Grid azimuth equals geodetic azimuth when the observation lies on the central meridian.

• A. central
• B. Observer's
• C. Greenwich
• D. prime

Answer: (A)

The essential idea is that in a transverse projected grid, the grid north lines up with true north exactly along the projection’s central meridian. The difference between grid azimuth and geodetic azimuth is called meridian convergence. Along the central meridian this convergence is zero, so directions measured on the grid match directions on the ellipsoid. That’s why when an observation lies on the central meridian, the grid azimuth equals the geodetic azimuth.

The term central refers to that central meridian of the projection, which is why this option is the correct one. The other terms don’t describe the line where the alignment occurs (observer’s line, the Greenwich/prime meridian) or are not the relevant reference in this context.