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<title>Re: [Proj] WGS84 to Sphere Inconsistency Between Proj Versions</title>
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<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Clifford wrote: “Absolute spatial positions between
disparate datums are lost when using datum-specific coordinates in a single
spherical projection.”<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Are you talking about moving a position between two
spherical datums, or any transformation which involves a spherical datum?”<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Clifford wrote: “I suggest you read some of my past
columns, in particular "The Basics of Datums."”<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Where could we find these columns?<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Do you also post messages on the POB forum occasionally?<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Thanks,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>The Sunburned Surveyor<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 color=blue face="Arial Narrow"><span
style='font-size:12.0pt;font-family:"Arial Narrow";color:blue'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 color=blue face="Arial Narrow"><span
style='font-size:12.0pt;font-family:"Arial Narrow";color:blue'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><b><font size=2 face=Tahoma><span style='font-size:10.0pt;
font-family:Tahoma;font-weight:bold'>From:</span></font></b><font size=2
face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'>
proj-bounces@lists.maptools.org [mailto:proj-bounces@lists.maptools.org] <b><span
style='font-weight:bold'>On Behalf Of </span></b>Clifford J Mugnier<br>
<b><span style='font-weight:bold'>Sent:</span></b> Monday, August 25, 2008 9:03
AM<br>
<b><span style='font-weight:bold'>To:</span></b> PROJ.4 and general Projections
Discussions<br>
<b><span style='font-weight:bold'>Subject:</span></b> RE: [Proj] WGS84 to
Sphere Inconsistency Between Proj Versions</span></font><o:p></o:p></p>
</div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<div id=idOWAReplyText24088>
<div>
<p class=MsoNormal><font size=2 color=black face="Times New Roman"><span
style='font-size:10.0pt;color:black'>Absolute spatial positions between
disparate datums are lost when using datum-specific coordinates in a single
spherical projection. I suggest you read some of my past columns, in
particular "The Basics of Datums." This is not an appropriate
venue to get into a detailed tutorial on geometric geodesy and it's historical
foundations.</span></font><o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
</div>
<div>
<p class=MsoNormal><font size=2 face="Times New Roman"><span style='font-size:
10.0pt'>In regards to your logic, I'll leave the rationalizations to you.
What I said stands as is.</span></font><o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
</div>
<div>
<p class=MsoNormal><font size=2 face="Times New Roman"><span style='font-size:
10.0pt'>C. Mugnier</span></font><o:p></o:p></p>
</div>
</div>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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face="Times New Roman"><span style='font-size:12.0pt'>
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<p class=MsoNormal style='margin-bottom:12.0pt'><b><font size=2 face=Tahoma><span
style='font-size:10.0pt;font-family:Tahoma;font-weight:bold'>From:</span></font></b><font
size=2 face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'>
proj-bounces@lists.maptools.org on behalf of Mikael Rittri<br>
<b><span style='font-weight:bold'>Sent:</span></b> Mon 25-Aug-08 10:26<br>
<b><span style='font-weight:bold'>To:</span></b> PROJ.4 and general Projections
Discussions<br>
<b><span style='font-weight:bold'>Subject:</span></b> RE: [Proj] WGS84 to
Sphere Inconsistency Between Proj Versions</span></font><o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>Dear Mr. Mugnier, </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>you wrote </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>>When diddling with spherical projections, the concept of
"DATUM" is entirely inappropriate. </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>which made me quite confused, because</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>1) I think what you wrote is absurd, and </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>2) I know well that you are an expert in geodesy. </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>But Hillel said, "the shamefast is not apt to
learn", so let me go on. </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>I suppose you require more properties of a geodetic datum
than I care about. </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>For me, a geodetic datum is essentially a way to
georeference a map (or at least </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>to georeference the graticule on the map.) Surely you
are not saying that if a map </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>uses a spherical projection, then its graticule cannot be
georeferenced. </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>I have also been wondering why the EPSG people, when
describing the web Mercator, </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>are careful to say that the geodetic datum is not WGS84
but something spherical </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>that is not a true datum. </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>As I see it, the projection machinery has to treat the earth
as a sphere, while the </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>datum shift machinery has to treat it as an ellipsoid.
