<HTML dir=ltr><HEAD><TITLE>Re: [Proj] "Double ellipsoid" case?</TITLE>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=2>Frank:</FONT></DIV>
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<DIV dir=ltr><FONT size=2>Ah, there's a BIG difference between the true coordinate system relations of geodesy used by national governments and one cooked up by an ignoramus at Google Maps that did not know what they were doing ... I guess there's a lot of that going around, too.</FONT></DIV>
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<DIV dir=ltr><FONT size=2>I suppose even twits help contribute to keep knowledgeable consultants in business.</FONT></DIV>
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<DIV dir=ltr><FONT size=2>Cliff Mugnier</FONT></DIV>
<DIV dir=ltr><FONT size=2>LSU</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> Fri 28-Nov-08 10:52<BR><B>To:</B> PROJ.4 and general Projections Discussions<BR><B>Subject:</B> Re: [Proj] "Double ellipsoid" case?<BR></FONT><BR></DIV>
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<P><FONT size=2>support.mn@elisanet.fi wrote:<BR>> Hello,<BR>><BR>> I just started to think about a situation where there might be a double<BR>> ellipsoid case.<BR>><BR>> 1) projection uses ellipsoid A independently<BR>> 2) datum shift uses ellipsoid B<BR>><BR>> Is this possible to be handled with proj.4? Since there is only one<BR>> ellipsoid definition available<BR><BR>Janne,<BR><BR>PROJ.4 does not currently handle this situation conveniently. One special<BR>case is where the projection uses a particular ellipsoid, but datum shifts<BR>should treat the corresponding lat/long values as being WGS84. In that<BR>special case you can use +nadgrids=@null to effectively say the datum shift<BR>to WGS84 is a no-op.<BR><BR>I have at times contemplated having a way of having a datum/ellipsoid<BR>definition used for datum shifting purposes that is independent of the<BR>normal ellipsoid used by the projection functions but I have not pursued it.<BR><BR>Clifford J Mugnier wrote:<BR> > I've seen a few projections and datums in my day, and I've never come<BR> > accross that. For there to be a different ellipsoid of reference for a<BR> > projection than there is for a datum is a contradiction in terms.<BR><BR>Cliff,<BR><BR>The very common case we see a lot these days is the google maps mercator<BR>projection. The mercator calculations are done based on a particular<BR>spherical earth model, but the resulting lat/long coordinates are intended<BR>to be treated as WGS84 after that.<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></BODY></HTML>