<HTML dir=ltr><HEAD><TITLE>[Proj] What about datum shift via direct projection?</TITLE>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=2>In regard to "direct projection," it happens to be a very old method of changing datums. With some variations, it is essentially how the European Datum of 1950 was developed by the U.S. Army Map Service (AMS) after WWII. They used the UTM Grid to accomplish the result on the complex plane. Designed by Geodesists at AMS, the paper and pencil work was done mostly by Europeans after the war. ED50 covered (and covers) all of Western Europe and the entire Mediterranean Region.</FONT></DIV>
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<DIV dir=ltr><FONT size=2>It's not that elegant, but it DOES work, especially if you are not too concerned about the geoid. Cartometrically speaking, it's "bang on."</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 Mikael Rittri<BR><B>Sent:</B> Thu 11-Dec-08 09:43<BR><B>To:</B> PROJ.4 and general Projections Discussions<BR><B>Subject:</B> [Proj] What about datum shift via direct projection?<BR></FONT><BR></DIV>
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<P><FONT size=2>Gerald wrote:<BR>> While there seems to be lull in the hot debate about separation of<BR>> church and state ... er ... datum and projection,<BR>> [...]<BR>> Thus, why is it so necessary to bind the two operations so tightly as done<BR>> in the proj.4 distribution? I cannot find a precedence for this concept.<BR><BR>This post is not specifically about the PROJ.4 design (so I changed<BR>the Subject line), but it is about how much datums and projections<BR>can and should be separated. <BR><BR>There is method for datum shift that uses a direct projection. <BR><BR>As an example, the old Swedish Grid is traditionally defined<BR>on the Swedish RT90 datum (ellipsoid: Bessel 1841) and using a<BR>Transverse Mercator projection with<BR> central meridian: 15° 48' 29.8" E<BR> scale factor: 1<BR> false easting: 1500000 m<BR> false northing: 0 m<BR>( <A href="http://www.lantmateriet.se/templates/LMV_Page.aspx?id=4766&lang=EN">http://www.lantmateriet.se/templates/LMV_Page.aspx?id=4766&lang=EN</A> )<BR><BR>With this definition, one would need some datum shift method<BR>to transform between RT90 lon/lat and WGS84 lon/lat.<BR><BR>However, a simpler method, now recommended by the Swedish Land Survey<BR>instead of a 7-parameter shift, is to start from the WGS84 datum, and than<BR>tweak the projection parameters a little: just use a Transverse Mercator<BR>with<BR> central meridian: 15° 48' 22.624306" E <BR> scale factor: 1.00000561024<BR> false easting: 1500064.274 m<BR> false northing: -667.711 m<BR>( <A href="http://www.lantmateriet.se/templates/LMV_Page.aspx?id=5197&lang=EN">http://www.lantmateriet.se/templates/LMV_Page.aspx?id=5197&lang=EN</A> )<BR><BR>A paper describing this technique is<BR><A href="http://www.fig.net/pub/fig2006/papers/ps05_03/ps05_03_04_engberg_lilje_0670.pdf">http://www.fig.net/pub/fig2006/papers/ps05_03/ps05_03_04_engberg_lilje_0670.pdf</A> .<BR><BR>So, I have some rather vague questions to the readers of this list:<BR>- What do you think of this technique?<BR>- Is anyone else using it?<BR>- Doesn't the technique imply that a projected coordinate system<BR> may have an ambiguous geographic coordinate system? For the Swedish Grid,<BR> I can think of the geographic coordinate system as RT90 lon/lat, if I use<BR> the traditional projection parameters. Or I can think of it as WGS84 lon/lat,<BR> if I use the direct projection instead.<BR>- If the correct answer to the previous question is "No, you fool", then what?<BR> If I wanted to express the Swedish Grid, datum-shifted by the direct projection,<BR> in Well-Know Text, then I would be forced to say that the geographic coordinate<BR> system is WGS84 lon/lat. But then the resulting CRS cannot be Swedish Grid,<BR> because Swedish Grid has traditionally RT90 lon/lat as its geographic coordinate<BR> system.<BR><BR>I think direct projections for datum shifts are efficient and easy to<BR>use, and normally as accurate as a 7-parameter shift. But when I try<BR>to fit this method into the traditional framework that separates datum<BR>shifts and projections, and which insists that each projected CRS<BR>has a unique geographic coordinate system, I run into problems.<BR><BR>Are these problems caused by inflexibility in the traditional framework?<BR>Or is the method of direct projection just weird?<BR>Or am I missing some good way to reconcile them?<BR> <BR>Best regards,<BR><BR>--<BR>Mikael Rittri<BR>Carmenta AB<BR>Box 11354<BR>SE-404 28 Göteborg<BR>Visitors: Sankt Eriksgatan 5<BR>SWEDEN<BR>mikael.rittri@carmenta.com<BR>www.carmenta.com<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>