Defining coordinate reference system with rotation in GeoServer?

I am using GeoServer and have a layer in EPSG:900913 ("Google Mercator").

I need to "rotate" the map around certain point (say, 1500000, 7000000) by certain degree (say, 30 degrees clockwise). How could I define such a coordinate system based on EPSG:900913?

GeoServer's angle vendor option does not work for my purposes as I need to tile the map later on.

As far as I understand this, my only option is to define an own coordinate system. For GeoServer I'd need to define it in WKT form. The configuration seems to be straightforward, but I have a difficulty defining my rotated CRS in WKT.

I am wondering how to apply a rotation around certain point onto a CRS like Google Mercator:

PROJCS["WGS84 / Google Mercator", 
  GEOGCS["WGS 84", 
    DATUM["World Geodetic System 1984", 
      SPHEROID["WGS 84", 6378137.0, 298.257223563, AUTHORITY["EPSG","7030"]], 

      AUTHORITY["EPSG","6326"]], 
    PRIMEM["Greenwich", 0.0, AUTHORITY["EPSG","8901"]], 
    UNIT["degree", 0.017453292519943295], 
    AXIS["Longitude", EAST], 
    AXIS["Latitude", NORTH], 
    AUTHORITY["EPSG","4326"]], 
  PROJECTION["Mercator_1SP"], 
  PARAMETER["semi_minor", 6378137.0], 
  PARAMETER["latitude_of_origin", 0.0], 
  PARAMETER["central_meridian", 0.0], 

  PARAMETER["scale_factor", 1.0], 
  PARAMETER["false_easting", 0.0], 
  PARAMETER["false_northing", 0.0], 
  UNIT["m", 1.0], 
  AXIS["x", EAST], 
  AXIS["y", NORTH], 
  AUTHORITY["EPSG","900913"]]

My questions, specifically:

• How to write a WKT which transform an existing CRS? My guess would be that I need a new PROJCS wrapping an existing one and adding a PROJECTION clause.


• How would I found out the projection id (like Mercator_1SP above) and the required parameters (the PARAMETER clauses)?


• Can I "reference" EPSG:900913 in CRS WKT instead of copy-pasting the whole PROJCS clause?

Answer

Ok, I've figured it out.

It is possible to apply an affine transform onto some existing CRS using FITTED_CS. Below is an example of rotation of 60 degrees counterclockwise and movement:

FITTED_CS["BPAF", 
  PARAM_MT["Affine", 
    PARAMETER["num_row", 3], 

    PARAMETER["num_col", 3], 
    PARAMETER["elt_0_0", -0.5], 
    PARAMETER["elt_0_1", -0.8660254037844386], 
    PARAMETER["elt_0_2", 1487816.0], 
    PARAMETER["elt_1_0", 0.8660254037844386], 
    PARAMETER["elt_1_1", -0.5], 
    PARAMETER["elt_1_2", 6886579.0]], 
  PROJCS["WGS84 / Google Mercator", 
    GEOGCS["WGS 84", 
      DATUM["World Geodetic System 1984", 

        SPHEROID["WGS 84", 6378137.0, 298.257223563, AUTHORITY["EPSG","7030"]], 
        AUTHORITY["EPSG","6326"]], 
      PRIMEM["Greenwich", 0.0, AUTHORITY["EPSG","8901"]], 
      UNIT["degree", 0.017453292519943295], 
      AXIS["Longitude", EAST], 
      AXIS["Latitude", NORTH], 
      AUTHORITY["EPSG","4326"]], 
    PROJECTION["Mercator_1SP"], 
    PARAMETER["semi_minor", 6378137.0], 
    PARAMETER["latitude_of_origin", 0.0], 

    PARAMETER["central_meridian", 0.0], 
    PARAMETER["scale_factor", 1.0], 
    PARAMETER["false_easting", 0.0], 
    PARAMETER["false_northing", 0.0], 
    UNIT["m", 1.0], 
    AXIS["x", EAST], 
    AXIS["y", NORTH], 
    AUTHORITY["EPSG","900913"]], 
  AUTHORITY["EPSG","8011113"]]

However I've found a bug in the current version of GeoTools (class cast exception when parsing this WKT). I've patched it and will also commit the fix.