Which statement is true about lines of constant bearing on a Mercator projection?

• A. They are curved lines
• B. They are straight only at the equator
• C. They are circles of latitude
• D. They are straight lines

Answer: (D)

On a Mercator map, lines of constant bearing, or rhumb lines, appear as straight lines because the projection preserves angles. This means the angle between the route and the meridians stays the same all along the path, so it maps to a straight line on the chart. This is what makes Mercator charts so handy for navigation: a constant compass bearing translates to a straight line on the map. In contrast, great-circle routes (shortest paths on the globe) don’t maintain a constant bearing and usually appear curved on a Mercator projection, with the exception that the equator itself is a special, straight path. Lines of latitude are horizontal circles and don’t generally correspond to constant bearing paths.