Convergence correction is added to the geodetic azimuth to determine the correct grid azimuth of a line if the line is ________ of the central meridian.

Convergence correction is added to the geodetic azimuth to determine the correct grid azimuth of a line if the line is ________ of the central meridian.

• A. West
• B. North
• C. East
• D. South

Answer: (C)

Convergence correction reflects the difference between grid north and true north caused by projecting onto a map grid like Transverse Mercator. The convergence angle is zero on the central meridian and becomes positive to the east (grid north rotated toward the east) and negative to the west. To convert a geodetic azimuth (from true north) to a grid azimuth (from grid north) you add the convergence angle when the line lies east of the central meridian, because grid north is already rotated toward the east. If the line were west of the central meridian, you’d subtract. The north/south position of the line isn’t what drives the correction—the longitude offset is. So, for a line east of the central meridian, you add the convergence correction to the geodetic azimuth to get the correct grid azimuth.

Convergence correction reflects the difference between grid north and true north caused by projecting onto a map grid like Transverse Mercator. The convergence angle is zero on the central meridian and becomes positive to the east (grid north rotated toward the east) and negative to the west. To convert a geodetic azimuth (from true north) to a grid azimuth (from grid north) you add the convergence angle when the line lies east of the central meridian, because grid north is already rotated toward the east. If the line were west of the central meridian, you’d subtract. The north/south position of the line isn’t what drives the correction—the longitude offset is. So, for a line east of the central meridian, you add the convergence correction to the geodetic azimuth to get the correct grid azimuth.