navigation training section 8 position lines and fixes

NAVIGATION TRAINING

Section 8

Position Lines and Fixes

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

Table of Contents

• 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.

• 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.

Position Lines

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

1230Insert fix circle on intersection of position lines, and time of fix

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

1230

In this example there is an error of 3ºE on the compass bearing of Point Atkinson Light and a cocked hat is formed.

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

1000

Insert fix symbol, and time

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

0945A 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

Symbol Type Meaning

Labeling Fixes

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.

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

1230

DR 1245

From ship’s known position at 1230, a future position is plotted for 1245, knowing vessel’s course and speed.

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

1230

EP 1245

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.

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

CIR 0.32’

015ºT

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.

Parallel IndexingOffset 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 IndexingThe 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 x T where:

D = distance traveled note: ( 1 nm = 2000 yds)

S = speed in knots(nautical miles per hour)T = time in hours

• 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.

Simple Rules