<html><head></head><body><div style="color:#000; background-color:#fff; font-family:Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif;font-size:13px"><div id="yui_3_16_0_ym19_1_1490185827564_329674">Hi Jochem,</div><div id="yui_3_16_0_ym19_1_1490185827564_329674"><br></div><div id="yui_3_16_0_ym19_1_1490185827564_329674" dir="ltr">if I understood well you need a method to transform your coordinates from one system to another.</div><div id="yui_3_16_0_ym19_1_1490185827564_329674" dir="ltr">7 parameters transformation (or Bursa-Wolf) is a geodetic coordinates transformation method, so the first question you must answer is what type of coordinates do you have on both systems and you must describe these systems.</div><div id="yui_3_16_0_ym19_1_1490185827564_329674" dir="ltr">A second issue that arises is the accuracy. Martin already explained you that 7 param. method gives transformations with errors of even >10 m big. So big errors are expected when we deal with datum (ellipsoids) that have parameters with big differences between them. </div><div id="yui_3_16_0_ym19_1_1490185827564_329674" dir="ltr">There is a confuse also, if I got the point, on the proj.4 calculation rounding errors that you say are 0.1 mm and the standard deviation of your transformed coordinates which you say are 4.5 cm.</div><div id="yui_3_16_0_ym19_1_1490185827564_329674" dir="ltr">In turn we can't help you unless you explain clearly : "what you have" and "what you need".</div><div id="yui_3_16_0_ym19_1_1490185827564_329674" dir="ltr"><br></div><div id="yui_3_16_0_ym19_1_1490185827564_329674" dir="ltr">regards Stefanos Beligiannis </div><div id="yui_3_16_0_ym19_1_1490185827564_329674" dir="ltr"> </div><div id="yui_3_16_0_ym19_1_1490185827564_329674" dir="ltr"><br></div><div class="qtdSeparateBR" id="yui_3_16_0_ym19_1_1490185827564_329709"><br><br></div><div class="yahoo_quoted" id="yui_3_16_0_ym19_1_1490185827564_329693" style="display: block;"> <div style="font-family: Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 13px;" id="yui_3_16_0_ym19_1_1490185827564_329692"> <div style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 16px;" id="yui_3_16_0_ym19_1_1490185827564_329691"> <div dir="ltr" id="yui_3_16_0_ym19_1_1490185827564_329708"> <font size="2" face="Arial" id="yui_3_16_0_ym19_1_1490185827564_329710"> <hr size="1" id="yui_3_16_0_ym19_1_1490185827564_329711"> <b id="yui_3_16_0_ym19_1_1490185827564_330219"><span style="font-weight:bold;" id="yui_3_16_0_ym19_1_1490185827564_330218">From:</span></b> Martin Desruisseaux <martin.desruisseaux@geomatys.com><br> <b id="yui_3_16_0_ym19_1_1490185827564_330221"><span style="font-weight: bold;" id="yui_3_16_0_ym19_1_1490185827564_330220">To:</span></b> proj@lists.maptools.org <br> <b><span style="font-weight: bold;">Sent:</span></b> Thursday, March 23, 2017 12:33 AM<br> <b id="yui_3_16_0_ym19_1_1490185827564_330184"><span style="font-weight: bold;" id="yui_3_16_0_ym19_1_1490185827564_330183">Subject:</span></b> Re: [Proj] +towgs84 approximation error<br> </font> </div> <div class="y_msg_container" id="yui_3_16_0_ym19_1_1490185827564_329690"><br><div dir="ltr" id="yui_3_16_0_ym19_1_1490185827564_329689">Hello Jochem<br clear="none"><br clear="none">The 7 parameter coordinate transformations are for datum shifts, which<br clear="none">are themselves approximative operations by nature. They have stochastic<br clear="none">error unrelated to the Proj.4 accuracy, much greater than the sine<br clear="none">approximation. For example the "NAD27 to WGS 84 (4)" transformation over<br clear="none">USA (EPSG:1173) has an accuracy of 10 metres. So in this context I<br clear="none">wonder what would be the purpose of aiming for an "accuracy" of 0.1 mm.<br clear="none">It can not be a positional accuracy (in the geodetic sense) at least,<br clear="none">but maybe the intend was something else?<br clear="none"><br clear="none"> Martin<br clear="none"><br clear="none"><div class="yqt7429681066" id="yqtfd98030"><br clear="none">Le 22/03/2017 à 22:41, Jochem a écrit :<br clear="none">> Hello, <br clear="none">><br clear="none">> I joined the mailing list because I noticed that the PROJ.4 library uses<br clear="none">> approximate formulas for the 7 parameter coordinate transformation defined<br clear="none">> by +towgs84 (technical details: The elements of the rotation matrix are the<br clear="none">> rotation angles instead of the product of sines and cosines of the angles).<br clear="none">> For really small rotation angles this gives the same result, but not for<br clear="none">> larger rotations. <br clear="none">><br clear="none">> For my current application I want to transform a map in QGIS with 1 mm<br clear="none">> accuracy. However, the approximation causes an error of 4 cm. Since PROJ.4<br clear="none">> normally gives results within 0.1 mm accuracy, I did not expect such<br clear="none">> approximation.<br clear="none">><br clear="none">> What can I do to help to fix this?<br clear="none">><br clear="none">> Regards, Jochem<br clear="none"><br clear="none">_______________________________________________<br clear="none">Proj mailing list<br clear="none"><a shape="rect" ymailto="mailto:Proj@lists.maptools.org" href="mailto:Proj@lists.maptools.org">Proj@lists.maptools.org</a><br clear="none"><a shape="rect" href="http://lists.maptools.org/mailman/listinfo/proj" target="_blank">http://lists.maptools.org/mailman/listinfo/proj</a></div></div><br><br></div> </div> </div> </div></div></body></html>