navigational equipment azimuthal mirror

7/24/2019 Navigational Equipment Azimuthal Mirror

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Navigational Equipment(Material courtesy of A.N.T .A. publications, edited html extracts Ranger Hope 2008,)

otting Position by Latitude and Longitudeefining Position by Bearing and Distancepplication of Error to a Compassariation

eviationules for applying Variation and Deviationompass Errorhe Gyro Compasselative Bearingshecking of a Deviation Cardy Transit Bearings

y Bearing from a Known Position

avigational Equipment

e navigator will require other equipment to work satisfactorily with the chart. A list of the basicuipment is appended below.

Parallel Rules - can be of the roller or Capt. Fields type.

A pair of dividers

2B pencils and soft erasorPencil compass - similar to dividers except one leg is designed to contain a 2B lea


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Compass with a means for taking bearings

Accurate clock

Sextant - for vertical angles

Sounding device - Echo sounder or hand lead line

emember, electronic instruments are classed as aids to navigation and their use forpositioning wiquire backing up using the more rudimentary equipment.

ore detailed information on wheelhouse instruments are discussed in the Module on Nauticalowledge.

e are more interested in this Module on the information we can receive from the instrument and noworks.

Figure 2.1 Figure 2.2

Dividers Parallel Rules


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otting Position by Latitude and Longitude

e will consider plotting our position on the chart from a given latitude and longitude. There are twoethods of carrying this out.

u will be able to follow the process by looking at Figure.2.7.

ace one edge of the parallel ruler along one of the parallels of latitude printed on the side of the chad walk the ruler until one edge passes through the given latitude.

ncil in the latitude line.

w line up the ruler with a longitude median and walk the ruler across the chart until one edge is thre correct longitude. Pencil in the line and where it crosses the latitude line is your position.

Figure 2.3 Figure 2.4

Magnetic Compas Pelorus

and Azimuth Mirror or Circle

Figure 2.5 Hand Held Compass

Figure 2.6 Sextant


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ure. 2.7: Plotting Lat. and Long. Using Parallel Rules

alternative method is shown in Figure 2.8.

e the ruler up on the correct latitude, as explained in 2.2.1 and then with a pair of dividers measuree required mark on the longitude.

is method can be worked with the ruler on the longitude and the dividers on the latitude.

s usual to express Latitude and Longitude in degrees, minutes and tenths of a minute.

g. Latitude 25 15.2 S

Longitude 150 25.9 E


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ure. 2.8: Plotting Lat. and Long. using Parallel Rules and Dividers

efining Position by Bearing and Distance

fore we can define our position by bearing and distance we require to be able to read distance on art.

ferring back to Outcome 1 (Page.11), distance on the chart is measured on the latitude scale, onenute being equal to one nautical mile.

e Figure 2.9.


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gure 2.9: Reading Chart Distance (Drawing by courtesy of Coastal Yacht Navigation)

ke the dividers and open them until the points are on the two places in question (See Figure 2.10)iders are moved to the side of the chart.

n a Mercator Chart the latitude scale expands with increased latitude and the measurementade at approximately the mean latitude of the interval being measured.

ure 2.10: Measuring Chart Distance Using Dividers

most coastal charts the minutes of latitude are subdivided into tenths and it is usual to expresstance in miles and decimals of a mile

g. 5.8 mile

a long distance is involved it is more convenient to set the dividers (at say 5 or 10 mile) and step ofng the line joining the two places.

ving understood the reading of distance we now require to lay off a bearing on the chart.

ection was explained in Outcome 1 and since the most common charts used by Mariners are Merc

ojections, directions and angles are correctly represented. The charts are orientated with North (0the top, other directions then being in their correct relation to North.

desired direction can be measured by placing the parallel rules along the line from the centre of a


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mpass rose to the circular graduation representing the desired direction.

an extra check on your accuracy the continuation of the rule passes through the reciprocal directio

e bearing required is 065T and its reciprocal is 245T. See Figure 2.11.

ure 2.11: Reading A Bearing or Course from the Compass Rose

e ruler is now moved across the chart until one of its outer edges passes through the point in ques

ine drawn through the point, along the rulers edge is the bearing.

e line drawn on the chart is a TRUE direction.

e example in Figure 2.12. shows the transfer of a bearing 065T to Coppersmith Pk. Light


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ure 2.12: Transferring a Bearing from a Compass Rose

e have now covered the laying off of a position by bearing and distance, it is now an easy step to laa course and measuring the distance between thetwo points.

course is the intended horizontal direction of travel through the water.

find the true course, lay the edge of the parallel rule between the start position A and the position aw a straight line between them. Now, the parallel rules are walked to the compass rose, the edge

e rule is placed through the centre point of the rose and the desired direction read off.

e edge will cut the compass rose in two places, make sure you read the correct direction at the reciprocal.

is course will be read as TRUE.

e distance between A and B is taken using dividers and placing them on the Latitude scale (see

ction 2.3.1)


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ure 2.13: Course and Distance Measurement

ing any chart, practice drawing lines with your parallel rules, transferring the line to thempass rose, reading off direction and measuring the distance with your dividers.

