How to use Compass

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manual how to use the Compass

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  • 1/9 Scout Skills Compass The Scout Association 2000 Item code: FS315074(Format Revision Aug 2000)The Scout Association, Information Centre, Gilwell Park, Bury Road, Chingford, London E4 7QW. Email: info.centre@scout.org.ukWebsite www.scoutbase.org.uk Direct: 020 8498 5400 Local rate call: 0845 300 1818 Fax: 020 8498 5407

    0845 300 1818

    Scout SkillsCompassINFORMATION SHEET

    A compass is an instrument with a magnetisedneedle which points to (magnetic) north and istherefore used for determining direction. Theycome in different shapes and sizes and indeed,the use of suspended magnetic ore (whichalways comes to rest in a north-south direction)was used many centuries ago as a primitive formof compass. Today, in one form or another,compasses are used on land, at sea or in the air,to help people to specify direction.

    Types of compass

    Air damped compass - This is the simplest andcheapest form of compass and does little morethan indicate the approximate direction ofmagnetic north. It takes a long time to stabiliseand the slightest movement makes the needlemove. This compass should never be used forany sort of hike or expedition.

    Simple map setting compass - It is a liquid filledcompass with only magnetic north marked on itand can be clipped onto the side of a map. It isuseful for positioning a map until whatever is infront of you in reality, is in front of you on themap. This can only be approximate as there isno allowance for magnetic variation, that is, thedifference between magnetic north and grid(map) north. (This is explained in more detaillater on).

    Prismatic compass - This is a more expensivetype of compass with a prism which enables acompass bearing to be taken while sighting yourobjective. It can be more accurate than othercompasses but it is harder to use and thereforeshould only be used once the basic principles ofmap and compass work have been mastered.

    Silva type compass - This consists of amagnetised needle suspended in an alcohol filledhousing. The liquid helps to 'dampen' movementof the needle enabling it to be read more quicklythan air damped compasses. The compass

    housing has etched orienting lines and anorienting arrow, whilst the baseplate (on whichthe housing is mounted) has the direction oftravel arrow and map scales etched onto it. Thiscompass allows for bearings, an accuratemethod of determining direction, to be workedout and is therefore the compass of choice forhiking and expedition type activities.

    Why use a compass?

    As you can see, it is possible to have a varyingquality of compass depending upon what job ithas to do and of course, ultimately, how muchyou pay for it!

    Apart from determining the direction of north, acompass enables you to work out a compassbearing. This is the angle measured in thenumber of degrees between 0 and 360 whichtells you the direction from one place to another.We call the direction north '0' and therefore, itfollows that east is 90 degrees, south-west is 225degrees and so on.

    If we just used the points of the compass, (north,south, east, west and so on) we would only get

  • 2/9 Scout Skills Compass The Scout Association 2000 Item code: FS315074 (Format Revision Aug 2000)The Scout Association, Information Centre, Gilwell Park, Bury Road, Chingford, London E4 7QW. Email: info.centre@scout.org.ukWebsite www.scoutbase.org.uk Direct: 020 8498 5400 Local rate call: 0845 300 1818 Fax: 020 8498 5407

    eight different directions (or possibly 16 or 32 atmost if we further divided the compass points,for example, south-southwest or north bynortheast and so on). By using bearings, we canhave 360, which enables us to be much moreaccurate.

    Once we have determined a direction (andbearing) in which to travel, it can then bechecked at regular intervals to confirm that weare still going in the correct direction whether ornot our destination can be seen.

    When using a compass proficiently, it isnecessary to be able to: Take a bearing - determine the angle

    between north and the direction of an objectin terms of degrees;

    Walk on a bearing - use a bearing to get to adestination without necessarily using a map;

    Set a map - use a compass to correctlyposition a map in order to represent what canactually be seen.

    (Details on how to do these are covered in theTeach Yourself section.)

    The three norths

    When working with a map and compass, thereare three different 'norths' to be considered!Fortunately, in the United Kingdom, for practicalpurposes, we only have to consider and workwith two of them.True north - Each day the Earth rotates about itsaxis once. The ends of the axis are the trueNorth and true South poles.Grid north - The grid lines, pointing to grid north,on Ordnance Survey maps divide Great Britaininto 100 kilometre sections. They are thenfurther sub-divided into one kilometre squares,east of an imaginary zero point in the Atlantic

    Ocean, west of Cornwall. The majority of gridlines are 1.5 degrees west of true north and aretherefore useful lines to refer to when takingbearings.Magnetic north - A compass needle points to themagnetic north pole. Unfortunately, it is not inthe same position as the true North pole. Themagnetic north pole is currently located in theBaffin Island region of Canada, and from theUnited Kingdom, it is west of true north. Thedifference between grid north and magnetic northis known as the magnetic variation and its valuecan be found in the orientation panel or marginof an Ordnance Survey map.

