A Position Line (or Line of Position, LOP) is a navigation term defining a line drawn on a nautical chart along which a vessel’s position must lie, determined by a single observation like a compass bearing, GPS range, or visual transit. A single LOP does not provide a fix; at least two are needed to pinpoint position.

Key Aspects of a Position Line:
Definition: It represents a set of points where the ship is located, generally obtained by measuring the direction or distance to a known object on land or a buoy.

Usage: If you take a compass bearing to a lighthouse and it is True, you draw a line from that lighthouse on the chart at seaward; your vessel is somewhere on that line.

Fixing Position: To get an accurate position, two or more LOPs are taken, ideally with a wide angle between them (ideally 90°) to minimize error.

Types:
Visual Bearing: Using a hand-bearing compass.

Transit/Range: Aligning two fixed, known objects ashore.

Distance Line: Using radar or a rangefinder to determine distance from an object, which creates an arc (a form of LOP).

Sumner Line: Historically refers to a method devised by Captain Thomas Sumner where a position line is determined by the altitude of the sun.