This article was originally sourced from Scouting Ireland CSI. The pictures have been modified to make them legible and some into colour! Enjoy!
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Navigation |
Travelling through wild countryside is achieved by the use of a map and a compass. The map conveys a detailed picture of the landscape and terrain we are travelling across and the compass provides us with a tool that will steer us in the correct direction. The secret of good navigation is a good knowledge of map reading and interpretation. The compass although important is secondary to good map reading skills.
Understanding your map
The map is a representation of the landscape. It is produced from satellite pictures and on the ground surveying. The map however is only two dimensional therefore it must employ a method known as contouring in order to show the rises and dents of the landscape. A number of symbols are also used to establish such features such as forests, churches, boggy ground, fences, train tracks etc. Roads and tracks are marked on the map using a number of different coloured lines and such methods as broken lines and chequered lines. Rivers and lakes are marked in blue.
Scale
In order to draw a map of manageable size we use a process of scaling to
ensure the correct miniaturization of the landscape on the map. Typical
scales used are half inch to the mile, one inch to the mile. In such a
scaling system one inch on the map represents one mile on the ground.
Therefore the bigger the scale the more information it is possible to draw
onto the map. For walking purposes you will be using the Discovery Series
of maps which have a scale of 1 : 50,000 or 2 centimetres to a kilometre (
one and a quarter inches to a mile)
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Grid lines
Overlaying all maps there is drawn a grid of light lines running from top to bottom and side to side on the map. The purpose of this grid is to allow us to identify every part of the map with a unique number system ( grid reference). These grid lines which correspond with the lines of longitude and latitude also enable us to identify the north of the map and aid with compass alignment. You will notice that each line is given a number, this will enable us to create the grid reference number.
Grid Reference
A grid reference is a series of numbers (coordinates) which gives us the exact location on a map. It is created by using the grid lines which appear on all ordinance survey maps using the following steps.
- Find your location on the map. If possible choose a recognisable
feature rather than a point in the middle of nowhere.
- Find the grid letter on the national grid by looking at your map.
These are printed in blue and are large in size. Quote the letter of
the sector your position is in.
- Start at the bottom left hand side of the map and move across the
grid lines till you arrive at the grid line nearest your location. The
number of the line is the first two numbers of your reference.
- You should then divide up the grid square into tenths. Half way is
.5, three quarter the way is .8 etc. State the location of your
position as a decimal. This number is the third number of the
reference.
- Repeat the same steps for the grid lines that cross the map and this
will give you the 3 figure reference for you location.
- You now have your six figure reference for your position.
A simple rule of thumb is the phrase that states - 'go in the door and up the stairs'
Which means that if you visualise a door at the left hand side of the
map - then you go in the door (give the bottom line first) then go up the
stairs (give the side numbers next)
Aligning a map
In order to read a map correctly you must first align the map. This is
done by moving the map around until the map and the landscape correspond.
This is usually done by selecting a landmark or feature, finding that
landmark or feature on the map, then aligning the map so that when you
look at the map and then at the landmark their is an imaginary line drawn
between the two points. When your map is correctly aligned you will be
able to identify other features from map to ground. The mountain peak on
your left or the stream junction on your right. If this exercise is done
correctly by aligning your map with two or three features rather than just
one you can now travel by using the map alone provided you establish
correctly where you are standing in relation to these features. As each
feature or new feature appears on the trail identify it on the map and re
establish your position.
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Contours
Contours are the method used to convey the shape of the terrain you are crossing. They are created by an imaginary line along which every point is the same distance above sea level. These lines are drawn at 10 metre intervals and allow us to see a representation of the shape of a hill or mountain. By looking at your map you can determine whether the mountain has a steep slope (contour lines close together) or a gentle slope ( contour lines spread apart). Contour lines are never or rarely circular in shape, because they are plotting a set ground level they allow us to see the gentle curves of a mountain as well as deep gullies. Practice on the ground with your map will provide you with hands on experience, and over a period of time you will get to know what variations of contour lines mean and what they translate to in reality on the ground.
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The Norths
A compass points to one north, your map is drawn with grid north, and the stars point to true north. What is the difference and how can you come to grips with then.
True North
True North is determined by sun readings and from the stars and is the
point we would call the north pole or very top of the earth where all the
lines of longitude converge.
