NAVIGATIONAL EQUIPMENT
(Extracts courtesy of A.N.T.A. publications, Ranger Hope © 2008 www.splashmaritime.com.au)
Plotting Position by Latitude and Longitude
Defining Position by Bearing and Distance
Application of Error to a Compass
Variation
Deviation
Navigational Instruments and Equipment
The navigator will require tools to work satisfactorily with the chart. A list of the basic equipment follows below:
A. Parallel Rules - can be of the roller or Capt. Fields type.
B. A pair of dividers
C. 2B pencils and soft erasor
D. Pencil compass - similar to dividers except one leg is designed to contain a 2B lead.
E. Compass with a means for taking bearings
F. Accurate clock
G. Sextant - for vertical angles
H. Sounding device - Echo sounder or hand lead line.
Electronic instruments and ECDIS are also aids to navigation but their use for positioning will require backing up using more rudimentary equipment and logs. Here we concentrate on the information we can receive from traditional instruments and not how they work.
Plotting Position by Latitude and Longitude
We will consider plotting our position on the chart from a given latitude and longitude. There are two methods of carrying this out.
You will be able to follow the process by looking at Figure.1.
Place one edge of the parallel ruler along one of the parallels of latitude printed on the side of the chart and walk the ruler until one edge passes through the given latitude.
Pencil in the latitude line.
Now line up the ruler with a longitude median and walk the ruler across the chart until one edge is through the correct longitude. Pencil in the line and where it crosses the latitude line is your position.
Figure. 1: Plotting Lat. and Long. Using Parallel Rules
An alternative method is shown in Figure 2.
Line the ruler up on the correct latitude, as explained in Fig. 1 and then with a pair of dividers measure to the required mark on the longitude.
This method can be worked with the ruler on the longitude and the dividers on the latitude.
It is usual to express Latitude and Longitude in degrees, minutes and tenths of a minute.
e.g.
Latitude 25° 15’.2 S
Longitude 150° 25’.9 E
Figure. 2: Plotting Lat. and Long. Using Parallel Rules and Dividers
Defining Position by Bearing and Distance
Before we can define our position by bearing and distance we require to be able to read distance on the chart.
Distance on the chart is measured on the latitude scale, one minute being equal to one nautical mile.
See Figure 3.
Figure 3: Reading Chart Distance (Drawing by courtesy of Coastal Yacht Navigation)
Take the dividers and open them until the points are on the two places in question (See Figure 4). The dividers are moved to the side of the chart.
On a Mercator Chart the latitude scale expands with increased latitude and the measurement made at approximately the mean latitude of the interval being measured.
Figure 4: Measuring Chart Distance Using Dividers
On most coastal charts the minutes of latitude are subdivided into tenths and it is usual to express distance in miles and decimals of a mile
e.g. 5.8 mile
If a long distance is involved it is more convenient to set the dividers (at say 5 or 10 mile) and step off along the line joining the two places.
Having understood the reading of distance we now require to lay off a bearing on the chart.
Direction was explained in Outcome 1 and since the most common charts used by Mariners are Mercator projections, directions and angles are correctly represented. The charts are orientated with North (000°) at the top, other directions then being in their correct relation to North.
A desired direction can be measured by placing the parallel rules along the line from the centre of a compass rose to the circular graduation representing the desired direction.
As an extra check on your accuracy the continuation of the rule passes through the reciprocal direction.
The bearing required is 065°T and its reciprocal is 245°T. See Figure 5.
Figure 5: Reading A Bearing or Course from the Compass Rose
The ruler is now moved across the chart until one of its outer edges passes through the point in question.
A line drawn through the point, along the rulers edge is the bearing.
The line drawn on the chart is a TRUE direction.
The example in Figure 6. shows the transfer of a bearing 065°T to Coppersmith Pk. Light
.
Figure 6: Transferring a Bearing from a Compass Rose
We have now covered the laying off of a position by bearing and distance, it is now an easy step to laying off a course and measuring the distance between thetwo points.
A course is the intended horizontal direction of travel through the water.
