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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=2>Mikael,</FONT></DIV>
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<DIV dir=ltr><FONT size=2>It is mandated for datum transformations in the Gulf of Mexico by the Minerals Management Service, part of the Bureau of Land Management, Department of the Interior. (See the Americas Petroleum Studies Group in Houston.)</FONT></DIV>
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<DIV class=MsoNormal style="MARGIN: 0in 0in 0pt"><SPAN style="FONT-SIZE: 10pt"><?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p>Cliff</o:p></SPAN></DIV>
<DIV class=MsoNormal style="MARGIN: 0in 0in 0pt"><SPAN style="FONT-SIZE: 10pt"><o:p></o:p></SPAN> </DIV>Louisiana State University<BR></SPAN></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 13-Jan-11 08:54<BR><B>To:</B> PROJ.4 and general Projections Discussions<BR><B>Subject:</B> Re: [Proj] cs2cs returns error then 0s with datum conversion<BR></FONT><BR></DIV>
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<P><FONT size=2>Clifford,<BR>you wrote:<BR><BR>> NADCON is a U.S. Federal standard. Converting its data files to NTv2 format<BR>> would be useless for applications within the U.S. as it would no longer be<BR>> compliant with the Federal Register mandate. <BR>> [...]<BR>> When working with National Coordinate Systems and Reference Frames,<BR>> one can get into trouble quickly through ignorance of local laws.<BR><BR>Right, I should have added "this suggestion is void where prohibited by law".<BR><BR>Well, seriously, I would have agreed with you if we had discussed Great Britain<BR>and its national grid shift file, for two reasons:<BR><BR>1) Great Britain has redefined its OSGB 1936 datum, so that<BR> their grid shift file is now exactly right by definition.<BR> <A href="http://www.ordnancesurvey.co.uk/oswebsite/gps/docs/Geomatics_world.pdf">http://www.ordnancesurvey.co.uk/oswebsite/gps/docs/Geomatics_world.pdf</A><BR><BR>2) The British grid shift file is rectangular in a projected plane,<BR> while NTv2 is rectangular in Lat/Long, so you cannot get millimeter<BR> agreement, unless you used a much denser NTv2 grid, perhaps.<BR> (I have tested Paul Kelly's NTv2 version for Great Britain,<BR> see <A href="http://lists.maptools.org/pipermail/proj/2006-November/002658.html">http://lists.maptools.org/pipermail/proj/2006-November/002658.html</A> ,<BR> and the errors were usually a couple of centimeters, if I remember right.)<BR><BR>But it's different for the U.S., isn't it? The NADCON does not redefine<BR>the meaning of NAD27, so any software-based datum shift will be slightly<BR>wrong anyway. And NTv2 and NADCON are both rectangular in Lat/Long<BR>and use the same bilinear interpolation technique, so I expect<BR>that you can get sub-millimeter agreement between their results.<BR>(Maybe Frank knows whether this is true.)<BR><BR>As for NADCON being a Federal Standard, I found the following notice:<BR><BR><A href="http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc90-18809.pdf">http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc90-18809.pdf</A><BR><BR>but it says<BR><BR> "It is not the intent of the notice to declare when to<BR> use a datum transformation or by what method, but only<BR> to declare that when a mathematical transformation is<BR> appropriate, NADCON is recommended."<BR><BR>As I understand this, other mathematical transformations (or<BR>other software) are still legal, though not recommended. And<BR>if an NTv2 version would give sub-millimeter agreement with<BR>NADCON, I would say that it would follow the spirit of the<BR>recommendation, anyway.<BR><BR>Just my 20 öre,<BR><BR>Mikael Rittri<BR>Carmenta<BR>Sweden<BR><A href="http://www.carmenta.com/">http://www.carmenta.com</A><BR><BR>________________________________<BR><BR>From: proj-bounces@lists.maptools.org [<A href="mailto:proj-bounces@lists.maptools.org">mailto:proj-bounces@lists.maptools.org</A>] On Behalf Of Clifford J Mugnier<BR>Sent: Wednesday, January 12, 2011 9:05 PM<BR>To: PROJ.4 and general Projections Discussions<BR>Subject: Re: [Proj] cs2cs returns error then 0s with datum conversion<BR><BR><BR>NADCON is a U.S. Federal standard. Converting its data files to NTv2 format would be useless for applications within the U.S. as it would no longer be compliant with the Federal Register mandate. <BR><BR>The converse would likely apply to some other countries, including Canada and the Union of South Africa. Other countries may have not (yet) enacted legislation that codifies the specific methodology, but I have never seen a study of such a topic.<BR><BR>When working with National Coordinate Systems and Reference Frames, one can get into trouble quickly through ignorance of local laws.<BR><BR>Clifford J. Mugnier, C.P., C.M.S.<BR>Chief of Geodesy,<BR>Center for GeoInformatics<BR>Department of Civil Engineering<BR>Patrick F. Taylor Hall 3223A<BR>LOUISIANA STATE UNIVERSITY<BR>Baton Rouge, LA 70803<BR>Voice and Facsimile: (225) 578-8536 [Academic]<BR>Voice and Facsimile: (225) 578-4578 [Research]<BR>Cell: (225) 328-8975 [Academic & Research]<BR>Honorary Life Member of the<BR>Louisiana Society of Professional Surveyors<BR>Fellow Emeritus of the ASPRS<BR>Member of the Americas Petroleum Survey Group<BR><BR><BR>________________________________<BR><BR>From: proj-bounces@lists.maptools.org on behalf of Frank Warmerdam<BR>Sent: Wed 12-Jan-11 13:25<BR>To: Mikael Rittri<BR>Cc: PROJ.4 and general Projections Discussions<BR>Subject: Re: [Proj] cs2cs returns error then 0s with datum conversion<BR><BR><BR><BR>On 11-01-12 04:31 AM, Mikael Rittri wrote:<BR>> Eric wrote:<BR>><BR>>> Maybe someday an enterprising soul will convert cs2cs to use National Geodetic Survey's<BR>>> current correction file format directly...<BR>><BR>> Or maybe Frank would be willing to convert these files (conus, alaska, etc.)<BR>> to the NTv2 format (and publish them), since he has already developed tools<BR>> for that conversion, and since cs2cs already understands that format.<BR>><BR>> (See <A href="http://lists.maptools.org/pipermail/proj/2010-August/005330.html">http://lists.maptools.org/pipermail/proj/2010-August/005330.html</A> )<BR>><BR>> Of course, this would help only if the NTv2 format is unaffected by<BR>> the 32-bit/64-bit issue. I don't know for certain if that's true.<BR><BR>Folks,<BR><BR>The NTv2 format is not affected by structure packing, byte order or word<BR>size and is a good candidate for a cross platform replacement for the<BR>binary format into which the conus and related files are currently translated.<BR>Unfortunately my work in GDAL with NTv2 format does not include support for<BR>the .lla format yet, but I would love to replace it. I think I have seen<BR>the same data in .los/.las format, so perhaps I could just find and translate<BR>those. It would be wonderful to cleanup this aspect of proj.4 datum handling.<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><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>