The line of position
A line of position (LOP) is a line established by observation or measurement on which a vessel can be expected to be located. A vessel can be at an infinite number of positions along any single LOP. In piloting, an LOP may be established by a measured bearing to a known and charted object, or by an alignment of two visible and charted objects, or by a measured distance from a charted object.
In the case of bearings, LOPs are straight lines. In the case of distances, LOPs are circles, and sometimes referred to as circles of position (COP). LOPs are used in this text to refer to both. These methods for determination of LOPs are discussed below. LOPs can also be determined by electronic means, such as radar.
Lop by bearing from charted object
One of the simplest and most common LOPs is developed from a single bearing on a charted object. The bearing can be taken using a pelorus (see Chapter 2), or a handbearing compass, or by orienting the boat so that the resulting relative bearing is dead ahead (000R), abeam (090R or 270R), or astern (180R). Bearings so determined could be compass bearings if the ship’s compass is used, magnetic bearings if a hand-bearing compass is used in a location where it is free of deviation, or relative bearings if a pelorus is used. If a relative bearing is taken, it must be converted to a true bearing by first determining the compass heading at the instant of the relative bearing observation, correcting this compass heading to a magnetic heading by applying deviation, and then to a true heading by allowing for variation.
Lop by distance From charted object
Distance to an object can be determined by measurement using a radar observation, by measurement of vertical angle with a sextant, stadimeter, or range finder. In this case, however, the LOP is a circle rather than a straight line. The resulting COP (or arc of position) is drawn as the circumference of a circle with origin at the observed object and radius equal to the measured distance.