<HTML dir=ltr><HEAD><TITLE>[Proj] trying to understand results of reprojecting from epsg 4326 to epsg 31370</TITLE>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=2>Consider not changing the signs of the rotation elements.</FONT></DIV>
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<DIV dir=ltr><FONT size=2>C. 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 Moritz Lennert<BR><B>Sent:</B> Tue 07-Jul-09 09:14<BR><B>To:</B> Proj@lists.maptools.org<BR><B>Subject:</B> [Proj] trying to understand results of reprojecting from epsg 4326 to epsg 31370<BR></FONT><BR></DIV>
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<P><FONT size=2>Hello,<BR><BR>I'm trying to reproject the following WGS84 Lat-Long (EPSG 4326)<BR>coordinates to the Belgian 1972 projection EPSG 31370:<BR><BR>4.468493 50.856057<BR><BR>Using the official software from the Belgian National Geographic<BR>Institute, I get the following coordinates (confirmed by a contact in<BR>the Institute):<BR><BR>157022.870 171745.084<BR><BR>Using cs2cs +init=epsg:4326 +to +init=epsg:31370, I get<BR><BR>157201.51 171803.24<BR><BR>i.e. way off the official response.<BR><BR>31370 is defined as<BR><BR>+proj=lcc +lat_1=51.16666723333333 +lat_2=49.8333339 +lat_0=90<BR>+lon_0=4.367486666666666 +x_0=150000.013 +y_0=5400088.438 +ellps=intl<BR>+towgs84=106.869,-52.2978,103.724,-0.33657,0.456955,-1.84218,1 +units=m<BR>+no_defs<BR><BR>so datum transformation is defined.<BR><BR>The transformation parameters are confirmed by the Belgian Institute<BR>(<A href="http://www.ign.be/FR/FR2-1-2.shtm">http://www.ign.be/FR/FR2-1-2.shtm</A>), which considers ETRS89 to be<BR>equivalent to WGS84:<BR><BR>Paramètres de ETRS89 vers BD72<BR>Tx 106.868628<BR>Ty -52.297783<BR>Tz 103.723893<BR>Rx 0.336570<BR>Ry -0.456955<BR>Rz 1.842183<BR>scale 1.0000012747<BR><BR>As these parameters are in the direction fromwgs84, they should be<BR>negated to get the inverse transformation. So, IIUC, 31370 should<BR>actually be defined as:<BR><BR>+proj=lcc +lat_1=51.16666723333333 +lat_2=49.8333339 +lat_0=90<BR>+lon_0=4.367486666666666 +x_0=150000.013 +y_0=5400088.438 +ellps=intl<BR>+towgs84=-106.869,52.2978,-103.724,-0.33657,0.456955,-1.84218,1 +units=m<BR>+no_defs<BR><BR>When I try this, I get:<BR>157080.62 171775.09<BR><BR>Much close, but still no good. The Institute also use a correction grid,<BR>but this should only change accuracy at the centimeter level.<BR><BR>Interestingly enough, when I use the 3-parameter transformation given by<BR>EPSG 1610:<BR><BR>+proj=lcc +lat_2=51.16666723333333 +lat_1=49.8333339 +lat_0=90<BR>+lon_0=4.367486666666666 +x_0=150000.013 +y_0=5400088.438 +ellps=intl<BR>+towgs84=-125.8,79.9,-100.5 +units=m +no_defs<BR><BR>I get perfect results:<BR>157022.80 171745.11<BR><BR>Does anyone have an explanation for this behaviour ?<BR><BR>Moritz<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>