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CANADIAN COAST GUARD AUXILIARY - PACIFIC
NAVIGATION TRAININGSection 8
Position Lines and Fixes
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Table of Contents
Section 1 Types of Navigation
Section 2 Terrestial Coordinates
Section 3 Charts
Section 4 Compass
Section 5 Nautical Publications Section 6 Navigational Aids
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Table of Contents
Section 7 Buoyage
Section 8 Position Lines and Fixes
Section 9 Tides
Section 10 Currents
Section 11 Weather
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Position Lines and Fixes
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Position LinesPosition Lines (P/L)- A single observation that
does not establish a fix, but does mean that
ships position is somewhere along that line.
Label - After the position line is drawn from a
charted object, a four digit time must be written
above and parallel to the position line.
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Position Lines
All Compass bearings that are to be plotted on
the chart, must be corrected to True Bearings,
allowing for any compass error, includingdeviation and variation, before plotting.
All True bearings/ courses taken from the chart,
must be corrected for any compass error to
obtain Compass Bearings/compass before use
on radar or vessels magnetic compass.
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Sources of Position Lines
A visual position line can be taken, using
charted fixed navigational aids such as tanks,
water towers, church steeples, spires, radioand TV towers, day marks, fixed navigation
lights, flagpoles, or tangents to points of land.
In general fixing off floating objects,especially buoys, should be avoided, if there
are fixed charted objects available.
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Visual
PositionLine
1000
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Radar
RangePosition
Line
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Position Line Measurement
Visual Bearings can be measured in:
1. Degrees Relative ( # # #0R )
2. Degrees per Gyro Compass ( # # # G )
3. Degrees Magnetic ( # # #0M )
The navigator must convert any of these
types of bearings to True before they can beplotted on the chart.
Degrees True ( # # #0T)
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Plotting and Labeling a Fix
Fix - The point where two or more positionlines, taken at the same time, 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 position lines are notwritten.
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Visual Position Fix
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Visual
PositionFix 1
Compass bearing
of Abode Island
bearing
009Compass,deviation 1W,
variation 23E,
gives 030 True
Bearing
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Visual
PositionFix 3
Compass bearing
of Pt. Atkinson
Light bearing
098Compass,deviation 1W,
variation 23 E,
gives True
Bearing of 120 T
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Visual
PositionFix 4
1230
Insert fix circle
on intersection
of position
lines, and timeof fix
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Cocked Hats
In a perfect world, with due allowance made forcompass error, the three position lines will crossat one point.
However depending on the speed of the vessel,the proximity of the object from which a vessel isbeing fixed, and the accuracy of the bearingwhen taken, and other factors, it is far more
likely that a cocked hat will be obtained.The larger the cocked hat, the larger an erroron one, two or all of the position lines is likely tobe.
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CockedHat
1230
In this example
there is an errorof 3E on the
compass
bearing of Point
Atkinson Lightand a cocked
hat is formed.
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Cocked Hats
Where a plotted position is a cocked hat, andthere is no obvious error (such as in calculation),it should be generally assumed the position of
the vessel is the point in the cocked hat closestto the nearest danger.
Another position should be taken a soon as
convenient to check on the position.An
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Reducing Errors
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Reducing Errors
When taking distances or ranges, always takethe ranges ahead or astern first, to minimise
errors (as these ranges will change quickestwith the speed of the vessel) before takingranges on the beam.When taking compass bearings, always take
the bearings on the beam first, to minimiseerrors (as these bearings will change quickestwith the speed of the vessel) before takingbearings ahead or astern.
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Radar Fixes
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Radar Fixes
Radar bearings are subject to compass error.
Therefore the best way to obtain a fix by radar,is to take three radar distances off charted andidentified objects.
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RadarPosition 1
Using radar:
Grebe Is
Electronic
Bearing Marker
showing 058 M
Variable Range
Marker showing
0.82
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RadarPosition 2
From radar, plot
position circle:
Grebe Is
Distance 0.49 nm
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RadarPosition 3
Grebe Is Range
0.82
A second range of
0.93 off Eagle Is.
would give fix
Mark fix position and
time. Best fix would
be have third range.
