|
|
 |
|
Navigation
Skills
Map & Compass
To find your way about,
the two most important items of kit you will need are: a map of the
area you will be travelling through, a good compass (the kind you
find at the bottom of a packet of cornflakes, or even the kind that
comes on the handle of so-called 'Rambo' survival knives are worse
than useless. They are far from accurate and can give you a false
sense of security). The best type to get are one of the Silva or
Suunto range, and they are quite reasonably priced.
Buy the best model that you can afford, and look after it. A sharp
pencil or pen and some paper are also essential.
Scale
Maps are produced to a number of different scales. A map is like a scaled down
picture of the ground. Some features are too small to be shown, but important
features are represented by conventional signs. On the 1:50,000 map, 2cm on the
map is equal to 1Km on the ground, while on the 1:25,000 outdoor leisure series
shown on the right, the scale is 4cm on the map to 1Km on the
ground.(recommended for walkers, or those travelling on foot as the contour
lines are better defined.)
Map Reference
Map reference or Grid reference both mean the same thing. I will refer here to
Grid References or GRs. GRs are normally given as a set of six figure numbers
which represent a grid square and a point within that square.
The map is crossed with vertical and horizontal grid lines. Each line is
identified by a two figure number, and these numbers give you the first two
numbers of your GR - two vertical numbers and two horizontal numbers.
The vertical lines advance across the map from left to right, and are called
'Eastings' - because they travel from west to east.
The horizontal lines travel up the map from bottom to top, or south to north and
are called 'Northings'.
Where the two lines join, you have a
grid point and you can identify this point by giving the relvant numbers,
easting first and then northings. This will identify a specific square on the
map and is known as a four-figure grid reference.
A four-figure GR is not a whole lot of use to us as it is, as each side of the
square is one kilometre long in real terms. By adding an extra number to the
easting, and an extra number to the northings, and if that extra number
represents one tenth of the square, we can narrow the GR down to just a few
metres.
It is ESSENTIAL that you give a
GR as Eastings before Northings or your GR will be completely out
(possibly as much as 100 kilometres).
The easiest way to remember it is "E comes before N in the alphabet", "In the
door before you go Up the stairs", or as we used to say in the Army, "
You have to get your leg over before you can get it up!!" :-)
Contour Lines
Contour lines are imaginary lines drawn on the map to give us an indication of
height. As we know, the land is not flat like the map is, and the only way the
map makers can indicate height is by drawing a series of lines, each one at its
own height, which follows the contours of the land, in and out of valleys and
re-entrants, and round spurs.
The contour lines are ten metres apart vertically, that means that the ground
has risen ten metres in height from one contour line to the next one. If the
contours are very close together then the land is either rising or falling very
sharply. If the contours are well spaced out, the land is flatter.
The way to tell if the land is rising
or falling along your chosen route, is to see which way the height numbers are
facing. They are marked on the thicker contour lines every 50 metres, and the
numbers always point uphill. The other way to see if you are looking at a spur
or a re-entrant, is to see if there is a thin blue line running through it. If
there is then it's a re-entrant as streams and rivers don't run along the tops
of spurs. Have a look at a map and see whart other features give you clues.
(If you still use the 1inch to 1
mile maps, the contour lines are 50 feet apart.)
Orienting the Map
Orienting your map means turning it round until the features on the map match
the features on the ground before you. This is sometines also referred to as
'setting' a map. This might mean that the map ends up upside down or sideways,
don't worry. It just means that objects on the ground will appear in the right
place. Quite often people go wrong when they are reading a map because they hold
it like they were reading a book, and they go east when they should be going
west - the ground can't move, so the map has to.
Orienting by Compass
This is the most common way of setting a map and is very useful in mountainous
country, or in woodland or forest, or featureless moorland. It is also the only
way to set your map in fog.
You need to know where north lies in order to set the map.
First you must set the 'Magnetic Variation' (more about this in a moment). On a
Silva compass there are some lines running down the length of the compass. These
lines are placed parallel to the lines running up and down the map (the
eastings). The compass housing is the part in the middle that rotates. There are
also some lines inside the compass housing.
Before you put the compass near the
map, you have to make an allowance for the difference in the Earth's magnetic
field. This is called Magnetic Variation.
If you can imagine the earth as you would see it from space, and look for where
the north pole would be, then that is Grid North. Grid North is what the lines
on the map are based on. Grid North never moves.
