The compass and its parts1:
Compass needle: Points towards magnetic North.
Simple navigation without a map:
The compass housing is marked with the numbers 0 to 360. These numbers represent direction in degrees (°). North is considered to be at 0° (or 360°), East is at 90°, and so on.
Let's say you would like to go in a certain direction, East for example. Turn the compass housing so that the 90° number (or the E) lines up with the direction of travel arrow. Next, hold the compass out in front of you, with the direction of travel arrow pointing straight ahead (away from you). Now, turn yourself until the North end of the compass needle is lined up with (or on top of) the orienteering arrow. Look where the compass's direction of travel arrow is pointing. This is where you want to go! Finally, just walk in a straight line following your compass's direction of travel arrow. You are walking East (magnetic East that is)! Your direction of travel (East or 90°) is also called your bearing.
True North versus Magnetic North:
True North is what is commonly thought of as the North Pole. Looking at a globe, true North would be at the very top, in the middle of an icy continent. True North never changes over time.
Magnetic North is more complicated. The Earth is not a simple solid magnet. Scientists believe that the Earth's magnetic field comes from its liquid core. The liquid core moves and generates electrical currents, and these currents create the Earth's magnetic field. Since the core is liquid, the core itself and its electrical currents change over time. This causes the Earth's magnetic field to change slowly, and we can observe this change by looking at historical positions of Magnetic North.
The North magnetic pole is in Canada! Early explorers started measuring
the position of the pole in 1831. In recent years, Canadian Government
scientists have measured the position of the pole every few years. The
maps below show how much the pole has moved 2.
Maps and Charts:
Maps and charts always use true North as their reference point. All of the latitude and longitude lines (maps and charts use lines of latitude and longitude to give us a way to express location) on a map or chart are drawn in relation to true North. The longitude lines (which are drawn North-South) all converge at the true North Pole.
So what good is a magnetic compass? The problem is magnetic compasses always point to magnetic North, while maps and charts always use true North. A bearing measured on a chart won't be the same as a bearing measured by a magnetic compass.
Fortunately, maps and charts provide us with a correction factor called magnetic declination (or sometimes called magnetic variation). The correction factor is simply added (or subtracted) to a bearing to convert the bearing from degrees magnetic to degrees true, or from degrees true to degrees magnetic.
Declination on Charts:
Charts intended for navigation will show the declination in different ways. Some charts use isogonic lines to show the amount of declination in a certain area. Isogonic lines join points on the map having the same declination. The following map shows the isogonic lines in Canada 3.
Other charts use a simpler way to show declination. The chart has two compasses drawn on top of each other. In the two-compass drawing, the North arrow on one compass points to true North, while the North arrow of the other compass points to magnetic North.
Some charts might not show any declination information. If
you are using one of these charts, you should find out the declination for
the area before you begin your trip.
Near the city of Burlington, in 2002, the declination is about 10°W. This means that if we stand in Burlington and look towards the true North Pole, magnetic North would appear to be 10°W (or 10° to the left) of where we were looking. The drawing below shows how we would see true North and magnetic North if we were standing in Burlington.
This means that to make use of a true bearing measured from a map, we must add 10° to it before we can follow the bearing with a compass. There is an easy way to remember this fact:
Declination West, Magnetic Best.
This saying reminds us that whenever the declination is West (as is the case for our area around Burlington), the magnetic bearing will always be larger than the true (chart) bearing. So, to convert from a true (chart) bearing, to a magnetic bearing, we always ADD 10°.
To convert a magnetic compass bearing to a true bearing (since charts use true North), we must remember that the magnetic bearing is always the largest. We would therefore SUBTRACT 10° from the compass bearing, and this would give us the bearing to be used on the chart.
This page, and all contents, Copyright © 2002 Scouts Canada, 15th Burlington and 2nd Strathcona Scouting Group, except where source noted.
Select graphics courtesy of the Web Diner
Photo Credit: Scouter Bill Kowalchyk Material Source Credits:
1 http://www.learn-orienteering.org/ 2 http://gsc.nrcan.gc.ca/geomag/nmp/long_mvt_nmp_e.php 3 http://gsc.nrcan.gc.ca/geomag/field/magdec_e.php
Page last revised Monday September 10, 2012.