A course of constant azimuth/bearing on the Earth is a spiral. What is it on a standard Mercator map?

• A. Curved line
• B. Straight line
• C. Circle
• D. Ellipse

Answer: (B)

A course with constant azimuth is a rhumb line (loxodrome) on the Earth, which on a globe would spiral toward the poles because meridians converge. The Mercator map is designed so that angles are preserved: meridians are vertical straight lines and the scale changes with latitude, but the bearing of a path remains the same across the map. That means a line that crosses all meridians at a fixed angle—i.e., a constant bearing—maps to a line with a constant slope, which is a straight line. So on a standard Mercator projection, a constant-azimuth path appears as a straight line.