lesson 11 lops and fixes
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Fundamentals ofNaval Science
Lines of Position & Fixes
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.
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Single LOP
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Visual LOP
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FL 6s 58ft 9M
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TWR
Visual Bearing LOP 2357
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Radar RangeLOP
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Radar Range LOP
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Lines of Position (LOP)Visual Range - The most accurate type of LOP, when two known objects line up.
Some ranges are man made: “intentional”
and others are natural: “coincidental.”
COINCIDENTAL RANGE
INTENTIONAL RANGE
Lines of Position (LOP’s) Visual Bearing can be measured in:
- Degrees Relative ( # # # oR )- Degrees per Gyro Compass ( # # # opgc )- Degrees Magnetic ( # # # opstgc or oM )- Degrees True ( # # # oT)
The navigator must convert any of these types of bearings to True before they can be plotted on the chart.
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Range
Tangent
Bearing
Distance Arcs0915
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 ConceptProblem - 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.
SPIRE TWRTANK
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Visual Fix
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Electronic Fix
Symbol Type Meaning
Labeling Fixes
Fix
Fix
DR
EP
Accurate Visual FixAccurate Fix obtained by electronic meansDead reckoning position, advanced from previous fix.Estimated position. Most probable position of ship.
TR
Visual Fix
1145
TR
Electronic Fix
1843
Optimum Spread of LOP’sBest Fix – LOP’s must be widely spaced, two LOP’s at nearly the same
angle will be less accurate.
Formula – Both sides: 3600
/ number LOP’s
– One side: 1800
/ number LOP’sExample: There are three LOP’s then
optimum separation is 1200.Same side: 600 .
“2”Fl R 4 sec
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Visual Fix
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0930
Visual Fix
Determining the Order of LOP’sBearings LOP’s
Shoot Bearings: “Beam to Bow”
Range LOP’s Shoot Ranges: “Bow to Beam”
Navigational Aid SelectionChoose 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.
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Visual FIX
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FL 6s 58ft 9M
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Electronic Fix
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6pa
FL 6s 58ft 9M
Tank
Long
Point
20
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Accurate / Inaccurate fix
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FL 6s 58ft 9M
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Frequency of Fixes• 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.
• Open water, a fix every 30 min. is normally plotted.