Piloting is a method of determining position and directing the movements of a vessel by reference to landmarks, navigational aids, or soundings. Piloting is usually used as a primary means of navigation when entering or leaving port and in coastal navigation. In piloting, the navigator obtains warnings of danger, fixes the position frequently and and determines the proper course of action.
LINES OF POSITION
A LOP is a line at some point of which a ship may be presumed to be on, as a result of observation or measurement. When piloting, LOPs are used to fix a ship’s position. An LOP is determined with reference to a landmark, which must be identified, and its position shown on the chart. There are three general types of LOPs: ranges, bearings including tangents, and distance arcs.
Note: An LOP should not be drawn through charted aids on the chart, because after a few erasures your symbols will become very difficult or impossible to see.
A LOP is a line at some point of which a ship may be presumed to be on, as a result of observation or measurement. When piloting, LOPs are used to fix a ship’s position. An LOP is determined with reference to a landmark, which must be identified, and its position shown on the chart. There are three general types of LOPs: ranges, bearings including tangents, and distance arcs.
Note: An LOP should not be drawn through charted aids on the chart, because after a few erasures your symbols will become very difficult or impossible to see.
A ship is on "range" when two landmarks are observed to be in line. This range is represented on a chart by means of a straight line which, if extended, would pass through the two related chart symbols. This line, labeled with the time expressed in four digits (above the line), is a fix of the ship’s position.
It is preferable to plot true bearings, although you can use either true or magnetic bearings. When the relative bearing of a landmark is observed, it should be converted to true bearing or direction by the addition of the ship’s true heading. Since a bearing indicates the direction of a terrestrial object from the observer, a LOP is plotted from the landmark in a reciprocal direction. Example, if a lighthouse bears 300° , the ship bears 120° from the lighthouse. A bearing LOP is labeled with the time expressed in four digits above the line and the bearing in three digits below the line.
It is preferable to plot true bearings, although you can use either true or magnetic bearings. When the relative bearing of a landmark is observed, it should be converted to true bearing or direction by the addition of the ship’s true heading. Since a bearing indicates the direction of a terrestrial object from the observer, a LOP is plotted from the landmark in a reciprocal direction. Example, if a lighthouse bears 300° , the ship bears 120° from the lighthouse. A bearing LOP is labeled with the time expressed in four digits above the line and the bearing in three digits below the line.
A special type of bearing is the tangent. When a bearing is observed on the right-hand edge of a projection of land, the bearing is a right tangent. A bearing on the left-hand edge of a projection of land as viewed by the observer is a left tangent. A tangent provides an accurate LOP if the point of land is sufficiently abrupt to provide a definite point for measurement.
A distance arc is a circular LOP. When the distance from an observer to a landmark is known, the fix of the observer’s position is a circle with the landmark as center having a radius equal to the distance. The entire circle need not be drawn, since in practice the navigator normally knows his position with enough accuracy as to require only the drawing of an arc of a circle. The arc is labeled with the time above expressed in four digits and the distance below in nautical miles (and tenths). The distance to a landmark may be measured using radar, the stadimeter, or the sextant along with TABLES 9 and 10 of the American Practical Navigator.
A distance arc is a circular LOP. When the distance from an observer to a landmark is known, the fix of the observer’s position is a circle with the landmark as center having a radius equal to the distance. The entire circle need not be drawn, since in practice the navigator normally knows his position with enough accuracy as to require only the drawing of an arc of a circle. The arc is labeled with the time above expressed in four digits and the distance below in nautical miles (and tenths). The distance to a landmark may be measured using radar, the stadimeter, or the sextant along with TABLES 9 and 10 of the American Practical Navigator.
