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

[Adrien Barbaste]

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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