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<DIV dir=ltr align=left><SPAN class=074540015-25082008><FONT face=Arial>Dear Mr.
Mugnier, </FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><FONT face=Arial>you
wrote </FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial></FONT></SPAN></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>>When diddling with spherical
projections, the concept of "DATUM" is entirely inappropriate.
</FONT></SPAN></SPAN></DIV></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><FONT
size=2></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>which made me quite confused,
because</FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial></FONT></SPAN></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>1) I think what you wrote is absurd,
and </FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008></SPAN></SPAN><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>2) I know well that you are an expert
in geodesy. </FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial></FONT></SPAN></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>But Hillel said, "the shamefast is not
apt to learn", so let me go on. </FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial></FONT></SPAN></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>I suppose you require more properties
of a geodetic datum than I care about. </FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>For me, a geodetic datum is
essentially a way to georeference a map (or at least </FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>to georeference the graticule on the
map.) Surely you are not saying that if a map </FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>uses a spherical projection, then its
graticule cannot be georeferenced. </FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial></FONT></SPAN></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>I have also been wondering why the
EPSG people, when describing the web Mercator, </FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>are careful to say that the
geodetic datum is not WGS84 but something spherical </FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>that is not a true datum.
</FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial></FONT></SPAN></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>As I see it, the projection machinery
has to treat the earth as a sphere, while the </FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>datum shift machinery has to treat it
as an ellipsoid. And why not? In Carmenta
Engine,</FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>the Mercator class has a switch that
lets you choose between ellipsoidal and spherical </FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>formulas. So the projection can
ignore the flattening of the ellipsoid, but the datum shift
</FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>machinery will not.
</FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial></FONT></SPAN></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>Are you (and EPSG) reasoning like
this: </FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT
face=Arial> (1.) all projections must be
implemented by ellipsoidal formulas, </FONT></SPAN></SPAN><SPAN
class=074540015-25082008><SPAN class=074540015-25082008><FONT face=Arial>so
</FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial> (2.)
the only way to </FONT></SPAN></SPAN><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>emulate spherical formulas is to
supply an earth </FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT
face=Arial>
model that is a </FONT></SPAN></SPAN><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>sphere from the beginning;
</FONT></SPAN></SPAN><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>and no proper datum
</FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT
face=Arial>
can be spherical. </FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>? </FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>If so, then I agree that (2) follows
from (1), and I agree that proper datums should </FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><SPAN
class=074540015-25082008><FONT face=Arial>not be spherical. But I do not
agree that (1) is true. </FONT></SPAN></SPAN></DIV>
<DIV dir=ltr align=left><FONT face=Arial></FONT> </DIV>
<DIV dir=ltr align=left><SPAN class=074540015-25082008><FONT face=Arial>Best
regards,</FONT></SPAN></DIV><!-- Converted from text/plain format -->
<P><FONT size=2>--<BR>Mikael Rittri<BR>Carmenta AB<BR>Box 11354<BR>SE-404 28
Göteborg<BR>Visitors: Sankt Eriksgatan 5<BR>SWEDEN<BR>Tel: +46-31-775 57
37<BR>Mob: +46-703-60 34
07<BR>mikael.rittri@carmenta.com<BR>www.carmenta.com</FONT> </P>
<DIV> </DIV><BR>
<DIV class=OutlookMessageHeader lang=en-us dir=ltr align=left>
<HR tabIndex=-1>
<FONT face=Tahoma size=2><B>From:</B> proj-bounces@lists.maptools.org
[mailto:proj-bounces@lists.maptools.org] <B>On Behalf Of </B>Clifford J
Mugnier<BR><B>Sent:</B> den 22 augusti 2008 05:45<BR><B>To:</B> PROJ.4 and
general Projections Discussions; PROJ.4 and general Projections
Discussions<BR><B>Subject:</B> RE: [Proj] WGS84 to Sphere Inconsistency Between
Proj Versions<BR></FONT><BR></DIV>
<DIV></DIV>
<DIV id=idOWAReplyText29000 dir=ltr>
<DIV dir=ltr><FONT face="Times New Roman" size=2>When didling with spherical
projections, the concept of "DATUM" is entirely inappropriate.</FONT></DIV>
<DIV dir=ltr><FONT size=2></FONT> </DIV>
<DIV dir=ltr><FONT size=2>Cliff Mugnier</FONT></DIV>
<DIV dir=ltr><FONT size=2>LOUISIANA STATE UNIVERSITY</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 Frank Warmerdam<BR><B>Sent:</B> Thu 21-Aug-08 22:38<BR><B>To:</B> PROJ.4 and
general Projections Discussions<BR><B>Subject:</B> Re: [Proj] WGS84 to Sphere
Inconsistency Between Proj Versions<BR></FONT><BR></DIV>
<DIV>
<P><FONT size=2>Faron Anslow wrote:<BR>> I just ran this:<BR>><BR>>
cs2cs +proj=latlong +datum=WGS84 +to +proj=lcc +lat_1=50 +lat_2=50<BR>>
+lat_0=50 +lon_0=-107 +a=6371200.0000000000 +es=0.0 +f=0.0<BR>>
+towgs84=0,0,0 -r<BR>><BR>> on:<BR>> 70.933 -8.667<BR>><BR>> and
got:<BR>> 2873633.37 4593659.18 -12148.43<BR>><BR>>
which is my original matlab answer and that from the old proj4.4.
Now,<BR>> the question is if forcing the datum shift with +towgs=0,0,0
is<BR>> appropriate? Doesn't +towgs48 conflict with/override the
spherical<BR>> projection definition?<BR><BR>Faron,<BR><BR>I can't imagine
any situation other than an effort to match old answers<BR>where it makes sense
to apply a plain lat/long on a sphere to lat/long on an<BR>ellipsoid conversion
the way this is doing.<BR><BR>If you want the lat/long values computed from the
lcc projection based on<BR>a sphere to treated as WGS84, then use
+nadgrids=@null (or in 4.6.0 just<BR>omit a datum specifier for the lcc
projection). This is *likely* want<BR>you want.<BR><BR>Best
regards,<BR>--<BR>---------------------------------------+--------------------------------------<BR>I
set the clouds in motion - turn up | Frank Warmerdam,
warmerdam@pobox.com<BR>light and sound - activate the windows | <A
href="http://pobox.com/~warmerdam">http://pobox.com/~warmerdam</A><BR>and watch
the world go round - Rush | Geospatial Programmer for
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