<HTML dir=ltr><HEAD><TITLE>[Proj] "Double ellipsoid" case?</TITLE>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=2>I've seen a few projections and datums in my day, and I've never come accross that. For there to be a different ellipsoid of reference for a projection than there is for a datum is a contradiction in terms. I'll believe that when I see it. The closest to that I have ever seen is the coordinate systems of Hong Kong. From one datum adjustment to another, they went to an older ellipsoid (Clarke 1880 to Clarke 1858) with a corresponding change in the defining parameters for their projection. I think it was Cassini-Soldner, it's been so long since I published on that place.</FONT></DIV>
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<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 support.mn@elisanet.fi<BR><B>Sent:</B> Fri 28-Nov-08 06:00<BR><B>To:</B> proj@lists.maptools.org<BR><B>Subject:</B> [Proj] "Double ellipsoid" case?<BR></FONT><BR></DIV>
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<P><FONT size=2>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 ellipsoid<BR>definition available<BR><BR>"ellps=something"<BR><BR>The user might want to have:<BR><BR>"ellps_proj=ellps1"<BR>"ellps_datum=ellps2"<BR><BR>This would mean (?) that the projection should be calculated using<BR>ellps1 and the datum shift using the ellps2. And if the user writes:<BR><BR>"ellps=ellpsX"<BR><BR>that might be interpreted as both ellipsoids beeing the same:<BR><BR>"ellps_proj=ellps_datum=ellpsX"<BR><BR>So to do a full datum conversion there might be maximum four<BR>ellipsoids involved. Two on both ends for projection and additional<BR>two for datum shifts to wgs84 on both ends. All together four<BR>ellipsoids. This might finally tame the "north jump" in all cases?<BR><BR>Regards: Janne.<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>