Determining a Back Azimuth.  A back azimuth is the reverse (plus or minus 180°) of an azimuth, either magnetic or grid.
 To determine a back azimuth, if the original azimuth is

                  ·       Less than 180°, add 180°

                                                        ·       More than 180°, subtract 180°

Examples:  

                 14°                                                   310°

             +180°                                                    -180°

               194°                                                    130°