Which pair of data is essential to establishing a parallel index line on a chart?

• A. Vessel speed and course
• B. Bearing to the NAVAID and the range to the charted line
• C. Current time and date
• D. Wind speed and direction

Answer: (B)

When you’re establishing a parallel index line on a chart, you need two specific pieces of information that work together to locate your position relative to a known reference. The bearing to a fixed aid to navigation tells you the direction toward that aid, anchoring your line of position to a real point on the chart. The range to the charted line gives you how far you are from that line, providing the offset needed to place a line on the chart parallel to the known chart line. With those two data—direction to the aid and distance from the charted line—you can draw a parallel index line that represents your possible positions. The other data—vessel speed and course, current time and date, wind speed and direction—don’t furnish the directional anchor and offset required for this plotting method.