Transcript No Slide Title

NAVIGATION TRAINING
Section 8
Position Lines and Fixes
Table of Contents
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Section 1 Types of Navigation
Section 2 Terrestial Coordinates
Section 3 Charts
Section 4 Compass
Section 5 Nautical Publications
Section 6 Navigational Aids
Table of Contents
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Section 7 Buoyage
Section 8 Position Lines and Fixes
Section 9 Tides
Section 10 Currents
Section 11 Weather
Position Lines and Fixes
Position Lines
• Position Lines (P/L) - A single observation that does
not establish a fix, but does mean that ship’s 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.
Position Lines
• All Compass bearings that are to be plotted on the
chart, must be corrected to True Bearings, allowing for
any compass error, including deviation 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 vessel’s
magnetic compass.
Sources of Position Lines
• A visual position line can be taken, using
charted fixed navigational aids such as tanks,
water towers, church steeples, spires, radio
and 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.
Visual
Position
Line
1000
Radar
Range
Position
Line
Position Line Measurement
• Visual Bearings can be measured in:
1. Degrees Relative ( # # # 0R )
2. Degrees per Gyro Compass ( # # # ºG )
3. Degrees Magnetic ( # # # 0M )
• The navigator must convert any of these types of
bearings to True before they can be plotted on the
chart.
Degrees True ( # # # 0T)
Plotting and Labeling a Fix
•Fix - The point where two or more position lines, taken
at the same time, 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 position lines are not written.
Visual Position Fix
Visual
Position
Fix 1
Compass bearing
of Abode Island
bearing
009°Compass,
deviation 1ºW,
variation 23ºE,
gives 030 º True
Bearing
Visual
Position
Fix 2
Compass bearing
of Grebe Island
Light bearing 058
º Compass,
deviation 1ºW,
variation 23ºE,
gives 080 º True
Bearing
Visual
Position
Fix 3
Compass bearing
of Pt. Atkinson
Light bearing
098ºCompass,
deviation 1ºW,
variation 23º E,
gives True
Bearing of 120 º T
Visual
Position
Fix 4
Insert fix circle
on intersection
of position
lines, and time
of fix
1230
Cocked Hats
• In a perfect world, with due allowance made for
compass error, the three position lines will cross at one
point.
• However depending on the speed of the vessel, the
proximity of the object from which a vessel is being
fixed, and the accuracy of the bearing when taken, and
other factors, it is far more likely that a cocked hat will be
obtained.
• The larger the cocked hat, the larger an error on one,
two or all of the position lines is likely to be.
Cocked
Hat
In this example
there is an error
of 3ºE on the
compass
bearing of Point
Atkinson Light
and a cocked
hat is formed.
1230
Cocked Hats
• Where a plotted position is a cocked hat, and
there 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 closest
to the nearest danger.
• Another position should be taken a soon as
convenient to check on the position.An
Reducing Errors
Reducing Errors
• When taking distances or ranges, always take the
ranges ahead or astern first, to minimise errors (as these
ranges will change quickest with the speed of the
vessel) before taking ranges on the beam.
• When taking compass bearings, always take the
bearings on the beam first, to minimise errors (as these
bearings will change quickest with the speed of the
vessel) before taking bearings ahead or astern.
Radar Fixes
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 and identified
objects.
Radar
Position 1
Using radar:
Grebe Is
Electronic
Bearing Marker
showing 058 º M
Variable Range
Marker showing
0.82’
Radar
Position 2
From radar, plot
position circle:
Grebe Is
Distance 0.49 nm
Radar
Position 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
Radar
Position 4
Radar bearing of
Grebe Is. is 058 º
compass
Deviation 1ºW
Variation 23ºE
True Bearing 080 ºT
which confirms
ranges
1000
Electronic Position
Electronic Position
• The GPS can give an accurate electronic
position.
•First check that the GPS information is live, and
not on Dead Reckoning (which GPS reverts to
with certain faults).
•Also check that the HDOP figure is low - 1 is
best.
Electronic
Position 1
Note down
Latitude and
Longitude
49º 20.38’N
123º 17.23’W
Electronic
Position 2
Plot Latitude
and Longitude
49º 20.38’N
123º 17.23’W
Electronic
Position 3
Insert fix
symbol, and
time
1000
Transits
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).
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.
Transits
• Good transit - Beacon in line with lighthouse
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.
Transits
A transit can give
either a position
line, or as shown,
a heading to steer
on from the
northwest, before
altering to about
045°T into
Fishermans Cove
0945
Labeling Fixes
Symbol
Type
Fix
Fix
DR
EP
Meaning
Accurate Visual Fix
Accurate Fix obtained by electronic
means
Dead reckon position, advanced from
previous fix.
Estimated position. Most probable
position of ship.
Dead Reckoning
Dead Reckoning
• Dead Reckoning is the process of determining a ship’s
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 ship’s speed and time, without
considering the effects of wind and current.
• From a known ship’s position, predicted future positions
are plotted.
Dead
Reckoning
From ship’s known
position at 1230, a
future position is
plotted for 1245,
knowing vessel’s
course and speed.
DR
1245
123
0
Dead Reckoning
• Dead Reckoning is derived from DEDUCED, or DED,
reckoning which was the process by which a vessel’s
position was computed trigonometrically in relation to a
known point of departure.
Estimated Position
Estimated
Position
From ship’s known
position at 1230, a
future position is
plotted for 1245,
knowing vessel’s
course and speed,
and allowing for set
and drift of tide.
EP
1245
123
0
Parallel Indexing
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 charted object.
• 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.
Parallel
Indexing
Find a radar
conspicuous object on
the chart. Draw a line
parallel 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.
015º
T
CIR
0.32’
Parallel
Indexing
Offset and set up the
Variable Range
Marker to the distance
off a conspicuous
point of land that is
required, and set the
Electronic Bearing
Marker to the required
compass course.
Course 017°C
VRM
0.18nm
EBL
017°C
Parallel
Indexing
The VRM should run
up the EBL if the
vessel is staying on
track.
Course 017°C
VRM
0.18nm
EBL
017°C
Time-Speed-Distance
Calculations
Time-Speed-Distance
Calculations
• These calculations can be made using a
nautical slide rule, electronic calculator, set of
pre-computed tables, or the speed nomogram.
D=S xT
where:
D = distance traveled
note: ( 1 nm = 2000 yds)
S = speed in knots(nautical miles per hour)
T = time in hours
Simple Rules
• 3 Minute Rule
Distance traveled in 3 minutes (yards) =
Ship’s speed (knots) X 100
• 6 Minute Rule
Distance traveled in 6 minutes (nm) =
Ship’s Speed (knots) divided by 10.