Environmental Monitoring
& Technology Series
Navigation &
Mapping
For Technicians
Study module 7
The compass
cffet.net/env
Navigation & mapping for technicians
Study Module 7
Assessment details
Purpose
Upon successful completion of this study module you will have an understanding of the
different types of reference systems, datum’s, coordinate systems and specific notations
used in navigation as well as being able to perform simple unit conversions.
Instructions
◗ Read the theory section to understand the topic.
◗ Complete the Student Declaration below prior to starting.
◗ Attempt to answer the questions and perform any associated tasks.
◗ Email, phone, book appointment or otherwise ask your teacher for help if required.
◗ When completed, submit task by email using rules found on last page.
Student declaration
I have read, agree to comply with and declare that;
◗ I know how to get assistance from my assessor if needed…
☐
◗ I have read and understood the SAG for this subject/unit…
☐
◗ I know the due date for this assessment task…
☐
◗ I understand how to complete this assessment task…
☐
◗ I understand how this assessment task is weighted…
☐
◗ I declare that this work, when submitted, is my own…
☐
Details
Student name
Type your name here
Assessor
Marker’s use only
Class code
N&M
Assessment name
Study module 7
Due Date
Speak with your assessor
Total Marks Available
23
Marks Gained
Marker’s use only
Final Mark (%)
Marker’s use only
Marker’s Initials
Marker’s use only
Date Marked
Click here to enter a date.
Weighting
This is one of twelve formative assessments that make up 40% of
the overall mark for this subject
Hunter TAFE - Chemical, Forensic, Food & Environmental Technology [cffet.net]
Course Notes for delivery of MSS11 Sustainability Training Package
Page | 1
Navigation & mapping for technicians
Study Module 7
Compasses
Compasses are rarely used by environmental technicians. Usually there are roads, and then
paths, and then visible markers, all of which can be obtained from navigations devices. As a
result, this section is purely informative and can be ignored if deemed irrelevant in your
workplace, and the student can be instructed to peruse the chapter in their own time.
Maps provide a lot of information, but used alone offer only positional information, as they
cannot give you accurate information about directions or bearings. For this you will need a
compass.
There are many types of magnetic compasses suitable for use in map reading and
navigation, and these days can be physical or electronic. Physical (or ‘hard’) compasses
differ in shape and size, but the principle of operation is essentially the same;
◗ Compasses consist of a magnetised needle accurately balanced on a pivot point set in
the centre of a non-ferrous or plastic box.
◗ Situated on the box of the compass is a datum point (north) and sometimes an aiming
point or protrusion along the same line.
◗ To take a bearing, simply rotate the compass until the aiming point or datum point is in
the direction of the object to which the bearing is to be taken.
◗ Read the bearing from the card where the datum point cuts it.
Figure 7.1 – Important parts of the compass (http://www.ussartf.org/compass_basics.htm accessed
2/5/13 - based on the general Silva design)
Hunter TAFE - Chemical, Forensic, Food & Environmental Technology [cffet.net]
Course Notes for delivery of MSS11 Sustainability Training Package
Page | 2
Navigation & mapping for technicians
Study Module 7
Types of compass
Prismatic Compass
The operation of the prismatic compass is similar to the Silva design but allow for greater
accuracy due to some added refinements. The box of the prismatic compass is filled with
liquid to dampen the movement of the card so it rotates gently and comes to rest quickly. A
card, graduated in degrees, is usually fixed to the top of the needle with the 360° mark
directly over the north point of the needle.
Also provided is a magnifying prism which enables the user to read the bearing on the card
with much more accuracy. Figure 4.2 below shows the main components of the prismatic
compass. Some clear plastic compasses can be used in ways similar to the prismatic as the
user sights through a lens and lines up a hairline on the object to which the bearing is
required.
Figure 7.2 – Typical prismatic compass layout (www.britishblades.com accessed 2/5/13)
Hunter TAFE - Chemical, Forensic, Food & Environmental Technology [cffet.net]
Course Notes for delivery of MSS11 Sustainability Training Package
Page | 3
Navigation & mapping for technicians
Study Module 7
Silva Compass
The Silva compass was developed in the 1930’s, and were originally designed for
orienteering. These compasses are now used widely throughout the world by everyone as a
general purpose compass.
