What is parallel indexing?
Parallel indexing is a radar technique used to keep the ship a planned distance off a coast, a buoy or a radar-conspicuous point. A line is drawn on the radar parallel to the planned track, offset by the required distance. If the echo of the object stays on that index line, the ship is on track. If the echo crosses the line, the ship is setting in or out.
How this calculator works
Distance off track = Range × sin(relative bearing). A beam bearing of 90° gives distance off equal to the range. A finer angle gives a smaller cross-track distance. If a planned distance off is entered, the tool shows how far the ship is inside or outside that plan. Always use a reliable radar object and check gyro error before setting the index line.
Who should use it
Useful for bridge officers preparing a coastal passage. This does not replace a full passage plan, ECDIS XTD limits or visual bearings.