<HTML dir=ltr><HEAD><TITLE>Re: [Proj] Accuracy Cassini Solder -> lat lon 200m?!</TITLE>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=2>Sebastian,</FONT></DIV>
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<DIV dir=ltr><FONT size=2>Whether you use EPSG parameters or any other set of parameters, you will never get exact conversion to the WGS84 datum from an old classical datum. For all intents and purposes, WGS84 is "perfect" and the old systems developed with classical triangulation and widely-spaced baselines physically measured on the ground with invar tapes were never "perfect." All of the datum shift algorithms are compromises ... none offer an exact conversion. Adding additional parameters such as changing from a three-parameter transformation to a seven-parameter transformation will give a better conversion, but will still fall short of perfection.</FONT></DIV>
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<DIV dir=ltr><FONT size=2>As a general rule, for a given number of parameters for a transformation from a classical datum to an inertial datum such as WGS84, <U>the smaller the area from which the parameters have been derived</U>, then the <EM><STRONG>higher </STRONG></EM>the accuracy of the transformation <U>for that given area</U>.</FONT></DIV>
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<DIV dir=ltr><FONT size=2>National governments commonly offer the best compromise for such datum transformations ... if they so choose to do so. That is because they have access to the entire database of all of <STRONG><EM><U>their</U></EM></STRONG> geodetic control, and rarely does anyone else. <BR></FONT></DIV>
<DIV dir=ltr><FONT size=2>C. Mugnier</FONT></DIV>
<DIV dir=ltr><FONT size=2>LOUISIANA STATE UNIVERSITY</FONT></DIV>
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<DIV dir=ltr><FONT size=2><EM>P.S. Consider reading some of my past columns - they may help understanding some of this material.</EM></DIV></FONT></DIV>
<DIV dir=ltr><A href="http://www.ASPRS.org/resources/grids">www.ASPRS.org/resources/grids</A> <BR>
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<FONT face=Tahoma size=2><B>From:</B> proj-bounces@lists.maptools.org on behalf of Sebastian Schubert<BR><B>Sent:</B> Mon 06-Jul-09 05:27<BR><B>To:</B> proj@lists.maptools.org<BR><B>Subject:</B> Re: [Proj] Accuracy Cassini Solder -> lat lon 200m?!<BR></FONT><BR></DIV>
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<P><FONT size=2>Dear all,<BR><BR>* On Friday 03 July 2009, 11:39:31, Jean-Claude Repetto wrote:<BR>> Sebastian Schubert a écrit :<BR>> > I want to convert from Solder system used in the city of Berlin<BR>> > (EPSG:3068) to the standard lat lon system.<BR>><BR>> What do you mean "standard lat lon system" ? If you mean WGS84, you<BR>> should try :<BR>> cs2cs -v +init=epsg:3068 +to +init=epsg:4326<BR><BR>I would like to understand this a bit more. I know this is probably not<BR>the perfect place to ask but I do not know where to ask. I would be<BR>also very happy to be given a hint where to find the solution myself.<BR><BR>As far as I understand, the standard settings, I applied before:<BR><BR>schubert@linux-8ubn:~> cs2cs -v +init=epsg:3068<BR>...<BR># ---- To Coordinate System ----<BR>#Lat/long (Geodetic alias)<BR>#<BR># +proj=latlong +datum=potsdam +ellps=bessel +towgs84=606.0,23.0,413.0<BR><BR>refer to the EPSG::6314 Deutsches Hauptdreiecksnetz. The not vanishing<BR>values of towgs84 show that this is not wgs84. This gives the same<BR>results as the formulas found in the epsg guide<BR><A href="http://www.epsg.org/guides/G7-2.html">http://www.epsg.org/guides/G7-2.html</A><BR>when using the parameters for the ellipsoid given in the epsg database<BR>for EPSG::6314 Deutsches Hauptdreiecksnetz.<BR><BR>However, I thought that when I replace these parameters with the ones of<BR>wgs84, I should actually get wgs84 coordinates. This does not seem to<BR>be true. Does this mean that eg the ellipsiod has another centre? So<BR>the formulas given in the guide are only applicable for exactly one<BR>Base Geodetic CRS (in this case DHDN)? So proj does a two step<BR>conversion when I want to convert to wgs84: convert to Base Geodatic<BR>system, then convert to wgs84? Are the formulas for the latter given<BR>somewhere?<BR><BR>Thanks a lot!<BR>Sebastian<BR></FONT></P></DIV></BODY></HTML>