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A
Guide to Compass Navigation
by Tony
Harding
In
open moorland and upland areas, the terrain may have few obvious features to
navigate from, map reading becomes more difficult. This is also true in poor
visibility like mist, low cloud or darkness. It is in these conditions that a
compass becomes essential.
Even
during normal conditions in lowland areas the compass can be an invaluable
tool when navigating through the countryside. Its main function is to confirm
that you are walking on the correct path or in the right direction. If
crossing large fields, which have recently been ploughed, or where the
footpath is not visible on the ground, the compass will provide the answer and
help prevent you going wrong or getting lost.
However
the compass is no substitute for good map reading, likewise a compass without
a map is also of no use to the walker.
Before
the modern walkers protractor compass was introduced, the traditional method
of obtaining grid bearings from a map was to use a 360º protractor and a
rule. The bearing obtained was then transferred to a magnetic compass to
determine the direction to travel. This method is still used by sailors for
plotting courses from sea charts. The following pages give information on the
protractor compass and how to use it to navigate with.
General
Navigation
Tips
Never
assume that the map is wrong or the compass isn't working properly. The
chances are, it's you and not the map/compass that is wrong!
Note:
New footpath diversions, forestry plantations, new roads and housing
developments all take time to appear on maps. Replace old maps to get the
latest information, particularly if the map is more than 5 years old.
Regularly
follow your route on the map, ensuring that features around you are where you
expect them to be on the map. This gives you confidence that you are on the
correct path. If unsure use the compass to confirm your direction of travel is
correct.
If you can not identify the features around you on the map and using the
compass doesn't help, never carry on, hoping that things will sort themselves out. They
won't and you will soon become totally lost. Go
back to the last place that you were confident of you position.
Then
re-check the map to see where you went wrong. Use the compass to confirm the
direction of the route you intend to walk. (Good reason for regularly
following the route on the map).
When out walking, regularly practice using the compass, don't wait
until you are lost before trying to remember how to use it.
Typical
Protractor Compass
So
called 'Protractor Compass' because it combines the function of a 360º
protractor, straight edge and a magnetic compass. Originally made by the Silva
Company in Sweden in the 1940's for the popular Scandinavian sport of
orienteering. There are several makes of these compasses available which are
ideal for walkers. The clear plastic Baseplate (this is the straight edge
part) may also include some additional features, such as:
-
Magnifying
glass for identifying small detail on maps, can also be used to light a
fire with the suns rays.
-
Romers,
of various scales, for quick and easy identification of 6 figure grid
references.
-
Millimetre
& inch scales for measuring distances on the map.
-
Luminous
points for night use.
The
rotatable compass housing is graduated with 360º around its edge (this is the
protractor part). North is at 0º (or 360º) East at 90º,
South at 180º and West at 270º.
The
compass housing has an Orienting Arrow which is used to point to Grid North
when used on the map. The Orienting Lines are similarly used to aid alignment
which can be lined up parallel to the maps blue north - south grid lines.
The
magnetic needle is pivoted on a sapphire bearing and housed in an antistatic
fluid. The red end of the needle points to Magnetic North and is accurate to
+/- 2º. Small air bubbles may form in the fluid at altitudes above 2000 feet,
a drop in atmospheric pressure or at low temperatures. The air bubbles will
not affect the magnetic bearings and will disappear when normal conditions
return.
Luminous
markings are for night use and contain the radio active material tritium which
very slowly decays. It will glow effectively for for 12 years then gradually
fade. The luminous markings need to be 'charged' by exposure to light i.e. the
flash of a torch, the glow will fade over a period of 20 minutes.
The
Protractor Compass is used to perform 4 main functions:
-
To walk
over open ground on a bearing taken from the map, this probably accounts for
75% of all compass use. Particularly useful where paths are not visible on the
ground or in poor visibility such as low cloud, mist or darkness. Essential if
walking across open moorland & upland areas like the Lake District.
-
To check
the direction of a path with its bearing taken either from the map &
transferred to the compass or a magnetic bearing which can be transferred to
the map.
