A basic introduction to this navigation aid providing an outline of what it is and what it does
What is it?
Basically except the GPS it is nothing more than a small, magnetised strip of metal protected by a plastic case, there are many variations with
different features and accessories designed for different purposes; a few examples are shown below.
||Basic Field Compass|
Suitable for basic
navigating along footpaths and trails.
Shows your location by
grid reference or a marker on their electronic map and needs to be "in view" of at least 3 satellites to provide a reasonable level of
accuracy so can be unreliable in woods, deep valleys and other similar areas. Being battery operated they may "run down" just when you need
them most, so are not a replacement for your map and compass but rather a useful additional aid.
||Military Style Compass|
For more accurate and micro navigation when you are venturing away from footpaths, where there aren't any easily identifiable features or landmarks
and/or in poor visibility.
Can be used for very general directions but not for navigating your way.
The Parts of a Compass
What does it do and what are its limitations?
- A magnetic north seeking needle (red in the diagram)
- A rotating dial with numbered bearings (dark blue in the diagram)
- A capsule within the dial with orienting lines and a broad orienting arrow (light blue in the diagram)
- A rectangular base plate with direction arrow and lines (black in the diagram)
The different norths
A compass is basically a tool for finding North but depending on the type you have can also be used for other purposes including:
- Measuring distances on a map with the aid of a ruler printed on the base plate.
- Working out Grid
- Setting a map
- Taking bearings
There are however a number of issues that you need to be aware of when using a compass the main ones being:
- It points to magnetic north whereas your map uses grid north (see "The different norths" below).
- It needs to be level when using otherwise the needle won't work properly – it will catch on the compass housing and give a false
- Magnetic north is not fixed but moves gradually over a period of time (see "magnetic variation" below).
- Being magnetic the needle can be affected by its surroundings such as certain rock formations like the Cuillins, electricity supply lines and objects
containing ferrous metals such as watchstraps, farm gates, other magnets including compasses and GPS units (their batteries have a magnetic field)
- It is a sensitive instrument that can be permanently damaged by sharp knocks(1), dropping it(1) or storing it with other
magnetic items including other compasses.
(1) = does not apply to GPS units
When we refer to North, where exactly do we mean? This question is important when navigating
because there are actually three different
North's in existence:
The North Pole where all lines of longitude converge and together with the lines of Latitude are used by Satellite
systems (Sat Navs).
Because it is difficult to represent the shape of the land and curvature of the earth on a flat map a square grid system of parallel lines was
devised - the grid lines on your map. The vertical lines, called "Eastings" are the ones that point up to 'Grid North'.
Confusingly the horizontal lines which point East are called "Northings"
The focus of the Earth's magnetic field generated by the Earth's iron core, it fluctuates steadily and predictably over time. This causes
the focus point (the Magnetic North) to gradually change its position, oscillating around the North Pole; at times very close to it but decades
later hundreds of miles away.
Setting A Map
The magnetic north changes by about ten minutes per year so as there are 60 minutes in a degree, it
takes about six years to make a one degree difference. Just to complicate matters this deviation also varies from place to place. For example, in June 2016
it was about 3¾ degrees in Lynmouth and 2¼ degrees in Coniston. Although for practical purposes you would use 4 degrees and 2 degrees respectively
as compasses are only marked in 2 degree intervals
To obtain the current value of the deviation for your location you need to do a calculation based on the value quoted in the legend of the relevant OS
or Harvey's map, or from the latest values on the internet.
Because it is relatively small this variation can be ignored for very basic navigation like confirming which path you need to take, or which
general direction you should be travelling in. But in situations requiring more advanced navigation it must always be included.
||It is often useful to keep the map open (suitably folded) and aligned with the ground so that the route you are following and any visible
landmarks line up with their positions on the map. This is known as "setting the map" and by keeping it "set" as you make your way along your
route, it is easier to keep to it at junctions etc., particularly where there are paths off that aren't shown on the map.
