I'm having trouble creating custom proj4 CRS files to work with existing .shp files on local project coordinates. (Local in that the coordinates are relative only to themselves and not the 'real world')
Right now we are wanting to export property lines from QGIS into AutoCAD. The property lines were provided to us with a .prj file using NAD83 State Plane Coordinates. In the past, we have utilized ArcGIS (or notepad) to create a projected local coordinate system based on our project parameters and assign its .prj to the data frame of our .shp file, and tell ArcGIS to use that .prj for the output. Done.
The idea is that we would like to be able to create custom CRS in proj4 and be able to utilize that to export 'data' from QGIS and have it relate to our local project coordinates. Vice versa, we would like be able to export CAD 'data' (obviously on the local coordinate system) and assign the custom CRS to have it relate to the rest of the world (or at least my state)
For example, I generally set up our project coordinate systems based around State Plane Coordinates on ground distances(for quantity purposes). I generally "lop off" the first couple digits off from the SPCS in an effort to make the coordinates a bit easier to work with(knowing that most people we work with find it easier with 4 or 5 digits rather than 6 or 7). The problem herein lays that our point of "origin"(aka the point where Northing = 0 and Easting = 0) does not fall down in Northeastern New Mexico where my state has its SPCS origin. I need to be able to tell QGIS a false northing/easting, a scale factor, a rotation, and a lat/long origin to accurately project our project work onto the 'real world'