Convergence correction is subtracted to the astronomic azimuth to determine the correct grid azimuth of a line if the line is ___________ of the central meridian.

• A. east
• B. west
• C. north
• D. south

Answer: (B)

The concept here is the grid convergence—the angle between true (astronomical) north and the map grid north in a Transverse Mercator/UTM projection. This convergence is zero on the central meridian and increases (with a sign) as you move away from it. When converting an azimuth from astronomical to grid, you adjust by this convergence angle, and the direction of the adjustment depends on which side of the central meridian you’re on.

If a line lies to the west of the central meridian, the grid north is rotated toward the west relative to true north. In this case, you subtract the convergence correction from the astronomical azimuth to get the grid azimuth. That’s why the correct line position is west of the central meridian: subtracting the convergence gives the proper grid-based direction.

For intuition, imagine some numbers: if the astronomical azimuth is 90° (east) and the convergence angle to the west is 4°, the grid azimuth would be 86° (90° - 4°). If the line were east of the central meridian, you would add the convergence angle instead (GA = AA + gamma).