Part one of a two-part series.
The weekend after I joined the Cub Scouts (a junior branch of the Boy Scouts of Canada), I went out and bought myself an official Boy Scout compass.
As I recall it cost me something like $3.95 at Gerlovin’s Hardware Store. I didn’t last very long in the Cubs, but I did get a fair amount of use out of the compass. I took it with me whenever I went camping, fishing or riding my bike, just in case.
While I may not have known too much about the proper use of a compass or how to use it in conjunction with a map, it did not stop me from venturing out into the great outdoors, and the fact that I am here today to write this column is proof that I found my way back.
There is, however, a certain amount of scientific knowledge required to properly read a compass. Understand the science and you are well on your way to know where you’re going.
A compass works on the principal of magnetic attraction.
Think of the needle of a compass as a bar magnet that is being attracted by the North Magnetic Pole – sort of. In actual fact, the needle of a compass aligns itself with the parallel lines of magnetism connecting the north and south magnetic poles, so that it lines up north and south, with its northern end pointed north and its southern end pointed south. The Earth’s magnetic field covers the entire earth and is generated by the movement of its iron core in relation to the crust.
The needle of a compass is marked with north and south ends – most usually with the north end painted red. There are four important points on a compass – north, south, east and west – known as cardinal points. A direction halfway between an cardinal point, say north and east would therefore be referred to as northeast. Subsequently, the three other inter-cardinal points – southeast, southeast and northwest. There are also secondary inter-cardinal points halfway between each cardinal point and inter-cardinal point. These are north-northeast, east-northeast, east-southeast, south-southeast and so on. With these points of direction, you can determine a fairly good idea of what direction you need to travel in order to reach your destination.
Because there is more often a need for precise directions, the face of a compass is further split into 360 marks called degrees, representing the circle of direction which always surrounds you. By aligning your compass with your direction of travel and using the N-S-E-W markings, you can get a good idea which direction you are facing or heading. (The direction you are travelling is called your heading – heading and bearing are essentially the same thing.)
Having said all of this, nothing is ever simple. So into the process we must now introduce magnetic declination.
Magnetic declination is the angle between magnetic north (the direction the north end of a compass needle points) and true north.
The declination is positive when the magnetic north is east of true north. Simply speaking, true north is the direction in which the north pole is located along the Earth’s rotational axis, while magnetic north is the direction toward which the compass needle points.
Magnetic declination varies both from place to place and with the passage of time. In most areas, the spatial variation reflects the irregularities of the flows deep within the earth.
(In some areas, deposits of iron ore or magnetite in the Earth’s crust may contribute strongly to the declination.)
Degrees of declination must be added or subtracted from the degree of bearing when determining precise direction, especially when using a compass in conjunction with a map.
We’ll get into all of this in next week’s column.