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How to Use a Compass for Hill Walking

July 10th, 2009

how-to-use-compass.jpgThis article by Amy Weber describes some of the issues one might encounter when trying to use a compass for navigation during a hiking or hill walking expedition. Apparently it’s not quite as simple as looking at the compass and assuming that the needle is always pointing to the north.

Magnetic Variation

Unfortunately taking a bearing is not quite as simple as you might think because there are three different north directions: true north, grid norm, and magnetic north. The first is where the North Pole is and, unless you are going there, is not important for hill walking. Grid north is where the grid lines on a map point while magnetic north is where your compass needle points. The variation between these two is called the magnetic variation or declination.

To make matters more confusing, magnetic norm moves about from year to year. All Ordnance Survey and Harvey maps give the variation at the date of publication and the rate of change per year. In the British Isles in the mid 1990s magnetic north is about 5 degrees to the west of grid north. For accurate navigation magnetic variation must be taken into account. When you transfer a bearing from the map to the compass, as described above, the variation must be added. So if the bearing from the map is 230 degrees and the variation is 5 degrees then the correct compass bearing is 235 degrees. (There are various mnemonics for remembering this. The one that stuck in my mind decades ago is ‘empty sea, add water’ - MTC, map to compass, add.)

If you go to a region such as the western United States where magnetic north is east of grid north you have to subtract the variation. In any area you should always check the variation on the map. Some compasses can be corrected for magnetic variation, which is very convenient though you must remember to alter the setting if you go to an area where the magnetic variation is different.

Following a Bearing

Once you have a compass bearing you can head off towards your destination. Even so, walking in a straight line while looking at a compass for any distance is difficult, especially over rough terrain. Also, obstacles such as cliffs and lakes may mean you cannot stay on your bearing. Rather than take a bearing on your ultimate objective it is better to take one on a much nearer point, ideally one that is visible and certainly one that you can see from the map you should be able to walk to directly. Once you reach that point take another bearing to another point. By connecting up these ‘legs’, as they are called, you can reach your destination.

Attack Points

It is unlikely that all your legs will be on the same bearing; in hill country being able to walk in a straight line to your destination is unusual. Normally you walk on several bearings between your start point and your destination, linking different points along the way. These are known to orienteering enthusiasts as attack points.

The best points to use are small, easily identifiable features such as cliffs, minor summits or tarns. In really poor visibility it can be better to head for such features even if they lie well off your route rather than do a long leg with no identifiable points along the way. Using a series of attack points makes it much less likely that you will get lost than if you aim directly for a distant feature.

Aiming Off

If your next objective, whether your final destination or a point on the way, lies on a linear feature such as a stream, ridge, path, or road and you take a bearing directly on it you have a problem if you reach the feature and cannot see your destination. Which way do you go? To avoid this happening you can ‘aim off’, another orienteering term. This means deliberately taking a bearing to one side or other of your objective. Then when you reach the feature you know which way to turn to get where you want to be.

Compass Problems

I was originally taught always to trust the compass. Then I found myself descending the wrong side of a mountain (Ben More on the island of Mull) in thick mist despite following the compass. The reason, I found out later, was that the summit rocks of Ben More are magnetic, causing the compass to give an incorrect bearing. There are a very few other places where this is so, such as the Cuillin hills on the Isle of Skye.

For the most part though, the advice to trust the compass is sensible. Do observe the land around too, so if you are going wrong you will notice as quickly as possible. Places where the compass will not work are usually very small so taking a few bearings and averaging them out may give you at least a rough idea as to the direction to go. Checking the compass regularly is worthwhile too. If I had looked at it a short while after leaving the top of Ben More I would have seen I was going the wrong way. Guide books usually give information about problem areas. Metal objects can also cause the compass needle to deviate from magnetic north so it is wise to take bearings away from watches, ice axes, and anything else metallic.

Air bubbles sometimes form in compasses due to changes in atmospheric pressure. This occurs most commonly at heights above 2000 feet (600m). It is not a problem and the bubbles usually soon disappear. If they do not and start to get bigger it could be that the housing has sprung a leak. This can be repaired by the maker but it is probably only worth doing this if you have an expensive model. Otherwise it is cheaper to buy a new compass.

If you travel widely you may find that in some countries your compass needle dips and presses against the housing so it cannot be used. This is called inclination. The earth has five magnetic zones running roughly parallel to the equator and compasses are balanced for one of these. The further you go from the zone for which your compass is balanced the more likely you are to have problems. Compasses balanced for the latitude zones in which the British Isles are located might still work fine in Alaska and Siberia but not in Australia or New Zealand where a differently balanced compass will be needed.

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