This information on How to Use a Compass is aimed at people who have not previously used compasses for hiking.
To become accomplished at taking bearings and converting them will require you to read carefully through the following directions and let the directions “sink in”. Think about an easy way for you to remember how the conversions work and what simple check you can think of to test the method. Do I add or subtract the magnetic variation? What information is on the map that could help you remember? Practice taking bearings on walks (ask the leader if you can practice your navigation skills) and become familiar with moving from a map to a compass and back again. Check your bearings with other walkers.
Become familiar with the features shown on maps (Eg. gullies, ridges, spurs, saddles, plateaux, rivers, cliffs, mountains) and learn to recognise them on the ground. Which features could provide bearings? Which types of terrain are easiest to walk through? Which routes do experienced walkers take? Why? Where could you seek emergency shelter in a storm? Is the creek water likely to be contaminated? Where is the nearest farm house to make a phone call to summon help in an emergency? Where might unpolluted drinking water be found?
Most of us usually start with a basic compass having a straight edge (costing around $20 to $30) which is quite adequate and will last for many years. Confidence in using it comes from continued practice when out walking. However, if you can afford a compass which has an offset adjustment (for compensating for the magnetic variation), then you may find it easier to use. The Silva brand is recommended. They simplify life considerably when taking bearings and reduce the risk of taking a wrong bearing due to miscalculation or poor memory. The amount of magnetic variation varies significantly across Australia and adjustments will be required depending on where you are walking.
Using a compass is a life saving skill. Your own life or someone in your group could be saved by your ability to read a map and navigate your way to safety.
- Remember that Grid North (GN) will usually differ from True North (TN) – the size of the variation will depend on the particular map, but is usually less than 2 degrees.
- Do not take compass readings directly under high voltage transmission lines, near anything containing iron (Eg. car bonnets), in areas with high iron content or close to watches, packs or other compasses. The compass needle will be diverted away from Magnetic North and give a false reading.
- When taking bearings from a map, it doesn’t matter which way the map is oriented. However, you may find that if you align the map facing TN then it is easier to relate to the geographical features around you. Eg. Creeks, valleys, ridges, spurs, saddles, power lines, farms, roads, etc.
- Protect your compass from heat and damage. Most navigators usually carry them on a loop of thin cord around their neck so that they can tuck it into their shirt pocket. Others carry it in their map cases which is also carried around their neck. This allows the navigator to frequently check their location on the move, rather than having to stop and put the map and compass back into their pack every time.
- There are many good books available on this subject. Find one that suits you.
- Practise. Practise. Practise.
1. Taking a Point to Point Grid Bearing
To take a grid bearing between one point A on a map and another point B, lay the edge of the compass along a line between the two points. Then rotate the bezel until the north orienting arrow on the bottom of the rotating bezel is pointing to the top of the map (Grid North). The compass manufacturers make it easier for us by also providing some lines on the bottom which are parallel to the north orienting arrow. This allows us to reduce the parallax error when trying to visually align one of these lines to become parallel to the vertical grid lines. The grid bearing can now be read over the bearing line. This bearing isn’t of any use however until it is converted into a magnetic bearing that can be set on the compass so that you can start walking towards the object at Point B.
2. Converting a Grid Bearing to a Magnetic Bearing
The rule is to take the Grid bearing and SUBTRACT the Variation.
You can derive the formula from the diagram shown on the bottom right-hand corner of every map which shows you the angular relationship between True North, Magnetic North and Grid North.
Consider that if an object was due north of you (0 degrees Grid – vertically up the grid line), then magnetically it would be 12 degrees west of Magnetic North. Thus we SUBTRACT the variation (12 degrees) to give the Magnetic Bearing.
Eg. If you calculated a bearing of Point B away from Point A on a map in Melbourne as 102 degrees, then you would subtract 12 degrees to give a magnetic bearing of 90 degrees.
The typical mistake is to add the magnetic variation instead of subtracting it, thus giving an error of twice the magnetic variation. (Eg. 24 degrees away at 114 degrees)
3. Walking Along a Magnetic Bearing
With the compass horizontal, rotate the bezel until the specified magnetic bearing angle is over the bearing line (with the arrow). Now turn yourself around slowly until the north end of the compass needle ends up between the two luminescent dots or lines or pointing at the N on the bezel. You will now be pointing along the bearing, ready to start walking. However, trying to walk in a straight line along a magnetic bearing will generally be difficult due to finding trees and other obstacles in your path or if you have a tendency to veer to the left or right. It becomes easier if we split the walk up into a series of shorter sections (and take bearings of intermediate objects along the way).
A simpler and easier method which usually saves time and energy is to take a bearing of a large prominent object that you can see from a distance (preferably up high when walking in hilly country so that you can check your location whilst walking along the easiest route towards this object). The object could be either natural (Eg. like a mountain or large prominent rock or tree) or man-made (Eg. like a church spire or a building or communication tower).
4. Taking a Magnetic Bearing of a Distant Object
Raise the compass to eye level then align the pointer or edge of the compass directly at the object you wish to take a bearing of. Rotate the bezel (the rotating part) until the north end of the floating magnetic needle lines up between the two luminescent dots or lines which are on the flat surface of the rotating part or directly over the red orienting arrow (depending on which brand of compass you are using). Read the magnetic bearing directly from the bezel opposite the pointer line. The accuracy is usually within +/- 5 degrees depending how carefully you hold the compass. (A sighting compass can improve the accuracy (or reduce the error) to within 1 degree.)
