In geographic coordinate systems, what is the central meridian?

• A. The reference longitude
• B. The equator
• C. The prime meridian
• D. The anti-meridian

Answer: (A)

The central meridian is the longitude line that serves as the origin and center for a map projection. It is the reference longitude used to define the projection’s grid and to compute the projected coordinates, with distortion minimized near this line. The equator is a latitude line (not a longitude), so it isn’t the central meridian. The prime meridian is a specific global reference (0° longitude) used in geographic coordinates, but the central meridian for a projection is chosen to center the mapping and can be any longitude, not necessarily Greenwich. The anti-meridian is the line 180° away from the central meridian and isn’t the central line itself. So the central meridian is best described as the reference longitude.