Radar Observer Unlimited (ROU) Practice Exam

A parallel index line is defined by where a charted feature sits relative to a fixed reference, so you can draw a line that runs parallel to that feature. You need both direction and distance to the reference feature: the bearing to the charted line tells you the orientation, and the range to the line gives the offset—the perpendicular distance from the reference to the line. With both pieces, you can reproduce a line on the radar that remains parallel to the charted line as you move.

Knowing only the bearing or only the range won’t fix the offset, and information like range to the shoreline or ship speed and course doesn’t establish the parallel line. Therefore, the essential information is the bearing and range between the charted line and the NAVAID.