-
To
identify distant features.
-
To find
your position from 2 identifiable features, then transferring these bearings
to the map. The point where they cross is your location (also known as
resection).
Types
of Bearing
Directions
are expressed as a bearing, this is an angular measurement in degrees (º)
with reference to North which is at zero degrees. When using the compass the
bearing is read at the Index Line, in the illustration shown above, this is 45º.
There
are two types of bearings used when navigating:
Grid
Bearing:
For
angular measurements taken from maps (not map bearings).
Magnetic
Bearings:
For
angular measurements taken with a compass (not compass bearings).
Grid
North & Magnetic North
Grid
North is the top of the map and is used as the reference point to which grid
bearings relate. The difference angle between Grid North and True North (the
actual North Pole) varies slightly throughout the UK but is sufficiently small
enough to be disregarded.
Magnetic
North is a region in the north of Canada with a natural strong magnetic
attraction, to which all compass needles point. The difference between Grid
& Magnetic North is known as the Magnetic Variation. All Ordnance Survey
maps include details of the grid, true and magnetic north's applicable for the
area covered by the map.
Magnetic
Variation
Magnetic
North or the Magnetic Pole, is not stationary but is slowly moving closer to
the North Pole, hence the Magnetic Variation is also slowly changing. The
current rate of change for the UK is about 1º every 8 years. The Magnetic
Variation for our Beds/Bucks area was 5º west of Grid North in 1996, this
will be 4º in 2004. It should be remembered that the Magnetic Variation
differs slightly throughout the UK, for example, for Cornwall it is 2º in
2003.
This
Magnetic Variation has to be taken into account when taking bearings for
navigation purposes, to reduce errors to a minimum. For example a 5º error
over a distance of 1 mile equals 154 yards off course. However over short
distances, less than 200 yards say, the Magnetic Variation can be disregarded,
as the errors will only be a few yards off course.
The
following mnemonics; GUMA & MUGS, is an easy way of remembering whether
the Magnetic Variation should be added or subtracted.
When
taking Grid Bearings from a map for transferring to Magnetic Bearings on a
compass, it is necessary to ADD the
Magnetic Variation i.e.
Grid
- Unto - Magnetic
- Add. (G-U-M-A)
When
taking Magnetic Bearings with a compass for transferring to Grid Bearings on a
map, it is necessary to SUBTRACT the Magnetic Variation i.e.
Magnetic
- Unto - Grid
- Subtract. (M-U-G-S)
Sighting
Compass
The
model 15 is the top of the Silva range and is known as a Sighting Compass. It
is similar to other protractor compasses except it is more accurate (+/- 1º)
and has a sighting mirror located inside a hinged lid which covers the compass
housing when not in use.
When
the compass is held horizontally at eye level, with the lid hinged at right
angles, the compass bearing and magnetic needle are reflected in the mirror. A
small notch or 'sight' is fitted in the top of the lid to enable more accurate
sighting of distant features.
Some
versions of sighting compasses also include a built in magnetic variation
adjuster and spirit level to aid accuracy.
Assume
you plan to walk from point A to point B, via the footbridge but the footpaths
have recently been ploughed out and are not visible on the ground. A grid
bearing is required before the compass can be used to follow the route as
follows:
Grid
bearing to Magnetic North Bearing
1. Place
the compass on the map with the long straight edge of the baseplate parallel
to the path to be followed. Ensure that the 'Direction of Travel' arrow points
in the direction you wish to walk i.e. from A to FB.
2. Without
moving the compass baseplate, rotate the compass housing until the 'Orienting
Arrow' points to the top of the map (Grid North). Ensure that the lines in the
compass housing are parallel with the maps north-south blue grid lines.
3. Take
the compass off the map, note the grid bearing at the 'Index Line' and add the
magnetic variation, by rotating the compass housing the appropriate number of
degrees (4º for 2002 - 2011), in an anticlockwise direction.
4. Hold
the compass in front of you at waist height, parallel to the ground, with the
'Direction of Travel' arrow pointing away from you.
