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I use Proj4j for a WebGIS and I have to create a new ESPG. I tried on QGIS to convert coordinates from a local system (in Greece) to a world coordinates system.

First, I have to do a 2D transformation: 2 translations (Tx ans Ty) and a rotation (Rz) of about 0.58931 radians around the "z axis" (in the plane of the local system) between the local coordinates and the Greek coordinates (EGSA87).

So I think that the rotation matrix is: [cos(Rz),-sin(Rz);sin(Rz),cos(Rz)] and without the cos and sin I have [0.8313242,-0.5557878;0.5557878,0.8313242].

Then, I can reuse the parameters of the EPSG:2100 definition:

+proj=tmerc +lat_0=0 +lon_0=24 +k=0.9996 +x_0=500000 +y_0=0 +ellps=GRS80 +towgs84=-199.87,74.79,246.62,0,0,0,0 +units=m +no_defs

I think I can insert the translations (Tx and Ty) in the x_0 and y_0, but I don’t know how to insert the rotation matrix. How can I insert a transformation with a matrix in a new EPSG?

I found on the internet that the +xform= command seems good, but I don’t know how to use it. I tried:

+proj=tmerc +lat_0=0 +lon_0=24 +k=0.9996 +x_0=-869388.894 +y_0=-4260031.458 +xform=[0.8313242,-0.5557878,0.5557878,0.8313242] +ellps=GRS80 +towgs84=-199.87,74.79,246.62,0,0,0,0 +units=m +no_defs

But apparently it isn’t a good solution. So, does anyone know how I can use this command in my new EPSG or where I can find the proj4 documentation?

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You can set up a local oblique mercator projection to get the rotation, as I described in my answer here:

Using customized Coordinate System in ArcGIS Desktop?

Your rotation parameter +alpha would be 33.765 degrees. You have to expriment about the sign to be used.

The Proj.4 parameters are listed here: http://trac.osgeo.org/proj/wiki/GenParms, but xform is not included.

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