And why not? In Carmenta Engine,</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>the Mercator class has a switch that lets you choose between
ellipsoidal and spherical </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>formulas. So the projection can ignore the flattening
of the ellipsoid, but the datum shift </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>machinery will not. </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>Are you (and EPSG) reasoning like this: </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'> (1.) all projections
must be implemented by ellipsoidal formulas, so </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'> (2.) the only way to emulate
spherical formulas is to supply an earth </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>
model that is a sphere from the beginning; and no proper datum </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>
can be spherical. </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>? </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>If so, then I agree that (2) follows from (1), and I agree
that proper datums should </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>not be spherical. But I do not agree that (1) is true.
</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>Best regards,</span></font><o:p></o:p></p>
<p><font size=2 face="Times New Roman"><span style='font-size:10.0pt'>--<br>
Mikael Rittri<br>
<st1:place w:st="on"><st1:City w:st="on">Carmenta</st1:City> <st1:State w:st="on">AB</st1:State></st1:place><br>
<st1:address w:st="on"><st1:Street w:st="on">Box</st1:Street> 11354</st1:address><br>
SE-404 28 Göteborg<br>
Visitors: Sankt Eriksgatan 5<br>
<st1:country-region w:st="on"><st1:place w:st="on">SWEDEN</st1:place></st1:country-region><br>
Tel: +46-31-775 57 37<br>
Mob: +46-703-60 34 07<br>
mikael.rittri@carmenta.com<br>
www.carmenta.com</span></font> <o:p></o:p></p>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<p class=MsoNormal style='margin-bottom:12.0pt'><b><font size=2 face=Tahoma><span
style='font-size:10.0pt;font-family:Tahoma;font-weight:bold'>From:</span></font></b><font
size=2 face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'> proj-bounces@lists.maptools.org
[mailto:proj-bounces@lists.maptools.org] <b><span style='font-weight:bold'>On
Behalf Of </span></b>Clifford J Mugnier<br>
<b><span style='font-weight:bold'>Sent:</span></b> den 22 augusti 2008 05:45<br>
<b><span style='font-weight:bold'>To:</span></b> PROJ.4 and general Projections
Discussions; PROJ.4 and general Projections Discussions<br>
<b><span style='font-weight:bold'>Subject:</span></b> RE: [Proj] WGS84 to
Sphere Inconsistency Between Proj Versions</span></font><o:p></o:p></p>
<div id=idOWAReplyText29000>
<div>
<p class=MsoNormal><font size=2 face="Times New Roman"><span style='font-size:
10.0pt'>When didling with spherical projections, the concept of
"DATUM" is entirely inappropriate.</span></font><o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
</div>
<div>
<p class=MsoNormal><font size=2 face="Times New Roman"><span style='font-size:
10.0pt'>Cliff Mugnier</span></font><o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><st1:place w:st="on"><st1:PlaceName w:st="on"><font size=2
face="Times New Roman"><span style='font-size:10.0pt'>LOUISIANA</span></font></st1:PlaceName><font
size=2><span style='font-size:10.0pt'> <st1:PlaceType w:st="on">STATE</st1:PlaceType>
<st1:PlaceType w:st="on">UNIVERSITY</st1:PlaceType></span></font></st1:place><o:p></o:p></p>
</div>
</div>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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face="Times New Roman"><span style='font-size:12.0pt'>
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<p class=MsoNormal style='margin-bottom:12.0pt'><b><font size=2 face=Tahoma><span
style='font-size:10.0pt;font-family:Tahoma;font-weight:bold'>From:</span></font></b><font
size=2 face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'>
proj-bounces@lists.maptools.org on behalf of Frank Warmerdam<br>
<b><span style='font-weight:bold'>Sent:</span></b> Thu 21-Aug-08 22:38<br>
<b><span style='font-weight:bold'>To:</span></b> PROJ.4 and general Projections
Discussions<br>
<b><span style='font-weight:bold'>Subject:</span></b> Re: [Proj] WGS84 to
Sphere Inconsistency Between Proj Versions</span></font><o:p></o:p></p>
</div>
<div>
<p><font size=2 face="Times New Roman"><span style='font-size:10.0pt'>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 Rent<br>
<br>
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