<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_352623">Hi Jochem,</div><div id="yui_3_16_0_ym19_1_1490185827564_352623"><br></div><div id="yui_3_16_0_ym19_1_1490185827564_352623">this is more clear : </div><div id="yui_3_16_0_ym19_1_1490185827564_352623">1. Usually <span id="yui_3_16_0_ym19_1_1490185827564_352965"><b id="yui_3_16_0_ym19_1_1490185827564_353496">these parameters are defined by the government by LSM </b>and</span> their (the parameters) computations are based on some CPs of the National HARN network with coordinates calculated on both systems. This means they enclose the national grid internal errors plus the least square methods calculation errors. In turn it is a big luck for you that you have only up to 4.5cm error.</div><div id="yui_3_16_0_ym19_1_1490185827564_352623">2. To my opinion, If you want better accuracy you must re-measure with big accuracy the coordinates of those (or at least, as many as possible) common points you have on the old Hayford datum, on WGS84 with GPS static Measurements and then define new "local" 7 parameters to transform the coordinates you have, and ignore the government for this area.</div><div id="yui_3_16_0_ym19_1_1490185827564_352623" dir="ltr">3. I had the same with you problem on Riyadh Airport (Ain Al Abd datum) and I solved it like above.</div><div id="yui_3_16_0_ym19_1_1490185827564_352623" dir="ltr">I find the problem very interesting and I want to kindly give me feedback how you solved it.</div><div id="yui_3_16_0_ym19_1_1490185827564_352623" dir="ltr">many thanks</div><div id="yui_3_16_0_ym19_1_1490185827564_352623" dir="ltr">Stefanos</div><div class="qtdSeparateBR" id="yui_3_16_0_ym19_1_1490185827564_352613"><br></div><div class="yahoo_quoted" id="yui_3_16_0_ym19_1_1490185827564_352617" 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_352616"> <div style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 16px;" id="yui_3_16_0_ym19_1_1490185827564_352615"> <div dir="ltr" id="yui_3_16_0_ym19_1_1490185827564_352624"> <font size="2" face="Arial" id="yui_3_16_0_ym19_1_1490185827564_352671"> <hr size="1" id="yui_3_16_0_ym19_1_1490185827564_352821"> <b id="yui_3_16_0_ym19_1_1490185827564_352702"><span style="font-weight:bold;" id="yui_3_16_0_ym19_1_1490185827564_352701">From:</span></b> Jochem <jochem.lesparre@kadaster.nl><br> <b id="yui_3_16_0_ym19_1_1490185827564_352700"><span style="font-weight: bold;" id="yui_3_16_0_ym19_1_1490185827564_352699">To:</span></b> proj@lists.maptools.org <br> <b id="yui_3_16_0_ym19_1_1490185827564_352698"><span style="font-weight: bold;" id="yui_3_16_0_ym19_1_1490185827564_352697">Sent:</span></b> Thursday, March 23, 2017 3:35 AM<br> <b id="yui_3_16_0_ym19_1_1490185827564_353353"><span style="font-weight: bold;" id="yui_3_16_0_ym19_1_1490185827564_353352">Subject:</span></b> Re: [Proj] +towgs84 approximation error<br> </font> </div> <div class="y_msg_container" id="yui_3_16_0_ym19_1_1490185827564_352614"><br><div dir="ltr" id="yui_3_16_0_ym19_1_1490185827564_352618">Hi Stefanos,<br clear="none"><br clear="none">I have coordinates in an old national grid and a new national grid. Both<br clear="none">grids have TM projections on the international Hayfort ellipsoid. Both grids<br clear="none">have 7 parameter transformations to WGS84 defined by the government but not<br clear="none">published in EPSG. I want to do:<br clear="none"><br clear="none">cs2cs +init=mygrids:old +to +init=mygrids:new -f %.4f <old_coords.txt<br clear="none">>new_coords.txt<br clear="none"><br clear="none">What I need is exactly what I explained in my first email: a 7 parameter<br clear="none">transformation without approximation error of 4 cm (this error is a bias,<br clear="none">not a standard deviation) in the formulas.<br clear="none"><br clear="none">Regards, Jochem<br clear="none"><br clear="none"><br clear="none"><br clear="none"><br clear="none">--<br clear="none">View this message in context: <a shape="rect" href="http://osgeo-org.1560.x6.nabble.com/towgs84-approximation-error-tp5313738p5313766.html" target="_blank" id="yui_3_16_0_ym19_1_1490185827564_352619">http://osgeo-org.1560.x6.nabble.com/towgs84-approximation-error-tp5313738p5313766.html</a><br clear="none">Sent from the PROJ.4 mailing list archive at Nabble.com.<div class="yqt5996692855" id="yqtfd97216"><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><br clear="none"></div></div><br><br></div> </div> </div> </div></div></body></html>