Map Reading

THE BASIS OF MAP READING
Introduction
Elements of a map
Grid systems and map references
Map scales and distances
Directions and bearings
Setting a map
Identification of own location

INTRODUCTION
The main aim of this lesson is basically to give cadets information on the basis of map reading. This is to help them in their orienteering practices/competition.
What are maps?
Maps are graphic representation of the Earth’s surface that are drawn to scale on a flat surface.

MAPS Physical Environment Human Environment
Land masses
and oceans
Major landforms
Major drainage systems
Components of climate
Global vegetation patterns
Agricultural patterns
Industrial patterns
Settlement patterns
Communication patterns

Elements Of A Map
Title
Scale
Legend
Grid Lines
North arrows

Grid Systems & Map References 10 70 60 50 50 40 40 30 30 20 EASTINGS NORTHINGS

The North to South grid lines are called EASTINGS whereas the East to West grid lines are called NORTHINGS.
A grid square can be named
4 grid reference (2030)
40 30 20 30

Map Scales & Distances
A scale is the ratio between a given map distance and the actual ground distance.
Type of scales:
Statement
Linear
Bar
As a representative fraction

How to measure distance on a map?
Straight line distance
Using ruler or a pair of dividers
Curved distance
Using ruler (Break the curved distance into small straight pieces)
A thread

Direction
4 Cardinal Points
North, South, East and West
Further divided in 16 divisions
Etc. North-East, North north-East…
Each division is approximately
22.5 degrees or 500 mils

True North- the direction of the Earth’s North Pole which is 90 degrees North. The North pole star is directly above the North Pole.
Magnetic North- the direction of the Earth’s Magnetic pole, i.e. the direction in which the needle of a compass point.
Grid North- the direction of the vertical grid lines called eastings .
Note- All three North do not point at the same direction.

Bearings
Equipment
Compass, protractor
Back bearing
If the direct bearing is between 0 to 180 (degrees) or 0 to 3200(mils), add 180 (degrees) or 3200 (mils) to obtain the back bearing.
If the direct bearing is between 180 to 360 (degrees) or 3200 to 6400(mils), subtract 180 (degrees) or 3200 (mils) to obtain the back bearing.

Setting Of A Map

Place the map in a horizontal position.
Place the compass parallel to a north-south grid line with the cover side of the compass pointing towards the top of the map. This will place the black index line on the dial of the compass parallel to grid north, we have a magnetic variation on the face of the compass formed by the index line and the compass needle.
Rotate the map together with the compass until the variation formed by the black index line and the compass needle matches the deviation shown on the declination diagram printed on the margin on the map. The map is then orientated.
In Singapore, because the deviation between the grid north, true north and magnetic north is very slight, you need not be concerned about the magnetic variation. You need only to rotate your map until the compass needle points towards grid north.

Known Location & Known Point Method
Draw a straight line on the map joining your location with the known point.
Rotate the map until the line points towards the known points on the ground.
Demonstrate: As an example, our position is on OCS HQ Building MGR 3185 4750. The known point is NTU road junction reflected in the map at MGR 316 483. You need to draw a straight line on the map from OCS HQ Building to NTU road junction. Once this has been completed, rotate your map until the line that you have drawn points towards NTU road junction as seen from your position. The map is then set.

Resection With Compass
Known location B Known location A
Unknown
Known location C location X

back
azimuth (a)
Unknown

back azimuth
(b) ideal angle
1600 mils
Known

back azimuth
Known
(c)

triangle of
error
Known
Known location C location Y

Resection Without Compass ( Straight Edge Method )

direction of move pylon
known location
bridge
stream
Known Line and Distance From A Known Feature

Known Line and Distance From A Known Feature
known location
x
bridge
200m
1500mils
stream

known location
bridge 150 m
200m 1200 mils
1500mils
stream
Known Line and Distance From A Known Feature y

Direction
of move Known
feature
Unknown
own location
Known Line and Azimuth From A Known Feature

Direction
of move Known
(a) feature
grid azimuth - 700 mils
Unknown
own location

Direction
of move Known
feature
back azimuth
Unknown
own location
Known Line and Azimuth From A Known Feature SLIDE 14/8c

Direction
of move Known
feature
Known
own location

a. Intersection
b. By azimuth and distance
METHODS OF IDENTIFYING OTHER LOCATIONS

INTERSECTION
Unknown
location X
Known
location A
Known
location B
Known location C

INTERSECTION
grid azimuth Unknown
location X
Known
location A
Known
location B
Known location C
(a)

INTERSECTION
Known
location X
Known
location A
grid
azimuth
Known
location B
Known location C
(b)

INTERSECTION
Known
location Y
Known
location A grid
azimuth
Known
location B
Known location C
(c)

BY AZIMUTH AND DISTANCE
Unknown
location X
Known
location A

Unknown
location X
grid
azimuth
Known
location A
(a)

Unknown
location X
Estimate
distance - 500 m
Known
location A

Known
location X
Known
location A

CONCLUSION
1. Method of identifying own location :
a. Resection
b. By known line and distance from known feature
c. By known line and azimuth from known feature

2. Methods of identifying other location :
a. Intersection
b. By azimuth and distance
CONCLUSION

Pacing
Pacing help us to judge how much distance we have covered.
Two ways
Single step
Double step

Pacing
Counting a double pace is easier than counting every single pace.
On a fairly level ground, paces determine distance as follows:
65 to 75 paces per 100m for men
80 to 90 paces per 100m for women
Pacing also depends on the terrain. i.e. fairly level ground, upward slope, downward slope.
Should we get lost or things do not follow as plan, we can roughly know the distance that we have covered.

END
Prepared by
2LT Muhd Nazir

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Map Reading

by Gan Seng on May 05, 2009

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