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You will find that every zone in a State Plane coordinate system has a central meridian. In Colorado, the central meridian is 105 degrees, 30 minutes West Longitude, as shown here, on the left you have station D266 and on the right, station Fink. Notice that, in both cases, that grid north is exactly parallel with the central meridian. This is the convention for any plane coordinate system, so that north is always up, and always the same direction. All the norths are parallel in the plane systems; this is, of course, unrealistic. Meridians converge on the surface of the Earth. Therefore, there is a value, shown here in blue, called convergence. Convergence is the difference between geodetic north and grid north. You see over here on the right at station Fink, the convergence is plus 17.2 arcseconds. All the convergence angles east of the Central Meridian are positive. On the left, at station D266, the convergence is minus 40.0 arcseconds. All the convergence angles west of the Central Meridian are negative.
Please note that the sum of the grid azimuths (in red) at station Fink and station D266 is 360 degrees. This is a direct result of the grid north lines being parallel with each other. However, the sum of the geodetic azimuths (in green) at station Fink and station D266 is not 360 degrees. This is a direct result of the geodetic north lines converging. On the Earth they are not parallel with each other. The bottom line is this. On the State Plane coordinate grid, north is always parallel with the central meridian, but at the points on the Earth, north is along the meridian that passes through them. They are certainly not parallel with one another and certainly not parallel with the central meridian.