<HTML dir=ltr><HEAD><TITLE>Re: [Proj] projection of Madagascar</TITLE>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=2>The problem with various software packages being unable to match the Laborde Oblique Mercator projection is not new. I mentioned the problem in my "Grids and Datums" column of <EM>Photogrammetric Engineering and Remote Sensing</EM> back in February of 2000. The Laborde projection is a triple projection on the Gaussian Sphere, and the Hotine Rectified Skew Orthomorphic projection is a triple projection on the aposphere. They are quite different. I spent many hours researching that projection back in the early 1980s, and I had to translate a lot from the original French, some of it in Laborde's original handwriting. It certainly is a tough nut to crack, and apparently too formidable for many programmers that don't read scientific French. </FONT></DIV>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=2>Some kluges have been worked up by ESRI in the past, but I thought Melita Kennedy told me that they (at ESRI) had indeed programmed Laborde's equations for one of their recent versions of Arc/View or Arc/Map. (However, I could be mistaken.)</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>
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<DIV dir=ltr><FONT size=2>For more info on the Grids and Datums of Madagascar, see: </FONT><A href="https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.ls" target=_blank><FONT face="Times New Roman" size=2>http://www.asprs.org/resources/GRIDS/</FONT></A><BR></DIV>
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<DIV dir=ltr><FONT face=Tahoma size=2><B>From:</B> proj-bounces@lists.maptools.org on behalf of OvV_HN<BR><B>Sent:</B> Tue 29-Apr-08 12:01<BR><B>To:</B> PROJ.4 and general Projections Discussions<BR><B>Subject:</B> Re: [Proj] projection of Madagascar<BR></FONT><BR></DIV></DIV>
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<P><FONT size=2>Clovis wrote:<BR><BR>> I have been quickly through the archive but didn't find any topic on this.<BR>> I have a projection issue concerning Madagascar.<BR><BR><BR>There are no good nor usable alternatives for the Laborde projection.<BR>Using the Oblique Mercator you'll get decimeters of error within the<BR>Madagascar territory.<BR>One example from my software:<BR><BR>Laborde Madagascar:<BR>lat = -16d 11m 23.280s; lon = 44d 27m 27.260s;<BR>lat0 = -18.9; lon0 = 46d 26m 14.025s;<BR>x0 = 4e5; y0 = 8e5; k0 = 0.9995; Az = 18.9;<BR>international 1924 ellipsoid<BR>-> x, y = 188333.848, 1098841.091<BR><BR>Oblique Mercator approximation with the following parameters:<BR>lat0 = -18.9; lon0 = 46d 26m 14.025s; lonc = lon0;<BR>x0 = 4e5; y0 = 8e5; k0 = 0.9995; alfac = 18.9; gamma = alfac;<BR>international 1924 ellipsoid<BR>-> x, y = 188333.847, 1098841.111<BR><BR>(angles in degrees wrt Greenwich, lengths in meters)<BR><BR>These differences may seem small, but wait until you get farther away<BR>from the central line.<BR><BR>See also section 1.3.6.1 "Laborde projection for Madagascar", EPSG<BR>Guidance Note 7-2, version April 2008.<BR>There you can also find omerc examples with an error of several decimeters.<BR><BR><BR><BR><BR><BR><BR><BR><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>