    As true north is only about 1.5 degrees off gridnorth, it is so small that it is normally disregardedand only grid north and magnetic north are used.

    Magnetic variation

    The magnetic variation, (the difference betweenmagnetic north and true north), is caused by theNorth and South poles not being directly'opposite' one another. The lines of the Earth'smagnetic field do not run in a regular pattern asthey are affected by other local magnetic forcesand the magnetic pole is always on the move.Some of these lines of magnetic variation areeast of true north and others west of true north.Between the east and west lines there is a line ofzero magnetic variation where the compass doespoint to true north - this line is known as theagonic line currently running through easternCanada, United States of America and SouthAmerica.

    However, not only does the magnetic variationchange as you move across the Earth's surface,it also changes with time. In the UnitedKingdom, the magnetic variation in 1991 wasabout 6 degrees and decreasing at anapproximate rate of 0.5 degrees every threeyears.

    It is important to check the magnetic variationregularly, and this can be found on a map'sorientation panel or margin. Remember also tocheck the year the map was printed, as a mapthat is 20 years or so old, could be up to 3 or 4degrees out! In fact, the magnetic variation alsovaries from side to side and top to bottom oneach and every map but these details can alsobe found on the map.

  • 3/9 Scout Skills Compass The Scout Association 2000 Item code: FS315074 (Format Revision Aug 2000)The Scout Association, Information Centre, Gilwell Park, Bury Road, Chingford, London E4 7QW. Email: info.centre@scout.org.ukWebsite www.scoutbase.org.uk Direct: 020 8498 5400 Local rate call: 0845 300 1818 Fax: 020 8498 5407

    This magnetic variation is important whencombining a map and compass as you need toconvert bearings from 'map to field'. To convertgrid bearings (which are indicated by a map) tomagnetic bearings (as per the compass pointingto magnetic north), add the current variation byturning the compass housing anti-clockwise. Forexample, if the current variation was 6 degrees,a grid bearing of 122 degrees would become 128degrees. This is what the dial should be set at.The reverse is true for converting a magneticbearing to a grid bearing; that is, subtract thecurrent variation.

    For expeditions abroad however, some parts ofthe world will not only have a different value, butmay also be east of true north, in which case,when converting from grid to magnetic bearings,the magnetic variation should be subtracted fromthe compass bearing.

    Further information and resources

    Ask other Leaders experienced in the use of themap and compass for advice and ideas. Do theyknow of opportunities for practising or learninghow to use them?

    There are also plenty of books available on thissubject, both at a beginner's level, and moreadvanced.

    Camping and Outdoor centres and other highstreet shops selling camping and hikingequipment may be able to offer advice.

  • 4/9 Scout Skills Compass The Scout Association 2000 Item code: FS315074 (Format Revision Aug 2000)The Scout Association, Information Centre, Gilwell Park, Bury Road, Chingford, London E4 7QW. Email: info.centre@scout.org.ukWebsite www.scoutbase.org.uk Direct: 020 8498 5400 Local rate call: 0845 300 1818 Fax: 020 8498 5407

    TEACH YOURSELF

    Understanding how to use a compass is likemany other activities; it's easy when you knowhow! Practice is also the only way to get it rightand remember it. Although this sheet can helpyou through the different stages, the onlyeffective way to learn is to go out and use thecompass for real. Ask experienced Leaders foradvice and also take part in a hike or expeditionto put the skills into practice.

    Time

    Up to one hour may be required to becomefamiliar with the parts of a compass and theprinciples of how to use it, especially inconjunction with a map, but more time will berequired in shorter sessions to put it into practice.

    Equipment

    A Silva (type) compass and an Ordnance Surveymap of the area you are in.

    Learning all about it

    Before having a go, you will need to read theInformation Sheet if you have not already doneso.

    Taking a bearing

    1. Hold the compass flat in your hand with thedirection of travel arrow pointing towardsyour destination or objective.

    2. Turn the compass housing until the compassneedle lines up over the orienting arrow.Ensure the north pole of the. needle, usuallyred, is used.

