I have been "gifted" a large cache of data (mostly CAD DWG, some TIF based photography) that is in an "arbitrary" coordinate system. I have determined that the system used is the correct NAD27 state plane zone for that area, except the first two digits of each coordinate pair have been clipped off. Is there a way to create a prj file or proj4 definition for this system to allow easy re-projection of the existing data?
If the area of interest is small enough so that it was always the same two digits removed (ex. 2411111 > 11111 and 122222 > 22222), create custom prj or proj4 definitions by updating the false easting and northing (x_0/y_0) values.
For instance, if the NAD27 state plane zone has a false easting of 2000000 US survey feet, and all coordinates are over 2100000, the new false easting would be -100000. If the original false northing was 0 US feet and coordinates are all above 320000, new false northing is -320000.
Just to clarify, you would assign or define this new coordinate system to the existing data. Do not reproject the data to it! The new definition describes the actual coordinate values/extents in the datasets. Once properly defined, they should line up with other datasets (if on-the-fly reprojection is enabled) or you can permanently reproject them to a standard coordinate system.