1000
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RadarPosition 4
Radar bearing of
Grebe Is. is 058 compass
Deviation 1W
Variation 23E
True Bearing 080 T
which confirms
ranges
1000
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Electronic Position
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Electronic Position
The GPS can give an accurate electronicposition.
First check that the GPS information is live, andnot on Dead Reckoning (which GPS reverts towith certain faults).
Also check that the HDOP figure is low - 1 isbest.
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Electronic
Position 1
Note down
Latitude and
Longitude
49 20.38N123 17.23W
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Electronic
Position 2
Plot Latitude
and Longitude
49 20.38N
123 17.23W
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Electronic
Position 3
1000
Insert fix
symbol, and
time
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Transits
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Transits
Transits are the most accurate type of
position line, when two charted objects line
up. Transits are one of the most valuable tools
when close to dangers or the land.
Some transits are man made (intentional)and others are natural (coincidental).
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Transits
The main benefits of transits are:
1. There is no compass deviation or variation.
2. They can be used when the vessel's motion
interferes with the use of a compass.
3. They are instantaneous and can be
monitored continuously.4.They occur frequently when in confined
waters.
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Transits
Good transit - Beacon in line with lighthouse
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Transits
Poor transit - Buoy in line with end of land. This may
be inaccurate due to land changing due to tidal height
and the buoy being set by tidal stream or current.
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Transits0945A transit can give
either a position
line, or as shown,a heading to steer
on from the
northwest, before
altering to about
045T into
Fishermans Cove
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Symbol Type Meaning
Labeling Fixes
Fix
Fix
DR
EP
Accurate Visual Fix
Accurate Fix obtained byelectronic means
Dead reckon position, advancedfrom previous fix.
Estimated position. Mostprobable positionof ship.
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Dead Reckoning
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Dead Reckoning
Dead Reckoning is the process of determining
a ships approximate position by applying, from
its last known position, a vector or a series of
consecutive vectors representing the true
courses steered and the distances run as
determined by the ships speed and time, without
considering the effects of wind and current.From a known ships position, predicted future
positions are plotted.
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DeadReckoning
1230
DR 1245
From ships
known position at
1230, a future
position is
plotted for 1245,knowing vessels
course and
speed.
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Dead Reckoning
Dead Reckoning is derived from DEDUCED, or
DED, reckoning which was the process by which a
vessels position was computed trigonometricallyin relation to a known point of departure.
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Estimated Position
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EstimatedPosition
1230
EP 1245
From ships
known position at1230, a future
position is plotted
for 1245, knowing
vessels courseand speed, and
allowing for set
and drift of tide.
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Parallel Indexing
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Parallel Indexing
Parallel indexing is using the radar to monitor
the track of a vessel along a preplanned course,
maintaining a distance off a known chartedobject.
Where using a magnetic compass input to a
radar, the true bearing will have to be corrected
for variation and deviation before setting the
Electronic Bearing Marker.
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Parallel
Indexing
CIR
0.32
015T
Find a radar
conspicuous object on
the chart. Draw a lineparallel to the required
course touching the
object. Measure the
distance between the
course line and the
parallel index line. That
is the Cross Index
range.
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Parallel
IndexingThe VRM should run
up the EBL if the
vessel is staying on
track.
Course 017C
VRM 0.18nm
EBL 017C
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Time-Speed-Distance
Calculations
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Time-Speed-Distance
CalculationsThese calculations can be made using a
nautical slide rule, electronic calculator, set of
pre-computed tables, or the speed nomogram.D = S x T
where:
D = distance traveled
note: ( 1 nm = 2000 yds)
S = speed in knots(nautical miles per hour)
T = time in hours
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3 Minute Rule
Distance traveled in 3 minutes (yards) =
Ships speed (knots) X 100
6 Minute Rule
Distance traveled in 6 minutes (nm) =Ships Speed (knots) divided by 10.
Simple Rules