From this point you will use a map to navigate and convert the position marked on the map to a bearing on which to travel. With knowledge of how to use a GPS and compass, how to convert bearings taken from the compass and plot them on a map you have all the power at your fingertips to navigate with full confidence in any terrain.
Not all compasses are the same and their features will determine their versatility. For use in conjunction with a GPS as well as for regular map navigation the prismatic type compass is ideal. The prismatic compass has the card (the part that rotates, indicating bearing) enclosed in a small case with a lid. The lid consists of a frame and a window with a hair line running vertically down it, and an extension on the opposite end to the hinge. The extension is known as the tongue and has an indentation in it that runs parallel to the hair line. Below the lid is another window which is marked in degrees. It can be rotated and has a pointer. On some models it can also be locked by a thumb screw clamp on the side of the compass body.
Below this is yet another window under which is the compass card, marked off in degrees 0˚ – 360˚, together with the four cardinal points; east, south, west and a pointer indicating magnetic north. On the outer rim of the compass card, the degrees are printed in reverse so they can be viewed through the prism the correct way up. On the inner ring of the compass card, the degrees are marked off in 20˚ intervals.
A divider is used by using measurements from the linear scale printed on the map to gauge distances on the ground. Although this can be done with a simple ruler, a divider is more accurate, faster and more versatile. They can be purchased from any stationery shop. Place the left point onto a whole number on the linear scale and the right leg on a whole number to the right of zero. Then by placing the left point on a place on the map, quick distance calculations can be made. To the left of zero on the linear scale, the distance is divided into fractions. These fractions are used in the same way.
Topographical maps are referred to in this chapter as they are the most useful type of map for ground navigation. Topographical maps are drawn from stereo aerial photographs. They represent an area’s topography, or the physical features of an area.
The scale of a map is the ratio between the distance represented on a map and the horizontal distance between the same two points on the ground.
The most common scale of topographical maps of Southern African is 1:50 000. The entire map normally represents a square measuring 25 X 25 kilometers. This totals 625 square kilometers. Maps are also available at a scale of 1:250 000, but these will lack some detail that may be of use to the ground navigator.
What the scale represents
For example, 1:50 000 means that for every 1mm represented on the map, 50 000 mm is represented on the ground. No matter which measurement system you are using, the same applies; for every 1 inch represented on the map, 50 000 inches is represented on the ground. This scale is also called the representative fraction, and in this case it is 1/50 000. With a 1:50 000 scale map, 2 centimeters represents 1 kilometer. This is obvious if one considers the calculation; 2cms = 20mm. 20 X 50 000 = 100 000 or 1000 meters = 1 kilometer. Don’t let this confuse you – just remember that 2 centimeters represents one kilometer on a 1:50 000 map.
The scale of a map is of great importance to the navigator. If you are working in a small area of ten kilometers, then a small scale map will be of greater use, because the smaller the scale, the more detailed the map will be. If you are working in a large area, for example 300 kilometers, then a larger scale map will be of more use as more area will be represented on the same map. Heights on a map are represented by contour lines. On a 1:50 000 map, they are normally drawn at intervals representing 20 meters. Intervals in feet are drawn on older maps. This interval will be stated in the map key or scale. Orthophoto maps are available for some areas, and are particularly useful. These are prints of aerial photographs with the contour lines over-printed. They combine the advantages of photographs and topographical maps.
Using the compass
Taking a bearing:
• Bearing: the angle measured clockwise from True North, Magnetic North or Grid North.
• Open the lid to a vertical position, the hair line running
• Fold the prism over so that it lies flat on the compass window. Place your thumb in the ring and hold the viewing prism up to your eye, supporting the compass with your forefinger. The compass must be held as horizontally as possible.
• Swing around and view the object on which you wish to take a bearing.
• Line up the hair line to the exact point on the landscape, and let the line cut through it.
• Cast your eye downward. You will see that the hair line also cuts through numbers written on the compass card. When the compass card has come to rest, read off the number. This number is the magnetic bearing of that object.
A magnetic compass can give a false reading if it is placed in the following places:
• Inside a vehicle.
• Close to a vehicle. Walk 20 meters away if it is a light car or truck, and 60 meters away if it is a large truck.
• Electrical power cables. Move at least 40 meters away.
• Spectacles and jewelery made from steel or other magnetic material.
If you are in any doubt that a reading may be inaccurate due to external influences, take more than one bearing. Walk some distance away from or towards the object on which you are taking a bearing, and the reading should be the same. If it is not, then you know that one of the readings is false. To confirm which one is false, you must then take a third bearing. If all three are different, then some common magnetic source is affecting all your readings and you should move a considerable distance away and start the process again. This may occur if you are in an area of rocks containing large deposits of magnetic material.
The three north’s
When working with bearings and maps it is essential to know about the three Norths: magnetic north, true north and grid north.
This is the direction to which the compass card pointer will always point. It is the direction on a map that is clearly marked ‘Magnetic North’. It is also the bearing which a GPS receiver will display, although true north can be selected in the GPS set-up menu.
True north never changes. The North Pole is the most northerly point and is situated at 0˚ true north.
Maps are covered with lines, running both vertically and horizontally, dividing the map into squares. In fact an entire country is divided up into squares on a grid system. It follows that if these squares are in fact square, and the earth is round, not all of the vertical lines will point to true north. (Remember a map is a three dimensional area represented in two dimensions). The difference between grid north and true north is very slight, so for practical purposes they will be regarded as the same.
The three bearings
Because of the three north’s, there are three types of bearing:
This is the direction in which the compass card pointer will read and relates to magnetic north.
This is measured on a map with a protractor.
Because of the curvature of the earth, the grid lines on a map do not always point to true North. For practical purposes we shall regard the grid bearing and the true bearing to be the same.
The magnetic variation is the difference between the north measured by a magnetic compass and true or grid north plotted on maps. This difference is written as the magnetic variation on all maps with a workable grid. For the exercises on the left it is 12˚ west. This means that whatever grid/true bearing you have measured on your map, will be 12˚ more or less than the magnetic bearing.
Plotting bearings on a map
The conversion of bearings is necessary if you wish to use your map, compass and GPS together. Another important tool to the navigator, the protractor, a link between the compass and the map, will be used. It enables a bearing taken in the field to be plotted on a map, or a bearing taken on a map to be measured and then used in the field to find your way with the aid of a compass or GPS.
To plot a bearing on a map:
• Draw a pencil line between two positions on a map from which you wish to take a bearing.
• Place the protractor on the map so that the base line is absolutely parallel to the grid lines on the map. If the bearing to be measured is between 0˚ and 180˚, place the protractor to the right of the
point on the map and if the bearing to be measured is between
181 and 360˚, place the protractor to the left of the point on the map as follows:
• Place the zero edge, or base line (from where the degrees marked is zero) over the pencil line so that it precisely cuts through it. It can be placed anywhere along the plotted line.
• Read off the degrees from the degrees scale on the protractor. This is the grid bearing from one point to the other.
If this bearing is now going to be used to travel to an object, it must first be converted from the grid bearing taken to a magnetic bearing so that a compass can be used to follow it. In the case of navigating with a GPS, the two positions can be stored as way points and the GPS will do the rest.