The direction of the movements of a vessel by reference to land and sea marks, by soundings, or by radar, is called piloting. All electronic methods of navigation are considered to be forms of piloting. The electronic methods, including radar, are visual piloting and with the use of depth information.
Piloting requires the greatest experience and good judgment of any form of navigation. Constant vigilance, mental alertness, and a thorough knowledge of the principles involved are essential. Mistakes in navigation on the open sea can generally be discovered and corrected before the next landfall. In piloting there is little or no opportunity to correct errors. Even a slight blunder may result in disaster involving, the loss of life. The problems of piloting are very simple, both in principle and in application. It is the proximity of danger which makes piloting so important. Avoiding a collision in the heavy traffic that exists in the harbors and along coast lines is essentially a problem of seamanship. The navigator is concerned with the problem of keeping his ship in navigable waters. The principles and procedures which will keep sufficient water under the keel of a large vessel will bring safety to a smaller one.
In piloting, the navigator must constantly realize that he is dealing with the past, the present, and the future. He must continually analyze the situation that existed in the recent past and exists at present in order to plan for the future. He should constantly use every logical means at his disposal to:
obtain warnings of approaching danger;
fix the position of the ship accurately and frequently and determine the proper course of action.
Probably the most important thing in piloting, as in almost all phases of navigation, is that of the line of position (LOP). A single observation does not establish a position; it does provide the observer with a line on some point of which he is located. This line is a segment of a great circle but in visual piloting the segment is so short that it may be plotted as a straight, or rhumb, line on a Mercator chart.
It should also be noted that a line of position can provide useful negative information. If the LOP is valid, then the observer, and his vessel, are not somewhere else, such as in shoal water. A single LOP of good quality, while not establishing a position, can at least rule out some worries if it crosses no hazards.
It must be kept in mind that there is no connection between the DR course line and lines of position. The DR course line and DR positions may be considered as statements of intention, or a graphic representation of ordered courses and speeds. The lines of position are statements of fact, as the ship is actually somewhere on the line of position, regardless of courses steered, and speeds used.
A single line of position, whether a bearing, a range, or a distance, is labeled on the upper side of the line with the time of observation expressed in four digits. A single line of position advanced to form a running fix is labeled with the original time of observation and the time to which it has been advanced. (Direction is not normally labeled on a LOP, but, it can be added as a three-digit number directly
under the time label, true direction is assumed unless "M" is used indicating magnetic direction.) Lines of position forming a fix need not be labeled, the time of the fix being sufficient. Similarly, the second line of position in a running fix is not labeled, taking its time as that of the running fix.
Every line must be labeled as soon as it is plotted; an unlabeled line can be a source of error, especially after a change of watch. There is enough uncertainty in piloting without adding to it by leaving doubt as to the meaning of a line. Care must be taken not to confuse a course line with a line of position.
Range - The simplest way to establish a line of position is to observe a range. If two fixed objects of known position appear to the observer to be in line, he must at that instant be somewhere on the line passing through the objects and extending beyond it. He can also take from the negatives of his line of position; if he is on the range line, he is not somewhere off of it such as in shoal water or headed into some other hazard.
Draw light lines on the chart and make them no longer than necessary. Avoid drawing them through the chart symbols for aids to navigation, which may be indistinct by erasures.
Most ranges used for navigation consist of two fixed aids to navigation, usually, but not always, lighted, and established to constitute a range. A navigator, however, must not overlook the possibility of using natural or manmade objects located to meet his needs.
In addition to using two objects in line to obtain a line of position, a steering range may be used to direct the course of a vessel. Aids to navigation are located in pairs to assist a vessel in staying within a channel. The danger in using any range, is that it can be used beyond its safe limits. A navigator must be careful to use any range over only that portion which is safe. You must be careful not to follow a range too far, either towards the front marker or away from it; you must be alert as to his position along the range line.
Plotting bearings- It is not usually possible to find two fixed known objects in line at the time the navigator wishes to make an observation. Consequently, the line of position is normally obtained by plotting a bearing on the chart. The observer sights across his pelorus, hand-bearing compass, bearing circle, or gyro repeater toward a fixed, known object and determines the direction of the line of sight to that object: this is the bearing of the object. You then plot this bearing to the known object on his chart. When you get two lines of postions they should cross at angles near 90 degrees. It is best two use 3 LOP,this defines the postion more accurate.