    3. Read off the magnetic bearing (that is, thenumber of degrees) from the mark on thecompass housing indicated by the indexpointer.

    4. Keep the housing in that position and checkyour bearing at regular intervals by lining upthe needle with the orienting arrow andwalking in the direction indicated by thedirection of travel arrow.

    Walking on a bearing

    This is used when you can initially see yourobjective or destination and don't need a map. Itis important to work out a compass bearingbefore the situation changes. This might be dueto the weather (rain, fog and so on), the terrainyou are in (valley, hills and so on) or a delayresulting in darkness. Any of these factors maymean you can no longer see where you areaiming for and, therefore, you will need to rely onthe compass bearing.

    1. Turn the housing of the compass until thebearing you require is against the indexpointer.

    2. Turn the compass until the needle lies overthe orienting arrow.

    3. Pick out a landmark along your direction oftravel line and walk towards it.

    4. Check your bearing and your objective atregular intervals.

    Setting a map with a compass

    This is for when you are using a map inconjunction with a compass to reach a givendestination, probably in unfamiliar territory.

    1. Turn the compass housing until the magneticvariation for the area is shown against theindex pointer.

    2. Place the direction of travel arrow pointingalong the vertical grid line with the directionof travel arrow pointing to the top of the map.3. Turn the map with the compass in thisposition until the compass needle points tothe north mark on the compass housing.

    3. Your map is now 'set' and you should be ableto recognise actual features from your mapin front of you.

    Combining map and compass

    1. Place the compass on the map so that onelong edge joins the start point and your des-tination, with the direction of travel arrowpointing towards the direction you wish totravel. (The direction of the map does notmatter for this exercise).

    2. Turn the compass housing until the orientingarrow points to the top of the map and theorienting lines are parallel to the grid lines.

  • 5/9 Scout Skills Compass The Scout Association 2000 Item code: FS315074 (Format Revision Aug 2000)The Scout Association, Information Centre, Gilwell Park, Bury Road, Chingford, London E4 7QW. Email: info.centre@scout.org.ukWebsite www.scoutbase.org.uk Direct: 020 8498 5400 Local rate call: 0845 300 1818 Fax: 020 8498 5407

    3. Take the compass off the map and read offthe bearing at the index pointer and add (orsubtract) the local magnetic variation.

    4. Turn the whole compass so that the needlecomes to rest over the orienting arrow, withthe red part to the north.

    5. Hold the compass in front of you, pick out alandmark along your line of travel and walktowards it.

    Common errors

    When first learning how to use a compass, thereseem to be many things to take intoconsideration - here are a few things which often'go wrong': Failing to add on the magnetic variation. If

    the magnetic variation is, for example, 6degrees, and you forget to add it on, you willbe 105 metres off course for every kilometretravelled in a straight line. This getsproportionally bigger over greater distances.

    Not having the direction of travel arrowpointing from your start to finish. If youmake this mistake you will walk 180 degreesout from your intended route.

    Orienting arrow pointing to the bottom of themap. Again, you will walk (180 degrees out)in the opposite direction.

    Not taking account of the magnetic effects ofiron and steel around you. For example,watches, steel buckles, cars, buried pipes,reinforced concrete, wire fences, railwaylines and other compasses (and evenmagnetic rocks!) can influence yourcompass. That is, these items might attractthe compass needle in preference to themagnetic north pole therefore giving you aninaccurate reading. If in doubt, try to moveaway from such objects.

    Avoiding obstacles

    Sometimes when using a map and compass youwill come across obstacles such as a lake, woodand so on that cannot be crossed and you mustget round them somehow. The problem is toavoid the obstacle without losing direction.

    The obstacle may be by-passed by going round itby a series of right angles; walk at 90 degrees toyour original route, count the number of pacesuntil you clear the object. Turn 90 degrees again,so that you are not parallel with your originalbearing and walk past the obstacle. Turn 90degrees again and walk the same number ofpaces, then, finally, turn through 90 degrees tobring you back on your original course.

    This may seem rather pedantic, but it does workproviding the number of paces and turns areaccurate. This can be vital if the weather takes aturn for the worse. An error of just 2 degreesover a journey of say, just six kilometres meansthat you will miss your target by 200 metreswhich if you find yourself fog-bound, and it's theonly habitation for 20 miles around, might befatal!

    Can you do it?

    When you feel confide...

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