Line of Position from a Horizontal Angle

Extract

A horizontal angle between two charted objects provides a line of position and two angles between three or four objects will provide a fix or running fix except in the unique case of a ‘swinger’ or ‘revolver’. The method is faster than the conventional three-arm protractor.

A pelorus is frequently used to obtain two or more bearings for a position fix with generally sufficient accuracy. In rough weather the fix may not be particularly accurate, especially if the charted objects are a long distance away, such as lighthouses. If greater accuracy is desired, one solution is to use a sextant held horizontally to measure the two angles between three charted objects, plotting the fix by means of a three-arm protractor. The angles must be less than the 120-degree capability of the sextant and must be taken essentially simultaneously unless the snip's speed is slow. In some cases, it may be found that two objects lie to starboard and two to port, but the angles between the pairs exceed 120 degrees. The three-arm protractor cannot be used in this case. The method to be described will provide a fix or running fix without using a three-arm protractor.