<HTML dir=ltr><HEAD><TITLE>Re: [Proj] EPSG:28992 to EPSG:23031 conversion</TITLE>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=2>I find these sorts of discussions disconcerting in that TR8350.2 is a military publication that offers crude relations that are sufficient for indirect artillery fire control. That means it's good enough for shooting cannon shells over the horizon to hit military targets with an accuracy good enough for the blast radius of a 155mm high explosive Howitzer round!</FONT></DIV>
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<DIV dir=ltr><FONT size=2>GIS applications are not the same venue, and one should recognize the differences.</FONT></DIV>
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<DIV dir=ltr><FONT size=2>The Dutch government publishes software that will provide the MAXIMUM possible accuracy for a generalized datum shift within their Kingdom. Nothing else will be superior, muchless its equal.</FONT></DIV>
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<DIV dir=ltr><FONT size=2>Even with such superb accuracy, one must realize that a "datum shift" will DEGRADE the accuracy of ANYTHING submitted for processing as a "datum shift" and it is a best guess. The only "perfect" way to change datums is to re-compute the original field observations with new starting and ending survey point coordinates.</FONT></DIV>
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<DIV dir=ltr><FONT size=2>Short of that, it's a guess, the quality of the guess dependant on the quality of the algorithm - the official nationally-published version being the absolute best guess of them all.</FONT></DIV>
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<DIV dir=ltr><FONT size=2>Wanna invade a country and provide indirect cannon fire to support your infantry? Then use TR8350.2. However, don't consider it the first choice - it should be the last choice. I quote it often in my columns when there's nothing better available. It's better than nothing, but don't compare it to a national geodetic organization's offering!</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>P.S. ED50 was computed by the U.S. Army Map Service in the 1950s to provide a good unified system for future indirect artillery fire control ... if needed. It is NOT the "last" word in geodetic elegance - it was the best guess for the ARMY ... because WE won the war.</FONT></DIV></DIV>
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<FONT face=Tahoma size=2><B>From:</B> proj-bounces@lists.maptools.org on behalf of Paul Kelly<BR><B>Sent:</B> Wed 08-Oct-08 16:32<BR><B>To:</B> PROJ.4 and general Projections Discussions<BR><B>Subject:</B> Re: [Proj] EPSG:28992 to EPSG:23031 conversion<BR></FONT><BR></DIV>
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<P><FONT size=2>On Wed, 8 Oct 2008, Bart van den Eijnden (OSGIS) wrote:<BR><BR>> Hi Jean-Claude,<BR>><BR>> I am using PROJ 4.4.9 so that should not be necessary or am I mistaken?<BR><BR>I think you are mistaken, but would be interested if you could explain<BR>your reasoning behind that understanding. There seems to be a lot of<BR>confusion about the differences in this area in PROJ 4.4.9 and it might<BR>help to understand what is causing this confusion.<BR><BR>Jean-Claude is indeed correct about the second transformation lacking<BR>datum transformation paramters. If I use +towgs84=-87,-98,-121 (a general<BR>solution for ED 1950 across Europe) with your command-line I get the<BR>output:<BR>530201.420293435 5708616.572244617 1.595183260<BR>which is much closer to the official results. I suspect if you found a<BR>datum tranformation set for ED 1950 specifically covering the Netherlands<BR>rather than Europe as a whole you could get very close. (The NIMA TR8350.2<BR>document might perhaps be useful here; I haven't looked.)<BR><BR>Best regards,<BR><BR>Paul<BR><BR>><BR>> Best regards,<BR>> Bart<BR>><BR>> Jean-Claude Repetto wrote:<BR>>> bartvde@osgis.nl wrote :<BR>>><BR>>>> when I use PROJ.4 to do a conversion between EPSG:28992 (the Dutch<BR>>>> national grid) and UTM zone 31 ED50 I get:<BR>>>><BR>>>> cs2cs -f %.9f +proj=sterea +lat_0=52.15616055555555<BR>>>> +lon_0=5.38763888888889 +k=0.999908 +x_0=155000 +y_0=463000 +ellps=bessel<BR>>>> +units=m<BR>>>> +towgs84=565.2369,50.0087,465.658,-0.406857330322398,0.350732676542563,-1.8703473836068,4.0812<BR>>>> +no_defs +to +proj=utm +zone=31 +ellps=intl +units=m +no_defs<BR>>>> 19465 394814<BR>>>> 530108.861080788 5708613.329898705 -150.818087836<BR>>>><BR>>>><BR>>>> Northing 5708615,754<BR>>>> Easting 530199,479<BR>>>><BR>>>> so especially the difference in easting is too large.<BR>>>><BR>>>> Is there a logical explanation for this difference? TIA.<BR>>><BR>>> Hi Bart,<BR>>><BR>>> I think you have forgotten the +towgs84 parameters to do the WGS84 -> ED50<BR>>> transformation.<BR>>><BR>>> Jean-Claude<BR>>><BR></FONT></P></DIV></BODY></HTML>