There are many different types of Silva style compasses available. However, the basic
construction for all types remains the same. The compass enables the user to plot and
calculate bearings rapidly and accurately on a map without the use of a protractor or Romer
card by combining, on a common base plate, both a compass and a protractor. Figure 7.2
below shows the major components of the compass.
Figure 7.3 – Typical ‘Silva’ type of compass.
So, now that you have your compass, you need to be aware that the needle points to
Magnetic North, but your map doesn’t, so you need to be able to correct the north points so
that you head in the right direction
North Points
When reading maps, you can make reference to three different types of north points, as
seen in the image below;
Figure 7.4 – The North points
Hunter TAFE - Chemical, Forensic, Food & Environmental Technology [cffet.net]
Course Notes for delivery of MSS11 Sustainability Training Package
Page | 4
Navigation & mapping for technicians
Study Module 7
True north (TN)
The earth spins on an axis which passes through the north and south poles. As these poles
are reference points, the North Pole is called Geographical North, but is properly termed
True North. True north is therefore the direction from any point on the earth’s surface to
the North Pole. Lines drawn from the North Pole to the South Pole are true north-south
lines, or meridians, as opposed to grid lines which are UTM constructs.
Grid north (GN)
The grid lines on a map do not lie true north and south (except along one standard easting.
Elsewhere on the map they make an angle with the true north-south line. Since the grid
lines are parallel, and since they are drawn on the map, it is very convenient to use them for
drawing or measuring bearings.
The direction of the north-south grid lines (easting’s) is therefore known as grid north, and is
used in preference to true north.
When working from a gridded map, all bearings plotted from the map are referred to as grid
bearings and not true bearings, even though they may only differ by 2°. The difference
between GN and TN is called the grid convergence and its value is shown in the north points
diagram.
Magnetic north
To say that a compass points north is only relatively true because a compass needle does
NOT point to the North Pole. It points to the magnetic pole which differs from the North
Pole and complicate things, the absolute position magnetic pole varies slightly from year to
year (which means that the date of the map becomes very important).
The direction a compass needle points when freely suspended is known as magnetic north.
Compass Errors
When using a magnetic compass, you should be aware of the two main causes of variation
in compass readings.
Individual compass error
Each compass has its individual deviation, i.e. does not point exactly to magnetic north. The
compass needle itself may not be quite true with the markings on the card and slight
differences may be caused in other ways. The error may be negligible or comparatively large
and therefore it is important to have compasses checked regularly.
Checking your compass may be done by comparing the magnetic bearing, converted from a
measured grid bearing, between two points at least 1 km apart, and their compass bearing.
Hunter TAFE - Chemical, Forensic, Food & Environmental Technology [cffet.net]
Course Notes for delivery of MSS11 Sustainability Training Package
Page | 5
Navigation & mapping for technicians
Study Module 7
This deviation east or west should be checked by repeating this procedure in different
directions. If one measurement is different there is probably a local magnetic attraction and
further checks should be carried out. Any known error should be noted on the compass and
when readings are taken, allowance must be made for the individual variation.
Local magnetic attraction
Local magnetic attraction is due to the presence of iron or iron ore nearby. The compass is a
delicate instrument and quite small quantities of iron have a surprisingly large effect on its
behaviour. A wrist watch or steel-framed spectacles will affect the compass reading.
Take the precaution of seeing that all iron or steel objects are at a safe distance before using
the compass. Small articles will be safe in your pocket but larger articles should be placed
two or three metres away.
Remember also to keep well clear of power lines, wire fences, vehicles and railway lines
when using a magnetic compass.
Safe distances from objects
Transmission high tension lines
80 metres
Large vehicles
75 metres
Car/utility
60 metres
Fencing wire/concrete
10 metres
Pick axe or shovel
3 metres
To check for local magnetic attraction:
◗ Select two points about 100 m apart.
◗ From one, take a bearing to the other.
◗ Then move to the other and take a back bearing to the first.
◗ The two bearings should differ by 180° (back bearing).
◗ If they do not there is a magnetic disturbance at one point or the other or at both.
Using a compass
Direction
Points of a Compass
North, south, east and west are the four cardinal points (or key points) of the compass.
There are, in all, thirty-two points of the compass but only sixteen are normally used in map
reading. The four cardinal points and twelve intermediate points are shown below.