To find the true course, lay the edge of the parallel rule between the start position A and the position B. Draw a straight line between them. Now, the parallel rules are walked to the compass rose, the edge of the rule is placed through the centre point of the rose and the desired direction read off.
The edge will cut the compass rose in two places, make sure you read the correct direction and not the reciprocal.
This course will be read as TRUE.
The distance between A and B is taken using dividers and placing them on the Latitude scale.
Figure 7: Course and Distance Measurement
Using any chart, practice drawing lines with your parallel rules, transferring the line to the compass rose, reading off direction and measuring the distance with your dividers.
Application of Error to a Compass
The courses and bearings laid on a chart are true, but we steer courses and take bearings using a compass.
The compass used in small vessels is more commonly a magnetic compass, although some may be fitted with a gyro compass.
The magnetic compass and the errors involved:
The difference between direction as measured by the compass and the true direction as measured on the chart is termed compass error, stated differently: - It is the angular difference between true north and compass north. It is named east or west to indicate the side of true north on which the compass north lies.
Figure
8: Direction of Compass Error
The Compass Error is a combination of two separate and distinct components, namely variation and deviation.
Variation
When influenced only by the earth’s magnetic field, a compass
needle will point towards the earth’s north magnetic pole. This pole is located
somewhere to the north of
Examination of a globe will show that from a position on the East Coast of Australia the compass will point in a direction to the east of true north. This is magnetic north, and the angle between it and true north is called variation. In our case variation is east.
To find the value of variation for any position simply consult the nearest compass rose on a marine chart. The variation will be given for a specified year, together with the rate of change, allowing calculation of variation for any subsequent year. See appendix for variation chart of the world.
Example:
Chart Aus 823 gives the following information on the compass rose to the south of St Bees Island:
Mag Var 8°40’E (1979) Increasing about 2’ annually.
In 1997 the variation will have increased by 2’ each year for 18 years, a total of 36. Adding this to 8°40’ we find that the variation for 1997 is 9°16’E.
Deviation
In the unlikely event that a vessel is constructed entirely from non magnetic materials and has no electronics close to the compass, variation is the only error which will need to be accounted for. In all other cases the vessel and/or its equipment will create magnetic fields of their own. Some of these will be built into the vessel on the slip, others will change as the vessel moves around within the influence of the earth’s magnetic field.
The compass adjuster is usually able to reduce the effect of the vessel’s magnetic fields, but the causes are so complex that it is inevitable that some effects remain. For the ship’s compass to work at all the effect of the ship’s magnetism must be less that the force of the earth’s magnetic field.
To illustrate the effect of the vessel’s own magnetic field, imagine a vessel on which the compass needle is attracted towards the stern. When that vessel is heading towards magnetic north the effect of the pull towards the stern is to reduce the directive force at the compass but not to deflect it from magnetic north. As the vessel turns onto easterly headings the compass needle is deflected towards the stern i.e. towards west. When the vessel heads west the compass needle is deflected to the east. There will be no deflection when the vessel heads south, but an increase in directive force.
This deflection of the compass away from magnetic north is called deviation. As with variation it is named East or West and the value will change according to the ship’s heading. A deviation card is produced by the compass adjuster when the vessel is first commissioned and at intervals throughout its life. It is displayed close to the compass position.
The relationship between compass, magnetic and true courses and bearings is shown in the following diagram.
Figure 9: Relationship between Compass, magnetic and true courses and
bearings.
Rules for applying Variation and Deviation.
To avoid drawing diagrams every time variation and deviation are applied, a number of memory aids have been developed to clarify the rules of application:
Television Makes Dull Company
(T V M D C) reminds us that to true we must apply the variation to find magnetic, and to this we apply deviation to arrive at compass (course or bearing).
If we start with a compass bearing and wish to convert it to true the order of operation is reversed (C D M V T).
Having decided the correct order in which to apply variation and deviation, we need to know whether the correction should be added or subtracted. This may be decided using the word:
C A D E T
This simply indicates that to get from compass to true (the end points) we Add East.
Given that we add east (deviation or variation) it follows that we must subtract west (deviation or variation).
It also follows that if we add east to get from compass to true, we should add west when going from true to compass.