LESSON 7
MAP READING
BEARINGS
Reference:
Aids:
Manual of Map Reading and Land Navigation
1.
Slide 7/1 to /9.
Place of Work:
2.
1:50,000 map sheet per student.
Classroom
3.
Pencil, paper and ruler per student.
Student Dress:
4.
Lightweight compass per student.
Working Dress
5.
RA Protractor per student
Equipment:
Administration:
Projector and Screen
1.
Read the Instructors Notes before the
lesson.
Time Allowed:
2.
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3.
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4.
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40 Minutes
7-1
BEGINNING OF LESSON
STAGE
Revision
CONTENT
Relate the following revision questions to the student’s map.
What is the straight line distance between the junction at:
GR ....................... and the cross-roads at GR .......................?
ANSWER: To be worked out on the students’ own map.
Which is the shortest route between these two points using only “A”
and “B” class roads, and how far is it?
ANSWER: To be worked out on the students’ own map.
See Instructors Notes
Aim
The aim of this lesson is to introduce you to bearings.
Objectives
By the end of this lesson students will know:
1.
The two Norths used in Map Reading.
2.
The Mils system of measuring angles.
3.
The Grid Magnetic Angle (GMA).
Reasons Why
You must understand these three things before you can use a compass.
As a soldier you must be able to use a compass to set your map, and to
take Magnetic Bearings and to march on a bearing.
Incentive
If you see enemy movement from your OP, but are unable to identify
the location precisely, you could find the direction using a compass, and
the distance by estimation.
Use of GMA will enable the position to be plotted on a map.
7-2
MIDDLE OF LESSON
INTRODUCTION
Think about what you now know.
You now understand scale and how to measure distance.
You know some of the map content by use of conventional signs.
You know something of landforms by studying key contour patterns.
SLIDE 7/1
Now you need to learn about direction.
THE THREE
NORTHS
TRUE NORTH
There are three different “Norths”.
1.
True North.
2.
Grid North.
3.
Magnetic North.
True North is the direction from where you are standing to the North
Pole.
SLIDE 7/2
True North is not generally used in map reading but, because it is so
close to Grid North, you can usually disregard it. Therefore only Grid
and Magnetic North concern the map reader.
GRID NORTH
MAGNETIC
NORTH
Grid North is the direction of the North-South grid lines on your
map. Every grid line points in the direction of Grid North:
SLIDE 7/3
Magnetic North is the direction to which the Northern red end of the
compass needle points. It is measured using a magnetic compass.
The Magnetic North Pole is situated not at the North Pole but about
1500km South, in Arctic Canada.
It is not marked on maps because the Magnetic North Pole is
continually moving.
SLIDE 7/4
Problems can arise because Magnetic and Grid North are in different
directions in Britain (and in many other countries).
DIRECTIONS
AND THE MILS
SYSTEM
When you were at school you used a semi-circular protractor divided
into 180 degrees to measure angles:
You may have also seen a compass that was a full circle divided into
360 degrees and marked:
North, South, East and West.
SLIDE 7/.5
You should know that:
East is 90 degrees
South is 180 degrees
West is 270 degrees and
North is 0 or 360 degrees.
However, the Army does not use degrees; it uses mils to measure
angles.
7-3
In the mils system a circle is divided into 6400 mils.
SLIDE 7/6
Looking at this slide you will see that:
East is 1600 mils
South is 3200 mils
West is 4800 mils and
North is 0 or 6400 mils.
The example readings on the slide show 900 mils and 5900 mils.
Notice the readings are from North, clockwise.
Mils can be shortened to “m” with a diagonal line through the 'm'.
Any Questions
Questions from the class.
Confirmation
Questions to the class on directions and the mils system.
How many Norths are there in map reading?
ANSWER: Two, Grid North and Magnetic North.
What is Magnetic North?
ANSWER: The direction to which the Northern end of the compass
needle points.
What is Grid North?
ANSWER: The Northern direction of the North-South grid lines on the
map.
To where does the Northern end of the compass needle point?
ANSWER: To the magnetic North Pole.
How many mils are there in a complete circle?
ANSWER: 6400 mils.
BEARINGS
When you use a compass you are Measuring a Magnetic Direction or
Bearing. (In the USA a Bearing is called an Azimuth)
A Bearing is measured clockwise from North, so a Magnetic Bearing
is measured clockwise from Magnetic North and a Grid Bearing is
measured clockwise from Grid North. Magnetic Bearings are
measured on the ground whilst Grid Bearings are measured on the
map.
