Lines of Position (LOP’s) LOP - A single observation that does not establish a fix but does mean the ship’s position is somewhere along that line. Label - a four digit time must be written above and parallel to the LOP.
Lines of Position (LOP’s) Visual LOP “shooting” tanks, water towers, church steeples, spires, radio and TV towers, daymarks, fixed navigation lights, flagpoles, or tangents to points of land.
Lines of Position (LOP’s) Visual Bearing can be measured in: - Degrees Relative ( # # # o R ) - Degrees per Gyro Compass ( # # # o pg c ) - Degrees Magnetic ( # # # o pstgc or o M ) - Degrees True ( # # # o T ) The navigator must convert any of these types of bearings to True before they can be plotted on the chart.
0815 0835 0900 Range Tangent Bearing Distance Arcs 0915 Tank
Plotting and Labeling a Fix Fix - The point where two or more LOP’s cross. This indicates the ship’s position on the chart. Label - Use the four digit time next to the fix, it should be parallel to the bottom of the chart. The times of the individual LOPs are not written on each LOP.
Using the Centroid Concept Problem - If a large triangle is formed by a 3 LOP fix, mark the center of the triangle as the fix. Danger - If the large triangle is near a hazard then choose the part of the triangle closest to the hazard .
Symbol Type Meaning Labeling Fixes Fix Fix DR EP Accurate Visual Fix Accurate Fix obtained by electronic means Dead reckon ing position, advanced from previous fix. Estimated position. Most probable position of ship.
Optimum Spread of LOP’s Best Fix – LOP’s must be widely spaced, two LOP’s at nearly the same angle will be less accurate. Formula – Both sides: 360 0 / number LOP’s – One side: 180 0 / number LOP’s Example: There are three LOP’s then optimum separation is 120 0 . Same side: 60 0 .
Determining the Order of LOP’s Bearings LOP’s Shoot Bearings: “Beam to Bow” Range LOP’s Shoot Ranges: “Bow to Beam”
Navigational Aid Selection Choose Nav Aids based on: - Provides the best bearing spread. - Lighted so that navaids can be used in low visibility and day or night. - Easily distinguishable by bearing takers.
Frequency of Fixes <ul><li>The primary concern is the proximity of hazards: - Every 15 min . coastal water with no immediate danger . - Every three min . when more accuracy is required . </li></ul><ul><li>Open water , a fix every 30 min . is normally plotted . </li></ul>