONT face=Tahoma size=2><B>From:</B> proj-bounces@lists.maptools.org on behalf
of Paul Ramsey<BR><B>Sent:</B> Fri 3/10/2006 10:58 AM<BR><B>To:</B> PROJ.4 and
general Projections Discussions<BR><B>Subject:</B> Re: [Proj] help with clark66
as datum<BR></FONT><BR></DIV>
<DIV>
<P><FONT size=2>I live for these nuggets! More, more! One of the most powerful
pieces <BR>of knowledge of a discipline is an understanding of its history
and <BR>evolution.<BR><BR>P<BR><BR>On 10-Mar-06, at 7:30 AM, Clifford J
Mugnier wrote:<BR><BR>> The DCW - Digital Chart of the World was HAND
digitized by the Defense<BR>> Mapping Agency in the early 80s from small
scale "ONC" Operational<BR>> Navigation Charts. That became the defacto
FREE dataset of the <BR>> world's<BR>> coastlines, rivers, etc used
by all software companies including <BR>> ESRI. (It<BR>> came
on a 9-Track tape from DMA as did the GCTP come on a 9-track <BR>> tape
from<BR>> the U.S. Geological Survey.) The ellipsoid used as a default
then <BR>> was the<BR>> Clarke 1866. That is how it wound up
being used for New Zealand. <BR>> (So was<BR>> the entire
world.)<BR>><BR>> You have to be old enough to be able to remember this
stuff!<BR>><BR>> -----------------------<BR>> Very early versions of
ESRI software were on Unix machines that were<BR>> running Arc/Info, long
before personal computers were invented. The<BR>> initial
implementations of projection math were based on GCTP, a <BR>>
Fortran<BR>> translation of John P. Snyer's first book (GCTP was written by
Dr. <BR>> Atef<BR>> Elassal). All examples used the Clarke 1866
ellipsoid, because <BR>> that was<BR>> the legal ellipsoid in use
(for the NAD27) by the U.S. Geological <BR>> Survey in<BR>> the U.S.
before 1983.<BR>><BR>> You merely have an ancient dataset, and you may
change it to any <BR>> ellipsoid<BR>> that you wish without degrading
any of the data. The Normal Mercator<BR>> projection is rarely used for
large-scale mapping where an actual <BR>> datum is<BR>> of
importance. The exceptions are for all of Indonesia and for
the <BR>> city of<BR>> Guyaquil, Ecuador.<BR>><BR>> It's not a
mistake, it was correct at the time. John P. Snyder NEVER<BR>>
concerned himself with datum transformations. He considered
that <BR>> datums<BR>> were geodesy and not cartography, so he
deferred to me on that stuff.<BR>> (John had a Master's Degree in Chemical
Engineering.)<BR>><BR>> Cliff Mugnier<BR>> LSU<BR>><BR>>
--------------------------------------<BR>><BR>> I would assume that the
"D_Clarke" stuff is just a mistake, and that<BR>> all they mean is a Clarke
1866 spheroid. Does this work?<BR>><BR>> +proj=merc +lon_0=100
+lat_ts=-46 +ellps=clrk66<BR>><BR>> Paul<BR>><BR>> On 3/9/06, Hamish
<hamish_nospam@yahoo.com> wrote:<BR>>> Hi,<BR>>><BR>>>
I'm trying to figure out PROJ.4 parameters for a newly published<BR>>>
dataset that has been widely distributed down here in New
Zealand.<BR>>><BR>>><BR>>> ERSI Shapefile .prj file that came
with it:<BR>>><BR>>>
PROJCS["Clarke_1866_Mercator",GEOGCS["GCS_Clarke_1866",<BR>>>
DATUM["D_Clarke_1866",SPHEROID["Clarke_1866",6378206.4,294.9786982]],<BR>>>
PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],<BR>>>
PROJECTION["Mercator"],PARAMETER["False_Easting",0.0],<BR>>>
PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",100.0],<BR>>>
PARAMETER["Standard_Parallel_1",-46.0],UNIT["Meter",1.0]]<BR>>><BR>>><BR>>>
Documentation that came with it:<BR>>><BR>>> The projection used
[...] is:<BR>>> Mercator Projection<BR>>> Central Meridian =
100<BR>>> Standard Parallel = -46<BR>>> False Easting =
0<BR>>> False Northing = 0<BR>>> Spheroid/Datum = Clarke
1866<BR>>><BR>>><BR>>> This confuses both me & the GRASS
GIS projection auto-import tool.<BR>>> Does clark66 define a
datum??<BR>>> Is this meaningful: DATUM["D_Clarke_1866",
??<BR>>> Should I give up and just assume +towgs84=0,0,0
?<BR>>><BR>>> I have no idea why they used clark66 or a point in the
ocean <BR>>> 1500km SW<BR>>> of Perth Australia as the center
of projection for a modern New<BR>>> Zealand<BR>>> dataset. But so
it is.<BR>>><BR>>> Hamish<BR>>
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