[Proj] How to create a new EPSG for a local system

[Stefanos Beligiannis]

Hello Adrien,
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.
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.
If you want more support mail me.
BrgdsStefanos Beligiannis
       From: Adrien Barbaste <Adrien.Barbaste at efa.gr>
 To: "proj at lists.maptools.org" <proj at lists.maptools.org> 
 Sent: Thursday, March 12, 2015 4:29 PM
 Subject: [Proj] How to create a new EPSG for a local system
   
 <!--#yiv2579247644 _filtered #yiv2579247644 {font-family:"Cambria Math";panose-1:2 4 5 3 5 4 6 3 2 4;} _filtered #yiv2579247644 {font-family:Calibri;panose-1:2 15 5 2 2 2 4 3 2 4;}#yiv2579247644 #yiv2579247644 p.yiv2579247644MsoNormal, #yiv2579247644 li.yiv2579247644MsoNormal, #yiv2579247644 div.yiv2579247644MsoNormal {margin:0in;margin-bottom:.0001pt;font-size:11.0pt;font-family:"Calibri", "sans-serif";}#yiv2579247644 a:link, #yiv2579247644 span.yiv2579247644MsoHyperlink {color:blue;text-decoration:underline;}#yiv2579247644 a:visited, #yiv2579247644 span.yiv2579247644MsoHyperlinkFollowed {color:purple;text-decoration:underline;}#yiv2579247644 span.yiv2579247644EmailStyle17 {font-family:"Calibri", "sans-serif";color:windowtext;}#yiv2579247644 .yiv2579247644MsoChpDefault {font-family:"Calibri", "sans-serif";} _filtered #yiv2579247644 {margin:1.0in 1.0in 1.0in 1.0in;}#yiv2579247644 div.yiv2579247644WordSection1 {}-->Hello,    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.    I use Proj4js to work on a WebGIS and I try to determine a new EPSG for the layers that are in local coordinates. 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).    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).    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) So, I tried to use an oblique mercator projection with "+alpha=" (the rotation angle) and "+gamma=" (the meridian convergence)    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.    I’d like to know how I can build a good EPSG for the local coordinates system ?    Thank you for your help    
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