SLIDE 7/7
Before you can use a Magnetic Bearing on the map you must change it
into a Grid Bearing.
GRID MAGNETIC The difference between Grid North and Magnetic North is termed the
GRID MAGNETIC ANGLE or GMA for short.
ANGLE (GMA)
SLIDE 7/8
The GMA can be found at the top of M726 maps as blue angle lines.
Show and explain: The GMA lines on the map and how they are
based on the Blue Datum Circle at the bottom of the map.
You can see that GMA is West of Grid North by ................. mils.
7-4
At present GMA is West of Grid North in Britain.
See Instructors Notes
Any Questions
Questions from the class.
USING THE GMA
You use the GMA to change a Magnetic Bearing taken with your
compass into a Grid Bearing which you can plot on a map. You also
use the GMA to change a Bearing read from the map into a Magnetic
Bearing to be set on your compass.
REPEAT SLIDE 7/8
In Britain : if you subtract GMA from the Magnetic Bearing, you are
left with the Grid Bearing.
Similarly, if you measure a Grid Bearing you must add the GMA to
obtain the Magnetic Bearing.
Here are some examples for you to try out.
See Instructors Notes
If you are still unsure you can use the following rhyme:
SLIDE 7/9
Mag to Grid - Get Rid (of GMA west)
Grid to Mag - Add (GMA west)
Look at the examples and try them again.
Any Questions
Questions from the class.
Confirmation
Practise the class in changing bearings from grid to magnetic, and from
magnetic to grid.
END OF LESSON
Any Questions
Questions from the class on the whole lesson.
Test on the Whole
Lesson
Write down or give verbal answers to the following questions.
Where does the Northern end of the compass needle point?
ANSWER: To the Magnetic North Pole.
How many mils is equivalent to the direction of west?
ANSWER: 4800 mils.
What do you call a direction measured with a magnetic compass?
ANSWER: A Magnetic Bearing.
What do you call a direction measured on a map?
ANSWER: A Grid Bearing.
Remembering the rhyme, what are the answers to these problems?
What is the Grid Bearing equivalent to a Magnetic Bearing of .............?
Pack Equipment
If the Magnetic Bearing is .................., what is the equivalent Grid
Bearing?
Pass answer papers to the front.
Hand in stores.
7-5
We normally use two Norths in map reading: Grid North and Magnetic
North.
Summary
A circle is divided into 6400 mils, and directions are given in Mils and
not in Degrees. Mils are denoted with a lower case 'm' with a diagonal
line drawn through the 'm'.
Directions are measured clockwise from North and are called Bearings.
Bearings can be measured from Magnetic North or Grid North and are
therefore Magnetic Bearings or Grid Bearings respectively.
The difference between Grid North and Magnetic North is the Grid
Magnetic Angle (GMA).
In Britain, you subtract the GMA from a Magnetic Bearing to get a Grid
Bearing, because GMA is west of Grid North. You add GMA to a Grid
Bearing to get a Magnetic Bearing.
Because Magnetic North is West of Grid North in Britain you can use
the following rhyme to apply GMA correctly.
Mag to Grid - Get Rid
Grid to Mag - Add
Look Forward
In your next map reading lesson you will learn how to use the
Lightweight Compass and how to take and plot Bearings.
INSTRUCTORS NOTES
1.
The instructor must select the start and finish points for revision questions before the
lesson, and have checked distances ready to confirm student answers.
2.
The instructor MUST prepare a sufficient number of checked examples of changing
bearings in both directions before the lesson.
3.
Although recruits are not taught how to use a RA protractor, its issue for this lesson
only, will help in the understanding of the Mils system instead of the Degrees they used
at school. It should be described as similar to a 180 degree school protractor. If time
allows, the following explanation can be given as to why ground troops use mils rather
than degrees.
a.
b.
For small angles 1 mil subtends 1 metre at a distance of 1 kilometre.
The fall of shot can easily be corrected using mils.
4.
Because of shortage of time in recruit training and the possibility of their becoming
confused, recruits are not taught GMA East.
5.
Remember that GMA can be either West or East depending on where you are in the
world, and that the rhyme only works in places where GMA is West. Recruits must
NOT be confused by GMA East - however, in continuation Map Reading a lesson
must be given on GMA East.
6.
Remember that GMA varies from map to map and from country to country. Get used to
checking every map for its GMA and whether it is East or West.
7.
When is GMA West and GMA East? Look at your map!
8.
Do not confuse recruits with the annual change. It has been ignored in this lesson
because it is usually small on British maps and for most purposes can be ignored.
British maps are updated every few years with a new GMA.
7-6