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.

Single LOP

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Visual LOP

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Visual Bearing LOP 2357

Radar RangeLOP

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.

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

Visual Fix

Electronic Fix

Symbol Type Meaning

Labeling Fixes

Accurate Visual FixAccurate Fix obtained by electronic meansDead reckoning position, advanced from previous fix.Estimated position. Most probable position of ship.

Visual Fix

Electronic Fix

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

Visual Fix

TWR TANK LT

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.

Visual FIX

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Electronic Fix

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Accurate / Inaccurate fix

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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.

Lesson 11 LoPs And Fixes

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