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 using a watch gives the 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 TRUE MAGNETIC GRID The position of the Magnetic North pole changes every year 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 1
Magnetic Variation is the angular difference between True North and Magnetic North and on an OS map information on the Magnetic Variation is located at the bottom right hand side Grid Magnetic Angle (GMA) is the angular difference between Grid North and Magnetic North and on an OS map information the GMA is located at the centre of the top margin Convergence is the approximate difference between True North and Grid North and is between 0 and 5 degrees but not more than 2 degrees in most places. It occurs because the converging lines of longitude do not exactly match the parallel grid lines Lines of Longitude get closer together as they approach the North Pole. Finding the Pole Star by using the 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 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 A Silva compass is used for navigating when walking On a Silva compass the magnetic needle is red and white. 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 then settle down quickly to steady it to take a reading The compass should be held horizontal when reading to allow the needle to swing freely 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 2
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 3
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 An Occluded Front 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) In addition, the Polar continental air mass via the short seatrack (not shown on diagram) brings cold dry weather with little or no cloud to the British Isles in winter* Remember in particular 4
Cloud Types and Formations Cloud Type Prefix Height Cirrus, Cirrostratus, Cirrocumulus Alto, Altocumulus Nimbostratus Stratocumulus, Cumulus Cirro Alto Nimbo Strato High Medium Medium/Low Low Remember CANS for descending cloud types Low Level Medium Level High Level Cumulus Altocumulus Cirrocumulus Stratus Cirrus Remember that Stratus clouds occur as a featureless layer 5