The usual mistake is to align the wrong end of the compass needle, thus giving an error of 180 degrees. The magnetised end which points towards north is usually coloured red or blue.
5. Converting a Magnetic Bearing to a Grid Bearing
If you wish to find or mark your position on a map, you will need to take a magnetic bearing of at least two (preferably three) prominent objects using a compass and then convert each magnetic bearing to a grid bearing.
The rule is to take the Magnetic bearing and ADD the Variation.
Some people may find it easier to use a mnemonic to remember whether to add or subtract the variation. One popular mnemonic which is listed on all the maps (so you don’t have to remember it) is CMA – which usually stands for the Central Mapping Authority – but can be used as “Compass – Map – Add”. That is, when converting a magnetic bearing from a Compass to use on a map – and thus give a grid (Map) bearing so that lines can be drawn on the map to determine your position (by taking back bearings) -you Add the variation to the magnetic bearing. Conversely, when converting from Map to Compass – you subtract.
Another simple method is to remember the compass diagram shown on the bottom right-hand corner of every map. Consider that if the object was magnetically due north of you (0 degrees magnetic), then physically it would have to be 12 degrees east of Grid North. Thus we ADD the variation (12 degrees) to give the Grid Bearing.
Eg. If you took a bearing of a distant object in Sydney as 78 degrees magnetic, then you would add 12 degrees to give a true bearing of 90 degrees – due east of your present location. This is because the Magnetic North is offset by 12 degrees east of Grid North.
The usual mistake is to subtract the magnetic variation instead of adding it, thus giving an error of twice the magnetic variation.
6. Finding Your Position by Taking Back Bearings
Take a magnetic bearing of a prominent object in the distance and which you can positively identify on the map. Convert that bearing into a grid bearing and draw a straight line on the map starting from that object (use the side of the compass or any straight item to draw the line). You are located somewhere along this line. Take another bearing from a different object, ideally at 90 degrees from the first object. Convert the magnetic bearing and draw the next line on the map. This second back bearing line should intersect with the first line. If your bearing objects are high, narrow, not too far away, lines are straight and your compass alignment good, then you are likely to be within 100 metres of the location shown by the intersection of the back bearing lines. If not, you could be located within a radius of up to 300 metres. If you have to walk out a specific spur or creek then the accuracy can be critical. You could walk up the wrong spur and be forced to stop and camp overnight due to night falling. If you take a bearing from a third object and draw in another line on the map, you will usually end up with a triangle being formed by the three intersecting lines.
7. Deliberately Off-setting Bearings (or Aiming Off)
This is done when approaching a long feature running at 90 degrees across your path which is directly in front of you and where the risk of not being sure whether to go left or right when reaching this feature (Eg a cliff line, road or river) could be critical if time is wasted by going the wrong way. The method is to determine from the map (if you cannot see the object or aiming point) an accurate magnetic bearing directly at the object or aiming point and then deliberately add an off-set (say 2 degrees to the left of your aiming point) so that you will then know when you reach the feature that you will definitely have to turn right to reach the aiming point (or vice versa).
8. Magnetic Variation (Declination)
Every topographic 1:25000 CMA map in Australia shows the magnetic variation (or declination) for that specific area. If you marked up a map of Australia showing all of these variations you would end up with the map below which shows that the magnetic variation varies significantly across Australia. The magnetic variation is very slowly moving easterly as noted on the bottom of each map.
Remember to allow for the change in magnetic variation depending on where you are going to walk.
9. Determining the Time to Get From A to B
This involves having a good understanding of the different types of terrain and vegetation cover that will be traversed and the probable speed of the group. Is the group fit and fast or tired and slow? Consider the pace and energy level of the average walker – high energy after breakfast, slowing down to lunch, slight increase after lunch but getting gradually tired and slower late in the afternoon (especially after climbing up hill all afternoon). What packs are they carrying – light day packs or heavy overnight packs?
A guide as to how fast a group of average bushwalkers (say 6) with overnight packs can travel.
1 kph – Climbing up a steep sloping spur with thick scrub
2 kph – Scrambling over large rocks along a steep sloping creek
3 kph – Walking down a steep sloping spur
4 kph – Walking along a flat track (this pace can be maintained all day)
5 kph – Walking with extra effort – Eg. Making an effort to catch the train
6 kph – Walking with considerable extra effort (only maintained for several minutes)
Modify pace according to:
Faster if – Fit, small light packs, less dense vegetation, flat ground, long legs, high energy levels, early in day,
Slower if – Unfit, heavy packs, thicker vegetation, steeper slopes, short legs, low energy levels, late in day, injuries/blisters,
To determine the expected time taken, divide the estimated distance travelled by the guesstimated pace of the group.
Eg. 2 klms to walk / 4 kph = 0.5 hr
Add up the times for each section of the walk, allow for rest periods (5 minutes/hr), tea breaks (additional 5 minutes every 2 hrs) to give a cumulative time for how long the walk is expected to take. Experience will fine-tune your guesstimating skills.
Finally, remember that most walks are not races (unless you are an extremely fit “tiger walker” trying to beat the clock). Get wet to cool off if you’re getting hot. Unwind. Relax and take the time to observe and appreciate the natural beauty around you. Learn to look for the birds, plants and animals. They also need water and food to survive. Your observations may one day save your life.