Grid north
Map makers create a grid system around every country called the National
Grid. Each box on this grid has a letter. It is this letter which we use
to identify by means of a grid reference our position on the grid. These
grid lines are shown on our map and are numbered thus allowing a grid
reference to be created. As the position of these grid lines are only
slightly out of line with true north we use them as our method of setting
our compasses. So for our purposes in Ireland and the British Isles Grid
north and True north are the same
Magnetic north
This is the north that is indicated on our compass. Magnetic north is the
location of a part of the earth which is magnetic and attracts the needle
of the compass. This north is located approximately north of Canada. (8
degrees west of grid north)
How is each used
If you get lost or do not have a compass then you will relay on the sun,
stars and nature signs to show you the direction to follow. The direction
you will seek is true or celestial north. It is only a general indicator
and no fine navigation will be done by this method. e.g. If you are lost
and you know that from the last time you looked at a map that a road was
to the east of you. What you would do is determine where north was by
using the stars or sun and create a compass in effect. If you are facing
north then east is to your right west is your left and south is to your
back.
Grid north
Grid north as we have said is the map makers north. Therefore every map is
created with the top of the map being north, bottom south. If you have
your map folded up and you can read the writing on the map ( it is not
upside down) it is turned in the right direction. This is an important
point to remember when it comes to taking a compass bearing from your map
(explained later)
Magnetic bearing
The magnetic north is the north that your compass needle will always point
to. This magnetic field is constantly moving so in Ireland it moves by a
number of degrees every couple of years. In Ireland the current variation
is 7 degrees west (1994) See the side panel of you map to see what the
current variation is. This means that your map and your compass or out of
line with each other. So in order to take a directional bearing from the
map and translate that to the compass for you to follow you have to add on
the variation of 7 degrees ( for convenience we use 8 degrees - 4 marks on
the compass ). This variation is different in every country so always
check the side panel of your map for variation particular to that
location.
The Compass
The compass is an instrument that tells us where north is. This is done by way of a magnetised needle that is allowed to float freely within the instrument housing. Around the edge of the compass is plotted a circle on which marking similar to that on a ruler are inscribed. Each marking determines the number of degrees it is from north to this point.
There are many compasses on sale from the simple floating needle type to
the more expensive plotting compasses. We will only be concerning
ourselves with one compass and that is the Silva Compass. This is the best
compass and only compass you should use for navigational purposes on land.
The Silva Compass is made up of three parts - the needle, the compass
housing and the base plate. The needle is coloured red and white and the
red end points to north the white to the south. The compass housing
revolves and determines any desired bearing or direction of travel. The
base plate is used to indicate line of travel.
How to use your compass
Your compass is a tool that is used in conjunction with your map. By using
the compass it is possible to navigate very precisely between points on a
map. Your compass can also be used to check your position on the map and
check the correctness of your line of travel.
Taking a bearing
Place the compass on the map with the edge of the base plate along the desired line of travel. The direction arrow on the compass should point to the place you wish to go.
Move the compass housing until the north - south lines on the transparent base of the compass housing are parallel with the grid lines on the map. The north arrow on the compass housing should be pointing north. You should be as accurate as possible when lining up these lines as a movement each way will add or subtract degrees from your final bearing and result in bad navigation and missing your destination by hundreds of yards.
Lift up the compass from the map and read the bearing indicated on the
compass dial. Say this number to yourself then add on the magnetic
variation (e.g. Bearing is 92 degrees add on variation 8 degrees result
100 degrees) now move the compass housing to this setting. It is a good
practice to do bearings this way rather than adding on by moving the
compass housing immediately so that you do not make a mistake. It is
better to confirm in your mind what the bearing is before you move the
compass.
Your compass is now set. Hold the compass in your hand and move your body
around until the needle of the compass is correctly aligned with the north
- south markings on the housing. The direction of travel arrow on the
compass now points in the direction you need to travel to your next
destination.
This exercise is repeated from point to point as you travel on your
journey.
Following a bearing
You travel on a bearing by sighting a recognisable landmark along its path
and then travelling to that point and repeat until you reach your
destination. It is not advisable to follow your bearing by looking at the
compass and watching the movement of the needle. As you walk you will have
to move from side to side to avoid obstacles so this method of following
the bearing is discouraged in favour of line of sight identification
method.
However, if you find yourself in heavy fog or at night you will use the
method of looking at the compass to find your way. In fog or at night you
could use members of your party to line up on the bearing under your
direction and you then travel to these members. This is a more accurate
method than looking at the compass.
