Often in the hard rock mines that I consult for, they use a local grid coordinate system. The area is typically small <20km x 20km so curvature of the earth is not a major problem. Typically I have a (x,y,z) point in the local grid and it's related UTM (east, north, elev) which is easy to deal with since it is just a issue of new false easting/northing.
But typically the local grid is not equal to true north. Local grid north can be up to +/-45 degrees and even sometimes more. This is typically done to optimize the linear nature of some deposit shapes for gridding but then sometimes it is just one of those strange things that just happened many years ago!
So how should I define a custom local grid SRID where local grid north is x degrees from true north?