Section 5 EO M390.05 – DETERMINE BEARINGS ON A MAP AND ON THE GROUND

The four cardinal points of the compass, measured at right angles clockwise from north are:

The four inter-cardinal points are located halfway between each of the cardinal points. Measured clockwise from north, they are:

The eight intermediate points are located halfway between each cardinal point and inter-cardinal point. Measured clockwise from north, they are:

As an aid to remember the different types of points: • cardinal points are designated by one letter; • inter-cardinal points are designated by two letters; and • intermediate points are designated by three letters.

The cardinal, inter-cardinal, and intermediate points describe directions only to within one-sixteenth of a full circle. For a more precise indication of direction it is necessary to use the sub-divisions of the circle called degrees. This measurement starts and ends at north (top) and is measured in a clockwise rotation.

Degrees. The most common method of dividing a circle is by degrees. These degrees represent 360 equal angles in a complete circle and they are represented by the symbol “°” (eg, 222°).

It is important to emphasize that degrees should always be measured clockwise and always using north as the start point.

The relationship between the three norths, especially grid and magnetic, is key to using a compass on both a map and on the ground.

True North. True north is located at the top of the earth where the geographic North Pole is located, where all lines of longitude meet. In the declination diagram on the map, true north is represented by the symbol of a star, which represents the North Star, Polaris.

Grid North. Grid north is the north indicated by the grid lines (eastings) on a topographical map. The easting lines run parallel to each other and will never meet at the geographic North Pole; because of this, grid north points off slightly from true north. In the declination diagram on the map, grid north is represented by a square, which represents a map grid.

Magnetic North. Magnetic north is the location of the north magnetic pole, where the Earth’s magnetic field bends back into the Earth toward the south magnetic pole. It is located in the Canadian arctic and is different from true north. It is the direction in which the compass needle points. In the declination diagram on the map, magnetic north is represented by a needle as on a compass.

The differences between the three norths affect navigation for the map and compass user, in the form of magnetic declination. Magnetic declination is the difference in bearing either between true north and magnetic north or between grid north and magnetic north.

Cadets will normally use the magnetic declination value between grid north and magnetic north when navigating using a map and compass. By setting the magnetic declination on the compass, magnetic bearings are converted to grid bearings which allow bearings taken from the map to be used on the ground and vice versa.

Bearing. A bearing is an angle that is measured clockwise, from north. It is measured in degrees and is relative to the observer.

In geometry, an angle is based on three points; a vertex, and two points, each of which designates a ray. For a bearing, the vertex is the point where the bearing is taken from, another point is north, and the last point is where the bearing is directed to. The north (either true, grid or magnetic) used identifies the type of bearing. In ground navigation, one ray of the angle points north (usually grid north) and the other ray, known as a plotting ray, points to the object/direction.

Director Cadets 3, 2008, Ottawa, ON: Department of National Defence

Figure 3  Types of Bearings

True Bearings. A true bearing is a bearing measured from true north. While map users rarely use them, directions determined using the sun, moon and stars are true bearings. Global Positioning System (GPS) receivers also use true bearings.

Grid Bearings. A grid bearing is a bearing measured from grid north. The ability to determine a bearing from a map allows a map user to plan routes or activities before going into the field, and allows an easy method of communicating information about movement or location.

Magnetic Bearings. A magnetic bearing is measured from magnetic north and is measured using a compass, which either has no option of setting magnetic declination or has the magnetic declination set to zero. A magnetic bearing is a quick and efficient method of describing a route when a map is not being used.

If a compass has its declination set to zero, bearings to objects on the ground determined by that compass are magnetic bearings. Setting the magnetic declination on a compass converts the magnetic bearings determined by that compass into grid bearings for the map being used.

Back Bearing. A back bearing is a bearing that is in exactly the opposite direction of the bearing that has been measured. A back bearing can be useful for different reasons: to return to the start location after a hike, or to calculate the bearing from an object to one’s current location. The steps to calculate a back bearing are:

The ability to determine a bearing from a map allows cadets to plan routes or activities before going into the field, and allows an easy method of communicating information about movement or location. When a compass is adjusted to compensate for magnetic declination, it will allow bearings taken on the map to be used on the ground and vice versa.

For this skill lesson, it is recommended that instruction take the following format: (1) Explain and demonstrate the complete skill while cadets observe. (2) Explain and demonstrate each step required to complete the skill. Monitor cadets as they imitate each step. (3) Monitor the cadets’ performance as they practice the complete skill. Note: Assistant instructors may be employed to monitor cadet performance.

Director Cadets 3, 2007, Ottawa, ON: Department of National Defence

Figure 4  Measuring a Bearing on a Map

A cadet will accidentally measure a back bearing if they determine the bearing in the wrong direction (eg, bearing from point B to point A instead of point A to point B). Prior to measuring a bearing on a map it is good practice to first estimate the bearing by drawing a compass rose and looking at where the bearing would be on the compass rose. This serves as a good check to ensure the cadet has not accidentally measured the back bearing.

To measure a bearing on a map:

1.Set the predetermined declination on the compass.

2.Identify and mark the start (point A) and finish (point B) points on a map.

3.Draw a plotting ray from point A to point B.

4.Lay the fully opened compass with the edge of the compass base plate along the plotting ray, and the sighting arrow pointed in the direction of travel (point A to point B).

5.Hold the compass in place, rotate the compass dial so that the compass meridian lines align with the easting lines on the map, ensuring north on the dial indicates north on the map.

6.Read the number on the compass dial at the luminous index pointer.

If the bearing is taken from point B to point A, the compass will be pointing 180 degrees in exactly the opposite direction of travel wanted. This is called a back bearing.

Have the cadets practice determining bearings on a map from the list created (before the lesson) from the topographical map of the exercise area.

A compass can be used to determine the bearing for a direction of travel and from one’s current location to a prominent object. The ability to take a bearing of a prominent object also allows the cadet to look for a prominent object as a steering point when they need to follow a given bearing. A bearing is a quick and accurate method for describing the direction of travel.

A prominent object is something that is usually tall and easily recognizable (eg, church steeple, tall tree or hilltop).

Director Cadets 3, 2008, Ottawa, ON: Department of National Defence

Figure 5  Determining a Bearing

After the cadets have demonstrated the skill, have them practice determining the bearings of other prominent objects. This location should be predetermined by the recce IAW the pre-lesson instructions.

To determine the bearing of a prominent object:

1.Set the predetermined declination on the compass.

2.Hold the compass at eye level and at arm’s length, and turn to face the prominent object (as illustrated in Figure 18-5-5).

3.Aim at the object using the compass sight, ensuring the sighting line is in line with the index pointer.

4.Adjust the compass cover so the compass dial is seen in the sighting mirror.

5.Look in the mirror and turn the compass dial until the magnetic needle is over the orienting arrow (put the red in the bed).

6.Read the number on the compass dial at the luminous index pointer.

Inform the cadets that when taking a bearing of a prominent object they will get different readings than other cadets unless they are all using the same line of sight to that prominent object (eg, standing in the same spot).