Application of Error to a Compass

e courses and bearings laid on a chart are true, but we steer courses and take bearings using ampass.

e compass used in small vessels is more commonly a magnetic compass, although some may be

h a gyro compass.

s suggested that this section be read in conjunction with Learning Outcome 7 on magnetic compasd azimuth circle.

stly, we will deal with the magnetic compass and the errors involved.

e difference between direction as measured by the compass and the true direction as measured oart is termed compass error, stated differently: - It is the angular difference between true north and

mpass north. It is named east or west to indicate the side of true north onwhich the compass norts.


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gure 2.14: Direction of Compass Error

he Compass Error is a combination of two separate and distinct components, namely variaand deviation.

ariation

hen influenced only by the earths magnetic field, a compass needle will point towards the earths n

agnetic pole. This pole is located somewhere to the north of Canada and is slowly moving.

amination of a globe will show that from a position on the East Coast of Australia the compass willa direction to the east of true north. This is magnetic north, and the angle between it and true nortled variation. In our case variation is east.

find the value of variation for any position simply consult the nearest compass rose on a marine che variation will be given for a specified year, together with the rate of change, allowing calculation oriation for any subsequent year. See appendix for variation chart of the world.

ample


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art Aus 823 gives the following information on the compass rose to the south of St Bees Island:

ag Var 840E (1979) Increasing about 2 annually.

1997 the variation will have increased by 2 each year for 18 years, a total of 36. Adding this to 8

find that the variation for 1997 is 916E.

w we will consider Deviation.

eviation

the unlikely event that a vessel is constructed entirely from non magnetic materials and has noctronics close to the compass, variation is the only error which will need to be accounted for. In a

her cases the vessel and/or its equipment will create magnetic fields of their own. Some of these wlt into the vessel on the slip, others will change as the vessel moves around within the influence of

rths magnetic field.

e compass adjuster is usually able to reduce the effect of the vessels magnetic fields, but the caue so complex that it is inevitable that some effects remain. For the ships compass to work at all thect of the ships magnetism must be less that the force of the earths magnetic field.

illustrate the effect of the vessels own magnetic field, imagine a vessel on which the compass neattracted towards the stern. When that vessel is heading towards magnetic north the effect of the wards the stern is to reduce the directive force at the compass but not to deflect it from magnetic n

the vessel turns onto easterly headings the compass needle is deflected towards the stern i.e. towst. When the vessel heads west the compass needle is deflected to the east. There will be noflection when the vessel heads south, but an increase in directive force.

is deflection of the compass away from magnetic north is called deviation. As with variation it is nast or West and the value will change according to the ships heading. A deviation card is produced

e compass adjuster when the vessel is first commissioned and at intervals throughout its life. It isplayed close to the compass position. An example of a deviation card can be found on Page 78.

e relationship between compass, magnetic and true courses and bearings is shown in the followingram.


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ure 2.15: Relationship between Compass, magnetic and truc courses and bearings.

ules for applying Variation and Deviation.

avoid drawing diagrams every time variation and deviation are applied, a number of memory aidsen developed to clarify the rules of application:

levision Makes Dull Company (T V M D C) reminds us that to true we must apply the variation to fi

agnetic, and to this we apply deviation to arrive at compass (course or bearing).we start with a compass bearing and wish to convert it to true so that it can be used on the chart thder of operation is reversed (C D M V T).

ving decided the correct order in which to apply variation and deviation, we need to know whetherrrection should be added or subtracted. This may be decided using the word:

C A D E T

is simply indicates that to get from compass to true (the end points) we Add East.

ven that we add east (deviation or variation) it follows that we must subtract west (deviation or

riation).also follows that if we add east to get from compass to true, we should add west when going from tcompass.

simple layout for applying these rules is shown on the next page.

ompass Error

we expect to take several bearings whilst steering the one steady course it makes good sense to aa single correction to apply to all those bearings. Remembering that the variation remains effectiv

e same whilst operating in one area, and that deviation only changes when we change course, we d the compass error for the course being steered and apply that to all bearings taken whilst on thaturse.

ample:

e are steering 076(C). The deviation from the deviation card for 076 is 3W and the variation from

art is 11E. The compass error (combined variation and deviation) is therefore 8E. Since we arenverting compass bearings to true we add East (CADET).

erefore whilst steering 076(C) add 8to any compass bearing to convert it to true.


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mple layout for applying variation and deviation.

eviation is taken from the deviation table at the end of this section.)