Hunter TAFE - Chemical, Forensic, Food & Environmental Technology [cffet.net]
Course Notes for delivery of MSS11 Sustainability Training Package
Page | 6
Navigation & mapping for technicians
Study Module 7
Figure 7.5 – Cardinal points
Degree System
The points of the compass give a rough indication of direction only. For greater accuracy,
the circle is divided into 360 degrees with 0 (or 360) being the north point. The four
quadrants of the circle are each 90 degrees and therefore the east, south and west points
are at 90, 180, and 270 degrees respectively.
Each degree is sub-divided into 60 minutes and each minute into 60 seconds. Degrees are
marked thus °, minutes ’and seconds”.
When the compass is being used, the subdivisions of a degree are too small for practical use
and readings to one half of a degree are generally sufficient.
Figure 7.6 – Relating the cardinal points to the degrees of a circle.
Mil System
In the Mil system, the circle is divided into 6400 mils with 0 (or 6400) being the north point.
East, south and west are at 1600, 3200, and 4800 mils respectively. Accuracy to the nearest
10 mils is normally sufficient in map reading. Mils are convenient for many practical uses as
Hunter TAFE - Chemical, Forensic, Food & Environmental Technology [cffet.net]
Course Notes for delivery of MSS11 Sustainability Training Package
Page | 7
Navigation & mapping for technicians
Study Module 7
1 mil extends under approximately one unit of length at a range of one thousand units. For
example, 1 mil extends less than 1 m at 1000 m.
Figure 7.7 – The Mil System
Bearings
The purpose of a bearing is to give an accurate indication of the direction of one point from
another. A bearing is the angle, measured clockwise, that a line makes with a fixed zero line.
Unless stated otherwise, the zero line is always taken to be north.
Considering figure 7.8 below, if you are standing at point O, and the bearing of A is 30°, it
means that the line OA makes an angle of 30° with the north-south line.
Figure 7.8 – Example of the use of bearings (from Public Safety ITAB SES resource)
Hunter TAFE - Chemical, Forensic, Food & Environmental Technology [cffet.net]
Course Notes for delivery of MSS11 Sustainability Training Package
Page | 8
Navigation & mapping for technicians
Study Module 7
The essential point to remember is that bearings are always measured clockwise from
north, and therefore bearings of;
◗
any direction to the east of the north-south line fall between 0° and 180°.
◗
any direction to the west of the north-south line fall between 180° and 360°.
◗
The bearing is always measured clockwise, thus bearing OD in Figure 7.8 is 225°.
Protractors
To measure a bearing on a map accurately, use a protractor. There are many different types
of protractors available and although they differ in design and shape, they are used in much
the same way.
Most protractors are made of a clear, solid plastic material or wood with a thin plastic
veneer. They may be circular, semicircular or rectangular in shape. The outer edge of the
protractor is graduated in degrees from 0 to 360, clockwise. Situated in a suitable position
on the protractor is a north line and a centre point or hole.
Magnetic Variation and Grid Magnetic Angle
The angle between the magnetic north line and the true north line at any place is the
magnetic variation (or magnetic declination) for that particular place. As GN is used in
preference to TN it is more useful to know the size of the angle between GN and MN. This is
called the grid/magnetic angle (GMA) and its value is shown in the north points diagram.
Figure 7.9– Reading bearings off a compass
The position of the magnetic pole is not fixed; it moves slightly from year to year.
Consequently, the direction of magnetic north and therefore the magnetic variation also
changes by a small amount each year. Though this change is not constant it can be forecast
Hunter TAFE - Chemical, Forensic, Food & Environmental Technology [cffet.net]
Course Notes for delivery of MSS11 Sustainability Training Package
Page | 9
Navigation & mapping for technicians
Study Module 7
with sufficient accuracy over a number of years and details of the annual change are given
in the marginal information on the topographic map.
Note carefully whether the annual change has to be added or subtracted. If the annual
change is the same direction as the magnetic variation it must be added; if they are opposite
directions the annual change must be subtracted.
Example
Calculating the grid magnetic angle for 1997
The number of years elapsed since the diagram was correct is: 1997 – 1991 = 6 years and as
the annual change is easterly by 0.1° every 3 years, 0.2° must be added to the 1991 grid
magnetic angle: 14.5° + 0.2° = 14.7°.
Conversion of Bearings
Compass bearings taken on the ground must be converted to grid bearings for plotting on a
map. Conversely, grid bearings taken from a map will have to be converted to magnetic
bearings before they can be used with a compass on the ground.