Magnetic North however, does move - about half a degree every five years or so.
It currently lies somewhere in the North West Territories of Canada. That puts
it some six and a bit degrees to the west of us here in the United Kingdom. This
then is our magnetic variation, six degrees west.
To set the compass, you turn the housing six degrees to the west. Then place the
compass on the map with the lines on the main part of the compass parallel to
the vertical grid lines on the map (the eastings). Now keeping the map and
compass together, rotate the map until the compass needle (the red part), points
to the 'N' on the compass housing.
Now you know the direction of north, try to identify features on the ground and
relate them to the map and vece-versa.
Note: If using navigation skills as described here in a country other than th
U.K. please be sure to identify the magnetic variation for that country. (It
will usually be marked somewhere on a topographic map).
Translating a Grid Bearing into a
Compass or Magnetic Bearing
In order to get a grid bearing, you first have to identify your location and the
location of the place you want to reach, on the map.
Connect these two points with the long edge of the compass, then turn the
compass housing until the lines inside the housing (they are called orienting
lines) are parallel with the vertical grid lines on the map. Don't bother about
the needle at the moment, we're just using the compass as a protractor for now.
Read the number of degrees on the
housing that says 'Read Bearing Here'. This then, is the grid bearing. between
where you are and where you want to be. Before you can use the bearing though,
you have to convert it into a magnetic bearing so you can follow the compass in
the right direction. To do this you have to ADD the magnetic variation. The easy
way to remember whether you add or subtract is "Magnetic Unto Grid = Subtract"
MUGS and "Grid Unto Magnetic = Add" GUMA. You only have to remember one of them
and you'll all feel like MUGS if you get it wrong!
If your bearing was 234°, you add the
magnetic variation of 6° and your magnetic or compass bearing becomes 240°.
Now put the map away. You should have set the compass housing to 240°. Take the
compass in your hand, and keeping it level, turn yourself round until the red
part of the needle is in line with the arrow inside the compass housing. By
keeping both arrows together, and by walking in the direction that the travel
arrow points (that's the arrow at the very front of the compass), you should
reach your objective.
Backbearings
If you are lost, or go too far, you can find your way back to where you last
made a bearing change by following a backbearing. It is easy to calculate.
There are 360° on a compass dial. 180° west and 180° east. To get a specific
backbearing, subtract 180° if your original bearing was more than 180°, or add
180° if the original bearing was less than 180°.
For example:
Bearing 260° - Backbearing (less 180) = 80°
Bearing 65° - Backbearing (plus 180) = 245°
Naismith's Rule
Allow 12 minutes for every kilometre travelled, and add a kilometre for every
100 metres of climb.
More about Naismith in the next section.
Distance & Time
To work out your distances, mark off the distance scale on your map (usually
along the bottom) onto a piece of paper. Place the paper on your map along the
route you intend to follow. Now count off the distance and multiply by 12. This
will give you the time it will take in minutes. You will have to mark each
change in direction along the route to get the right length. Now add all the
minutes together and it will give you the time it will take. The only problem
here is, you have calculated the time over a flat route; the map. In real life,
nothing is that flat. You have to take into account the hills you are going to
climb.
Remember Naismith's Rule? It states
that you add a kilometre for every 100 metres of ascent. To do this you should
start at the very beginning of your route and count the number of contour lines
you cross. Just count the ones going UPHILL only. The downhill ones are treated
like flat ground. (No, you don't subtract for the downhill ones). For each 10
contour lines crossed it means that you will have climbed 100 metres. You should
add a kilometre to your first calculation. For example:
If you first estimate was 10 Km, and
on counting the contour lines along that 10 Km route, you have a total of 500
Metres of climbing, you would have to add (100m = 1Km, 500m = 5Km) 5 Kilometres
to your original estimate. Therefore your total adjusted route would cover the
equivalent of 15 Kilometres rather than 10. Or to put it another way, it would
take you 3 hours to walk at 12 mins per Km rather than 2 hours.
These times should be your best
unlaiden times. If you are carrying a load, increase your time per Km
accordingly. This can be worked out in advance by walking a measured Kilometre
both laiden with kit and unlaiden and noting how long it takes. You can then
make accurate route estimates for yourself.
This site is owned by Bushcraft Educational Society © 2005
Email:
info@bushcraft-educational-society.co.uk
|
|
|
|
|
|