1. Changing from compass to true.

Compass Course 215(C)

Deviation 6 W (in this case between 210 and 220)

Magnetic Course 209

(M) (add E so subtract W)Variation 11 E (from chart)

True course 220(T) (add E)

2. Changing from true to compass

True Course 220 (T)

Variation - 11 E (from chart)

Magnetic Course 209 (M) (from T to C subtract E)

Deviation + 5 W (from table for 210)

Compass course 214.5 (C) (add W)

r most practical purposes this compass course is sufficiently accurate, but to be strictly correct we

ould re-enter the deviation table with a Compass Course of 214, rather than the magnetic course

9. In this case deviation would be 6W and the total working as follows:-

True Course 220 (T)

Variation 11 E

Magnetic Course 209 (M)

Deviation 5.5 W ( Deviation (2) 6 W)

Compass Course 215 (C) 214.5 (C) Compass Course

e difference is rarely likely to be great, but beware a deviation card which indicates rapid changes viation.

hen working with bearings the same lay-out is followed but remember that the deviation table is enh Ships Head, Not Bearing.

he Gyro Compass

hough it is popularly believed that the gyro compass indicates true North, this is not necessarily th

se.


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e gyro is subject to course, speed and latitude errors. These are kept to the minimum by inputrrections.

e gyro error is rarely more than one or two degrees for a correctly maintained gyro.

avoid confusion with magnetic errors, gyro error is named high (H) or low (L).

ro error is named high when the gyro course or bearing is higher than the true course or bearing.erefore high error must be subtracted from the gyro reading to obtain the true reading. The oppos

plies to low error.

ample

ro error is known to be 2high. The true course to be steered is 076. What is the gyro course?

ue Course 076

ro error 2 H

ro Course 078

bearing taken with the same gyro gives 246 (G). What is the true bearing?

ro bearing 246

ro error 2 H

ue bearing 244

elative Bearings

hen bearings are taken using a pelorus, or by radar with an unstabilised display (ships head up), tharing taken will be measured relative to the ships head, rather than north.

ch a relative bearing must be applied to the true ships head to arrive at a true bearing for plotting oe chart. This will entail changing the compass course to true and then adding the relative bearing (0notation).

ing 360notation is now normal practice, but if the relative bearing is a given number of degrees Garboard) then it is also added. If the relative bearing is a given number of degrees Red (port), then

btracted. In the example below the relative bearing would have been Red 86. To subtract this fro

e true course it would have been necessary to first add 360. The answer is again 352 (T).


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gure 2.16: Relationship between True and Relative Bearings

Example for Relative Bearing Working

A vessel is steering 073(C) Compass 073(C)

Deviation is 4W Magnetic 069 (M)

Variation 9

E True 078

(T)

The relative bearing of Edward Island Light is 274(Rel)

What is the true bearing of Edward Island Light?

Rel. Brng. 274(R)

True Brng. 352(T)

e now have to consider the checking of a Deviation Card, and keepincord book.

find the Compass Error by Observation

here is some doubt as to the accuracy of the deviation card, or in any case as a periodical check, tviation can be determined by one of two simple methods :

y Transit Bearings


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hen two well charted objects are in transit a compass bearing is taken.

e true bearing is taken from the chart using parallel rulers.

mparison of the compass bearing and the true bearing gives the compass error.

ransit bearing when two or more objects lie on the same line, as ingure 2.17.

he deviation is required it is calculated by applying the charted variation to the compass error.

y checking the compass error using an azimuth circle on your compass and a Transit Bearing. A mficult one is using a pelorus for a Relative Bearing, then converting this to a Compass Bearing.

k the Master to show you. Then practice.

ample:

this case a bearing has been taken of Leading Lts 045

rue Bearing 045

ompass Bearing 048

Compass Error 3W

ariation 11 Eherefore Deviation 8 E

gure 2.17: Compass Error by Transit using a Azimuth Circle.

Drawing by courtesy of Small Ships Manual)


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the second method you require to know your exact position, this can be obtained by :-

. Corrected G.P.S.

. Ranges from radar

C. Ranges from Vertical Sextant Angles

e object should be on the chart and well distanced from the vessel.

y Bearing from a Known Position

hen the vessels position is accurately fixed, a compass bearing may be taken of a well charted andtant object, say a tower (as in Figure 2.18).

e true bearing can again be taken from the chart using parallel rulers. Calculation made as above

y small change in the vessels position whilst swinging will have a negligible effect on the true beae chosen object is sufficiently distant.

gure 2.18: Compass Error from a known position

ample:

rue bearing 050

Compass bearing 033

Compass error 17 E


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ariation 11 E

eviation 6 E

ecord of Compass Errors and Deviation should be kept in a Compass Record Book (see U.S.L.quirements).

navigational equipment azimuthal mirror

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