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 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.

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 vessels

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 ( # # # G )

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 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 not written.

Visual Position Fix

Visual

Position

Fix 1

Compass bearing

of Abode Island

bearing

009Compass, deviation 1W, variation 23E, gives 030 True Bearing

Visual

Position

Fix 2

Compass bearing

of Grebe Island

Light bearing 058

Compass, deviation 1W, variation 23E, gives 080 True Bearing

Visual

Position

Fix 3

Compass bearing

of Pt. Atkinson

Light bearing

098Compass, deviation 1W, variation 23 E, gives True

Bearing of 120 T

Visual

Position

Fix 4

1230 Insert 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 3E 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 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 compass

Deviation 1W

Variation 23E

True Bearing 080 T 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

49 20.38N

123 17.23W

Electronic

Position 2

Plot Latitude

and Longitude

49 20.38N

123 17.23W

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

0945 A 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

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 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.

Dead

Reckoning

123

0

DR

1245

From ships known position at 1230, a

future position is

plotted for 1245,

knowing vessels course and speed.

Dead Reckoning

Dead Reckoning is derived from DEDUCED, or DED,

reckoning which was the process by which a vessels

position was computed trigonometrically in relation to a

known point of departure.

Estimated Position

Estimated

Position

123

0

EP

1245

From ships known position at 1230, a

future position is

plotted for 1245,

knowing vessels 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

015T

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

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 017C

VRM

0.18nm

EBL 017C

Parallel

Indexing The VRM should run

up the EBL if the

vessel is staying on

track.

Course 017C

VRM

0.18nm

EBL 017C

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) =

Ships speed (knots) X 100

6 Minute Rule

Distance traveled in 6 minutes (nm) =

Ships Speed (knots) divided by 10.

Simple Rules