<HTML dir=ltr><HEAD><TITLE>Re: [Proj] WGS84 to Sphere Inconsistency Between Proj Versions</TITLE>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=2>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.</FONT></DIV>
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<DIV dir=ltr><FONT size=2>In regards to your logic, I'll leave the rationalizations to you. What I said stands as is.</FONT></DIV>
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<DIV dir=ltr><FONT size=2>C. Mugnier</FONT></DIV></DIV>
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<FONT face=Tahoma size=2><B>From:</B> proj-bounces@lists.maptools.org on behalf of Mikael Rittri<BR><B>Sent:</B> Mon 25-Aug-08 10:26<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>
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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>
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<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>
<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>
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<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>
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<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>
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<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>
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<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 Rent<BR><BR>_______________________________________________<BR>Proj mailing list<BR>Proj@lists.maptools.org<BR><A href="http://lists.maptools.org/mailman/listinfo/proj">http://lists.maptools.org/mailman/listinfo/proj</A><BR></FONT></P></DIV></DIV></BODY></HTML>