<html><body><div style="color:#000; background-color:#fff; font-family:HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif;font-size:13px"><div><span></span></div><div id="yui_3_16_0_1_1426098655357_272427" dir="ltr">Hello Adrien,</div><div id="yui_3_16_0_1_1426098655357_272523"><br></div><div id="yui_3_16_0_1_1426098655357_272515">In Greece we don't use the 2D Helmert or the similarity transformation to compute coordinates between a local system and from and to EGSA87.</div><div id="yui_3_16_0_1_1426098655357_272426"><br></div><div id="yui_3_16_0_1_1426098655357_272524" dir="ltr">We use instead a 2nd degree polynomial transformation. Try this it will cover a 50km side rectangle with less than 2-3 cm error. This method applied by Prof. Agantza to compute parameters between the old HATT system and EGSA87.</div><div id="yui_3_16_0_1_1426098655357_272525" dir="ltr"><br></div><div id="yui_3_16_0_1_1426098655357_272526" dir="ltr">If you want more support mail me.</div><div id="yui_3_16_0_1_1426098655357_272527" dir="ltr"><br></div><div id="yui_3_16_0_1_1426098655357_272530" dir="ltr">Brgds</div><div id="yui_3_16_0_1_1426098655357_272528" dir="ltr">Stefanos Beligiannis</div><div id="yui_3_16_0_1_1426098655357_272529" dir="ltr"><br> </div><div id="yui_3_16_0_1_1426098655357_272357" style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 13px;"> <div id="yui_3_16_0_1_1426098655357_272356" style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 16px;"> <div id="yui_3_16_0_1_1426098655357_272355" dir="ltr"> <hr size="1" id="yui_3_16_0_1_1426098655357_272425"> <font id="yui_3_16_0_1_1426098655357_272359" face="Arial" size="2"> <b><span style="font-weight: bold;">From:</span></b> Adrien Barbaste <Adrien.Barbaste@efa.gr><br> <b><span style="font-weight: bold;">To:</span></b> "proj@lists.maptools.org" <proj@lists.maptools.org> <br> <b><span style="font-weight: bold;">Sent:</span></b> Thursday, March 12, 2015 4:29 PM<br> <b><span style="font-weight: bold;">Subject:</span></b> [Proj] How to create a new EPSG for a local system<br> </font> </div> <div class="y_msg_container"><br><div id="yiv2579247644">
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<div class="yiv2579247644MsoNormal">Hello,</div>
<div class="yiv2579247644MsoNormal"> </div>
<div class="yiv2579247644MsoNormal">I’m doing an internship for an archaeology school in Greece and I have a problem in creating of a new EPSG for local coordinates.</div>
<div class="yiv2579247644MsoNormal"> </div>
<div class="yiv2579247644MsoNormal">I use Proj4js to work on a WebGIS and I try to determine a new EPSG for the layers that are in local coordinates.</div>
<div class="yiv2579247644MsoNormal">I've all the layers in a local system built by the archeologists and some of this points in the greek coordinates system too (EGSA87).</div>
<div class="yiv2579247644MsoNormal"> </div>
<div class="yiv2579247644MsoNormal">First, I can calculate a 2D transformation between the local coordinates and the greek coordinates. So I obtained the 5 parameters of transformation : the 3 translations (Tx Ty and Tz), the scale factor (K) and the angle of rotation (Rz).</div>
<div class="yiv2579247644MsoNormal"> </div>
<div class="yiv2579247644MsoNormal">Then, I don’t know how to create a new EPSG, because I tried with a transverse mercator projection as the greek projection, but I think that it’s impossible to insert a rotation in a proj4js (only with "+axis=", but I cannot choose the
angle)</div>
<div class="yiv2579247644MsoNormal">So, I tried to use an oblique mercator projection with "+alpha=" (the rotation angle) and "+gamma=" (the meridian convergence)</div>
<div class="yiv2579247644MsoNormal"> </div>
<div class="yiv2579247644MsoNormal">The results aren't good. I've an error of less than 3cm on an archaeological site when the rotation is very low, but when I try this method on another site where the rotation angle is about 33 degrees, I have an error that can reach 15-20
centimeters.</div>
<div class="yiv2579247644MsoNormal"> </div>
<div class="yiv2579247644MsoNormal">I’d like to know how I can build a good EPSG for the local coordinates system ?</div>
<div class="yiv2579247644MsoNormal"> </div>
<div class="yiv2579247644MsoNormal">Thank you for your help</div>
<div class="yiv2579247644MsoNormal"> </div>
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