Back bearing
If you think you have erred from your line of travel you can check your
bearing by using a back bearing. To do this you turn around and point the
compass back to your last location. The white part of the needle should
now point north. If it is slightly out then it is possible, by walking
left or right until the needle lines up, to correct your line of travel.
Finding your position on a map
To find your position an a map we use a process called resection. This is
preformed by plotting at least two points on the map to determine your
position.
First select a landmark that you can identify on the map and from the
position you are standing.
Point the compass at the landmark and move the housing until the needle
and north - south marking align. Read off the bearing on the dial. Now
subtract 8 degrees from that bearing. (e.g. bearing of 88 degrees less 8
degrees total 80 degrees). You then place the compass on the map with the
edge of the base plate on the symbol for identifiable feature. Without
adjusting the compass move the whole compass round this point until north
- south lines are parallel with grid lines. If you have a pencil drawn a
light line along the side of the base plate. Your position is somewhere
along this line.
You now select another feature which can be seen and identified from your
position and repeat the process. If possible choose a feature which is
nearly 90 degrees from your position. By doing this your new line while
precisely cross the line drawn from the other feature. If the two points
selected are two close to each other then the lines will tend to merge and
will result in a less accurate determination of your position. Where the
two lines cross in your position.
Normally, two bearing are all that is required however, if you wish you
can use three to confirm exactly where you stand.
Nasmiths Rule
Nasmiths rule is a method of determining our speed of travel over the
countryside. It states that we walk at 3 miles per hour and that we must
add on to this calculation half an hour for every 1000 ft climbed. This
calculation can be converted into a metric measurement thus - we walk at 5
kilometres per hour and allow 30 minutes for every 300 meters climbed. For
the purposes of calculating time travelled with young people it is better
to use a figure of 4 kms per hour.
These calculations can be simplified
- 15 mins per 1 km
- 7.5 mins per half kilometre
- or in the height 1 minute for every 10 meters
These simplifications allow us to calculate with ease. Measure the distance with the rule on your compass 2 centimetres = 1 kilometre = 15 mins travel time. Count the number of contour lines you pass on your way up each contour line = 10 meters = 1 minute extra to your travel time. You do not add on time if you are descending a mountain or high ground. Be careful reading contour lines on the map. You may have to calculate the height gained in meters or feet depending on the map used. ( If you are using the discovery series of maps it will be meters) Your start of position maybe at the 150 meter line and you may travel through to the 250 meter line you have therefore climbed 100 meters meaning you have to add on 10 minutes to your distance travelled time to give you your correct arrival time.
4 kilometres per hour is a suggested average for hiking across easy ground
with a light pack. If you intend to carry heavy packs or if you are
travelling through rough countryside then you will have to adjust this
figure. The chart opposite will give you some guide, 2.5 kms per hour is
the suggested figure for planning your route if you are participating in a
Mountain Pursuit Challenge. In determining the correct speed of your Troop
it is a good exercise to measure out a set distance and time your Troop
over this distance walking at an average pace, with packs, etc.
This simply rule enable us to navigate across rough ground with precision.
We can also use this calculation to plan hikes and adventures into wild
country without leaving our sitting rooms.
Route Cards/Planners
Route cards are a device we use to plan our adventures across rough countryside. The route card/planner tells us essential information about our purposed route, number in the party, etc. The card also serves as a safety device as we should leave a copy of the route card with a responsible person who is not taking part in the hike or trip. If an emergency arises then this person can advise the rescue services of your route and aid your rescue. In preparing a route card we break up our journey into convenient sections or 'legs'. Each leg is then treated separately to calculate distances , bearings etc.
Filling out a route card
Each point of reference on the route card is identified with a grid
reference. So you will be travelling from grid reference to grid reference
rather than from the edge of the forest to the river. Grid references give
us precise position on the map whereas the edge of the forest is open to
interpretation. The direction of travel between two points is determined
by a compass bearing. These bearing are obtained from your map ( don't
forget to add on magnetic variation) The next steps are to determine what
distances you will travel between points and the height gained and
calculate the total time for each leg of your journey. You will also need
to add in such things as - stopping to admire the scenery, and rest time.
A general rule is to allow 15 mins per hour. This 15 mins maybe spread
over a number of legs. In arriving at your total time you should also add
in time for lunch or meals as required.
You should take note of the actual time that it takes to complete each section and put this figure down on your card. This information will be useful if you decide to do the route again at a later stage. It is also useful in determining your accuracy, which will improve with practice.