When converting astronomic azimuth to grid azimuth, convergence correction is subtracted if the line is west of the central meridian.

• A. Always added
• B. Always subtracted
• C. Subtracted for lines east of the central meridian
• D. Subtracted for lines west of the central meridian

Answer: (D)

Convergence angle is the angle between true north and grid north at a given location on a map projection. Since grid north is not aligned with geographic north everywhere, converting an astronomical azimuth (measured from true north) to a grid azimuth (measured from grid north) requires adjusting by that convergence angle. The central meridian of a transverse projection has zero convergence, so true north and grid north line up there, but away from it they diverge in opposite directions on opposite sides.

West of the central meridian, the grid north tilts toward the west relative to true north, so you subtract the convergence angle from the astronomical azimuth to get the grid azimuth. East of the central meridian, you would add the convergence angle. This is why subtracting is correct for lines west of the central meridian. For example, with a convergence of 2 degrees, an astronomical azimuth of 90 degrees becomes 88 degrees when west of the central meridian. Subtracting in that region aligns the azimuth with the grid direction, while adding would misalign it.