To convert a bearing from grid to magnetic or magnetic to grid is a simple matter of adding
or subtracting the magnetic variation. Unfortunately, the magnetic variation across the
continent of Australia is not constant; the majority of the country is subject to easterly
variation of up to 15° but the western part of Western Australia has a westerly variation of
up to 4°.
It becomes easy to make a mistake and add when you should subtract, or subtract when
you should add. Therefore, the only safe way to avoid this is to draw a rough diagram based
on the relationship between grid and magnetic north (as depicted in the map’s north point’s
diagram) and the required bearing.
Conversion of bearings easterly variation
Figure 7.10- West declination
Hunter TAFE - Chemical, Forensic, Food & Environmental Technology [cffet.net]
Course Notes for delivery of MSS11 Sustainability Training Package
Page | 10
Navigation & mapping for technicians
Study Module 7
Example 1
A magnetic bearing of 50° is to be converted to a grid bearing. The magnetic variation is 14°
east. So 50° + 14° = 64° which is the grid bearing.
Example 2
A grid bearing of 89° is being converted to a magnetic bearing. The magnetic variation is 14°
east, so 89° - 14° = 75° which is the magnetic bearing.
Conversion of bearings westerly variation
Figure 7.11– East declination
Example 1
A magnetic bearing of 80° is to be converted to a grid bearing. The magnetic variation is 4°
west. So 80° - 4° = 76° which is the grid bearing.
Example 2
A grid bearing of 89° is to be converted to a magnetic bearing. The magnetic variation is 4°
west, so 89° + 4° = 93° which is the magnetic bearing.
Hunter TAFE - Chemical, Forensic, Food & Environmental Technology [cffet.net]
Course Notes for delivery of MSS11 Sustainability Training Package
Page | 11
Navigation & mapping for technicians
Study Module 7
Assessment task
After reading the theory above, answer the questions below. Note that;

Marks are allocated to each question.

Keep answers to short paragraphs only, no essays.

Make sure you have access to the references (last page)

If a question is not referenced, use the supplied notes for answers
1. What are the two types of compass? 2 mk
Type your answer here
Leave blank for assessor feedback
2. List the three North points and explain the difference between each one? 6 mk
Type your answer here
Leave blank for assessor feedback
3. Identify the two main types of compass error and provide one example of each. 4mk
Type you answer here
Leave blank for assessor feedback
4. Explain the difference between the cardinal, degree and Mil systems of direction. 3 mk
Type your answer here
Leave blank for assessor feedback
5. What is a bearing? Use your favourite search engine to explain the term ‘azimuth’ and
how it relates to a bearing. 4 mk
Hunter TAFE - Chemical, Forensic, Food & Environmental Technology [cffet.net]
Course Notes for delivery of MSS11 Sustainability Training Package
Page | 12
Navigation & mapping for technicians
Study Module 7
Type your answer here
Leave blank for assessor feedback
6. What is meant by the term ‘declination’? How do we correct for this? 4 mk
Type your answer here
Leave blank for assessor feedback
Assessment for this topic is mainly done during the practical session.
Hunter TAFE - Chemical, Forensic, Food & Environmental Technology [cffet.net]
Course Notes for delivery of MSS11 Sustainability Training Package
Page | 13
Navigation & mapping for technicians
Study Module 7
Assessment & submission rules
Answers
◗ Attempt all questions and tasks
◗ Write answers in the text-fields provided
Submission
◗ Use the documents ‘Save As…’ function to save the document to your computer using
the file name format of;
name-classcode-assessmentname
Note that class code and assessment code are on Page 1 of this document.
◗ email the document back to your teacher
Penalties
◗ If this assessment task is received greater than seven (7) days after the due date (located
on the cover page), it may not be considered for marking without justification.
Results
◗ Your submitted work will be returned to you within 3 weeks of submission by email fully
graded with feedback.
◗ You have the right to appeal your results within 3 weeks of receipt of the marked work.
Problems?
If you are having study related or technical problems with this document, make sure you
contact your assessor at the earliest convenience to get the problem resolved. The name of
your assessor is located on Page 1, and the contact details can be found at;
www.cffet.net/env/contacts
Resources & references
1. These notes
2. Kjellstrom, B. 2010. Be expert with map and compass: The complete orienteering
handbook. 3rd Ed. Wiley. US.
Hunter TAFE - Chemical, Forensic, Food & Environmental Technology [cffet.net]
Course Notes for delivery of MSS11 Sustainability Training Package
Page | 14