NS310

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Tank
NS100
Light
FL 6s 58ft 9M
Long
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Point
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Fundamentals of Naval Science
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Lines of Position Fixes
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Objectives
1. Describe how to obtain , plot, and label
lines of position visual and radar. 2. Define
a fix. Discuss fix ambiguity and how to
resolve it. 3. Understand the optimum spread
for two and three LOP fixes.
3
Objectives
4. Understand the sequence of taking LOPs
both visually and by radar. 5. Discuss the
concept of assuming the centroid of the
triangle or point nearest the hazard as the
actual fix position. 6. Mention that depth
readings are taken with each fix and are
used to check fix accuracy.
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Lines of Position (LOPs)
LOP - A single observation that does not
establish a fix but does mean that ships
position somewhere along that line.
Label - After the LOP is drawn from a Navaid, a
four digit time must be written above and
parallel to the Line Of Position.
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Lines of Position (LOPs) (contd)
A visual LOP can be taken, or shot, using other
fixed navigational aids such as tanks, water
towers, church steeples, spires, radio and TV
towers, daymarks, fixed navigation lights,
flagpoles, or tangents to points of land.
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Single LOP
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Tank
Light
FL 6s 58ft 9M
Long
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Tank
Visual LOP
Light
FL 6s 58ft 9M
Long
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Point
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2357
Visual Bearing LOP
TWR
Radar Range LOP
2246
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Tank
Radar Range LOP
Light
FL 6s 58ft 9M
Long
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Lines of Position (LOP) (contd)
Visual Range - The most accurate type of LOP,
when two known objects line
up. Visual Ranges - Some ranges are man made
intentional and others
are natural
coincidental.

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COINCIDENTAL RANGE
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RTWR
TANK
N
Coincidental Ranges
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INTENTIONAL RANGE
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INTENTIONAL RANGE
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Lines of Position (LOPs) (contd)

• Visual Bearing can be measured in
• - Degrees Relative ( 0R )
• - Degrees per Gyro Compass ( pgc )
• - Degrees Magnetic ( pstgc OR 0M )
• - Degrees True ( 0T)
• The navigator must convert any of these types
  of bearings to True before they can be plotted on
  the chart.

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Plotting and Labeling a Fix
Fix - The point where two or more LOPs
cross. This indicates the
ships 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.
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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.
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Visual Fix
Tank
0733
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SPIRE
TWR
TANK
1145
wk
Visual Fix
1036
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12
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Tank
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Electronic Fix
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Labeling Fixes
Symbol Type Meaning
Fix Fix DR EP
Accurate Visual Fix
Accurate Fix obtained by electronic means
Dead reckon position, advanced from previous fix.
Estimated position. Most probable position of
ship.
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TR
Visual Fix
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TR
Range
Electronic Fix
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TR
Electronic Fix
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0835
0815 LOP
Range
Bearing
0915
0900
Distance Arcs
Tangent
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Optimum Spread of LOPs
Best Fix - LOPs must be widely spaced, two
LOPs at nearly the same angle will be less
accurate. Formula - 3600 / number of
LOPs Example There are three LOPs then
optimum separation is 1200.
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Visual Fix
2 Fl R 4 sec
1412
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Visual Fix
0930
60
60
LT
TWR
TANK
30
TWR
SPIRE
90
1036
1039
TANK
Visual Fix
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Determining the Order of LOPs
Bearings LOPs - Shoot visual bearings
that are on the beam first because they
change the fastest. Shoot Bearings
Beam to Bow Range LOPs - Shoot radar ranges
on the bow/stern first because their
range changes the fastest. Shoot Ranges Bow
to Beam
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Tank
Light
FL 6s 58ft 9M
Long
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Point
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1503
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C 090º
C 090º
S 10
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1506
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1503
1506
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S 10
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1500
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R FL 2s 38ft 10M
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Navigational Aid Selection
Choose Nav Aids based on - Provides the best
bearing spread. - Lighted so that Nav Aids can
be used in low visibility and day or
night. - Easily distinguishable by bearing
takers. Example Which radio tower?
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Tank
Visual FIX
Light
FL 6s 58ft 9M
Long
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Point
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1000
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Tank
Electronic Fix
Light
FL 6s 58ft 9M
Long
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Point
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Accurate / Inaccurate fix
Tank
Light
FL 6s 58ft 9M
Long
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Point
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1015
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Frequency of Fixes

• The frequency of fixes will vary with the
  situation , the primary concern being the 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
  

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Review Questions
1. What are the two types of LOPs ? 2. How do
you label a single LOP ? 3. The most accurate
type of LOP is called? 4. Name three ways visual
bearings can be measured? And what must
they be converted to in order to plot them
on the chart? 5. Explain how to choose the
centroid of a triangle from a poor fix?