You may also find it helpful to place your thumb on the map at your last known location, moving it each time you check your progress, quicker
than trying to relocate your position on the map each time you refer to it. Don't worry if the writing is the wrong way up, having the map set
properly so that you can correctly follow the features on the ground is far more important than being able to read the writing.
||If the features or landmarks on the ground aren't visible you can set the map with your compass by placing it on the map with the
direction arrow pointing ahead along your path / route and turning the map making sure the compass is held firmly in place (i.e. it moves with it)
until the red north end of the compass needle lines up with the grid lines on the map and points north.|
As you are basically using the compass to find north (i.e. using the map and compass as a rough guide) you can ignore the lines in the compass
housing and there is no need to correct for magnetic variation.
Remember to keep the map "set" when you turn a corner on a path or change direction by turning the map as well - i.e. If you turn right, turn the
map to the left.
There are 3 basic ways of using compass bearings
1. Plotting your route on the map and following it on the ground
- In poor visibility or open remote terrain with few landmarks or features you will need to navigate on compass bearings that is plotting your
route on the map and following it on the ground.
- To identify a feature or landmark that you can see by taking its bearing on the ground and plotting it on your map.
- To confirm your location.
2. Identify a feature or landmark that you can see.
- Place your compass on the map with one of the base plates long edges lined up between your current location on the map (point "A" in the diagram)
and your next destination (point "B" in the diagram).
- Then keeping the base plate perfectly still turn the capsule to make the light blue orienting arrow and lines in it line up with the vertical
grid lines on the map, making sure the orienting arrow is pointing north (top or grid north end of the map). Ignore the magnetic needle for the
moment and read the bearing at the index point (the X in the diagram).
- To apply the magnetic variation add it by turning the capsule anticlockwise the required number of degrees.
- Lift the compass off the map, don't turn the capsule.
- Stand with the direction arrow pointing away from you and turn around until the north seeking red end of the needle lines up with the light blue
orienting arrow in the capsule. The bearing you read off the map is now indicated on the ground by the direction arrow on the base plate.
- Walk in this direction towards a landmark, feature or other identifiable point. Don't simply follow the compass as you will end up
walking in a curve
3. Confirm your location
- Hold your compass out flat with the black base plate direction arrow pointing away from you and towards the landmark or feature you wish to
- Without moving the baseplate turn the capsule until the north seeking red end of the magnetic needle lines up with the light blue orienting arrow
within the capsule.
- Read the bearing at the index point (the X in diagram 2a), then apply the magnetic variation by turning the
capsule clockwise the required number of degrees (you are subtracting it).
- Without moving the capsule setting, place the compass on your map aligning a bottom corner of the baseplate with your current location (point "C"
in diagram 2b).
- Swivel the compass around this point "C" (do not adjust the capsule setting) until the light blue orienting arrow and lines in the capsule line
up with the vertical grid lines on the map, making sure the orienting arrow is pointing north (top or grid north end of the map). The edge of the
compass will now be in line with the landmark / feature you are trying to identify (point "D" in diagram 2b).
- Follow steps for diagram 2a above, then, without moving the capsule setting, place the compass on your map aligning a top corner of the baseplate
with the landmark (point "D" in the diagram for 2b above).
- Swivel the compass around this point "D" (do not adjust the capsule setting) until the light blue orienting arrow and lines in the capsule line
up with the vertical grid lines on the map, making sure the orienting arrow is pointing north (top or grid north end of the map).
- Where the edge of the compass (or a line extended from it) crosses the path or track you are on is your current location (point "C" in the
diagram 2b above).
- If you are in open country and / or not following a path on the ground, repeat the procedure 3 times using 3 landmarks such as points "E" (a
local sumit), "F" (a woodland corner) and "G" (the top end of a tarn) in the diagram. Actually draw a line from each to just past your
approximate location (use a pencil to avoid permanently marking the map) along the compass edge extending it as necessary.
- You should now have 3 lines that form a triangle around your current position as shown in the diagram.
This is obviously an ideal example with your 3 landmarks fairly evenly spaced out around the compass points. If you can only see 2 landmarks (in which
case your location will be where the 2 lines cross), or they are closer together be aware that your position will be located less accurately.