5.
Holding the compass steady, rotate yourself around until the red end of
the magnetic needle, points to North on the compass housing.
6. Look
along the 'Direction of Travel' arrow and sight a distant feature along this
direction. Put the compass away and walk to the distant feature. In poor
visibility it may be necessary to send a second person off in front, along the
direction to travel then wait there until you walk to join him/her.
7.
On reaching the distant feature/second person, repeat the process at
para's 5 & 6 until you reach your destination. It will of course be
necessary to take a new bearing between the footbridge (FB) and point B.
Magnetic
Bearing to Grid Bearing
Assume
you are unsure of your exact position along a footpath but can recognise a
landmark or feature both on the map and in the area around you (e.g. a
church). The following procedure will enable you to identify your location.
1.
Hold the compass in front of you at waist height, parallel to the
ground, pointing the 'Direction of
Travel' arrow at the landmark or distant feature.
2.
Hold the compass steady, rotate the compass housing until the red end
of the magnetic needle points to North (N) on the compass housing.
3.
Double-check that the 'Direction of Travel' arrow still points to the landmark
and that the compass is still on North.
4.
Ignore the position of the 'Orienting Arrow', it is not used when taking
magnetic bearings.
5.
Note the bearing at the 'Index Line' and subtract the magnetic variation, by
rotating the compass housing the appropriate number of degrees (4º for 2002 -
2011) in a clockwise direction.
6.
Lay the compass on the map with one of the corners on the 'Direction of Travel
Arrow' end, on the landmark from which the bearing was taken.
7.
Swivel
the whole compass around until the 'Orienting Arrow' points to the top of the
map (Grid North). Check that the 'Orienting Lines' are parallel to the maps
north-south grid lines.
8.
At the point where the straight edge of the compass baseplate (in line with
the landmark from which the bearing was taken), crosses the footpath, is your
location. It may be necessary to draw a line on the map, as shown below, if
the baseplate does not reach far enough to reveal your location.
Note:
If you are completely unsure of your position, a second bearing taken on
another landmark (about 90º away) and transferred to the map, will indicate
your location where the two lines cross (also known as resection).
Practical
Compass Test
The
following compass test relies on the fact that a triangle has 3 internal
angles. By adding 120º to the bearing at points A & B and if each leg is
walked with the same number of paces, you should return to the start. This
will test the ability of a person to set the compass and walk on a magnetic
bearing. Try placing a £1 coin at the start to focus the mind!
-
Choose a
suitable unobstructed large, flat grass area and place a marker at your feet.
-
Set the
compass for a bearing of 40º (other bearings can be used but any start
bearing greater than 120º may cause confusion as this takes the final setting
beyond 360º).
-
Hold the
compass in front of you, parallel with the ground at waist height. Rotate
yourself around until the red magnetic end of the needle points to North on
the compass housing. Look along the Direction of Travel arrow and identify a
feature or object on this line, then walk 40 paces towards the object. It is
not necessary to look at the compass whilst walking.
-
Stop at
the 40th pace and add 120º to the 40º bearing, i.e. set to 160º.
-
Turn
around until the red end of the compass needle points to North again and
identify another object on this new bearing. Walk 40 paces in the new
direction towards the object.
-
Stop at
the 40th pace and add 120º to the 160º bearing, i.e. set to 280º.
-
Turn
around until the red end of the compass needle points to North again and
identify another object on this new bearing. Walk 40 paces in the new
direction towards the object.
-
Stop at
the 40th pace, look down and you should find your marker, or at
least fairly close by!
Also
try using a second person instead of looking for a distant feature or object.
Send the second person off along the direction of travel for about 50 paces to
point A, then instruct them to go left or right as appropriate until they are
on the correct line. Walk towards the second person for exactly 40 paces, stop
and add the 120º. Send the second person off towards point B and repeat the
process. Repeat the process once more, adding 120º again until you arrive
back at the start point.
If
satisfactory results are obtained, try extending the length of each leg walked
to 80 paces.
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