The forward and back azimuths of a geodetic line do not differ by exactly 180 degrees due to the _____.

• A. location of the observer
• B. length of observation
• C. convergence of the meridians
• D. difference in instruments

Answer: (C)

Meridian convergence is the reason. The azimuth at a point is measured relative to the local north, which follows the local meridian. Meridians are not parallel everywhere—they bend toward the poles. So the local reference directions at the two ends of a geodetic line aren’t aligned, which means the forward azimuth (from the first point toward the second) and the back azimuth (from the second toward the first) aren’t exactly opposite by 180 degrees. The discrepancy grows with distance and latitude, though it’s often small for short lines.