LEADING CADET
Navigation with Map and Compass
Navigation is about knowing where you are on the map
Revision Notes
Learning Outcome 1
Be able to find a position with an Ordnance Survey map using the physical features of the
land
Two Methods of finding North without a Compass (LEARN)
Remember method 1 gives SOUTH direction initially
When using a map the first action is to set or orientate it by positioning it relative to features on the
ground
Learning Outcome 2
Know the different types of north
Remember the following symbols
Magnetic North in the UK differs from both True North
and Grid North
The difference between True North and Grid North arises because
the lines of latitude and longitude do not match the grid lines exactly
The position of the Magnetic North pole changes every year
TRUE
NORTH
MAGNETIC
NORTH
GRID
NORTH
The Magnetic Pole is currently located to the north of Hudson Bay in Canada
When navigating with a map Grid North is always used for reference
(Magnetic) Variation is the angular difference between True North and Magnetic North
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Grid Magnetic Angle (GMA) is the angular difference between Grid North and Magnetic North
The approximate difference between True North and Grid North is between 0 and 5 degrees
The difference between Grid North and True North in the UK is not more than 2 degrees in most places
Lines of Longitude get closer together as they approach the North Pole.
On an OS map information on the GMA is located at the centre of the top margin
On an OS map information on the Magnetic Variation is located at the bottom right hand side
Finding of the Pole Star (by using Great Bear (or Plough) star group) enables North to be located at night
Grid lines on a map point to Grid North
Learning Outcome 3
Be able to use a lightweight compass for practical navigation
A Silva compass is used for navigating when walking
On a Silva compass, which is used for navigating when walking, the magnetic needle is red and white.
The compass should be held horizontal when reading to allow the needle to swing freely
The scale used on the compass is in millimetres
The purpose of the liquid in the compass capsule is to allow the needle to move freely and settle down
quickly
When navigating with a compass we are making use of the Earth’s magnetic field
A freely suspended magnet will point to the Magnetic North pole
Magnetic Deviation is the effect on the compass needle of nearby ferrous objects or computer screens
A Grid bearing is converted to a Magnetic bearing by adding the GMA (GRID to MAG – ADD)
A Magnetic bearing is converted to a Grid bearing by subtracting the GMA (MAG to GRID – GET RID)
When setting a map with a compass the first step is to set the GMA against the compass index mark
The final step in setting a map with a compass is to turn the map and compass together until the
needle is inside the orientating arrow
To take a bearing on a distant object you first point the compass direction of travel arrow at the object
To take a bearing between 2 features on a map you first place the compass on the map so its longest
edge runs through both features you then turn the compass capsule until the orientating lines are
parallel with the north-south grid lines
When walking on a bearing in good visibility the best technique is to select a distant feature that is along
your intended route and walk towards it
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Resection means fixing your position on a map using three prominent landmarks. The landmark
bearings are taken with a compass. These bearings are then plotted on a map and your position is in the
middle of the triangle formed by the three intersecting plot lines
The best way to measure distances on a map without a pair of dividers is to use the straight edge of a
piece of paper and mark it with a pencil
Learning Outcome 4
Know land navigation methods
Measuring distances accurately helps you to keep on schedule and to reduce the uncertainty of your
position
The best way to measure distances on a map without a pair of dividers is to use the straight edge of a
piece of paper and mark it with a pencil
Handrailing is following linear features to get to your destination
Countouring means walking around a hill without gaining or losing height
An attack point is an easily identifiable feature that is close to your objective
The estimated speed of a group on level ground is 4kph
To estimate distance covered from last check point - measure the time taken and calculate at 4km/hr
Distance from your last check point can be estimated by measuring the time taken and calculate at 4kph
Pacing should only be used over short distances
Naismith’s Rule applies to speed of advance in mountainous terrain (steep terrain) when under such
conditions:


20 minutes should be added for every 200 metres ascended in a journey (1min per 10m)
10 minutes should be added for every 200 metres descended in a journey
A Roamer is used to read grid references and ensures better accuracy
A 4 figure grid reference refers to the SW corner of the grid square
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Learning Outcome 5
Be able to recognise weather conditions that affect land navigation
The relationship of the Upper and Lower winds can be used to indicate the weather shortly to reach you
Weather Fronts
1 – Cold
2 – Warm
3 – Stationary
4 – Occluded
Fronts occur when a cold
air mass meets a warm air
mass
A Depression is an area of Low Pressure; it moves under the influence of Upper winds
An Anticyclone is an area of High Pressure; it tends to bring long periods of fine weather
Fronts occurs when a cold air mass meets a warm air mass
Isobars join points of equal atmospheric pressure
Air Masses
W – Polar Maritime (brings cool winds and heavy showers to
the UK in both summer and winter)
X – Tropical Maritime (originates around the equator and
brings warm, cloudy weather to the UK in both summer and
winter)
Y – Artic Maritime
Z- Tropical Continental (Hot and very dry, originating from
the arid and desert regions during summer. Can bring
record heat to the UK, particularly in late-summer.
W - Returning Polar Maritime (mass starts off very cold with
dry air but arrives in the UK warm and wet)
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In addition, the Polar continental air mass via the short sea-track (not shown on diagram) brings cold
dry weather with little or no cloud to the British Isles in winter
Cloud Types and Formations
Cloud Type
Prefix
Height
Cirrus, Cirrostratus, Cirrocumulus
Cirro
High
Alto, Altocumulus
Alto
Medium
Nimbostratus
Nimbo
Medium/Low
Cumulus, Stratocumulus
Strato
Low
Remember CANS for descending cloud types
High level
Medium level
Low level
Cirrocumulus
Altocumulus
Cumulus
Cirrus
Stratus
Remember that Stratus clouds
occur as a featureless layer
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