A compass is an instrument that is used for navigation and mapping because it measures the geographic direction between two points. It is a fairly simple instrument that uses a magnet, mounted on a pivot that turns in response to the earth’s magnetic field, to determine direction (but not position). The magnetic needle points to the magnetic North Pole, which is different from geographic North Pole. A compass bearing, which is typically expressed as an angle (degrees), refers to the horizontal direction to or from any point. Τhe term “bearing” is used interchangeably with the term “azimuth” and practically they are the same. A compass is used for several different purposes such as:
•Determine direction to a destination or a landmark.
•Stay on a straight course to a destination or a landmark, even if you cannot see it.
•Avoid obstacles in the path to the destination or the landmark.
•Return to your starting point.
•Pinpoint locations on the map and in the field.
•Identify what you are looking at in the field or on the map.
•Orient a map.
•Mark points on a map.
•Plot the route of a hike on the map.
There are two major categories of compasses:
• Compasses with movable dial (Ranger type), as in Fig. 3, in which the magnetic needle is integrated in the dial. They have a “sight” so they are more accurate in measuring azimuth and we read the indication immediately. To draw an azimuth on the map, you must first orient the map.
• Compasses with manual dial (Silva type), as in Fig. 1. They are easier to use, cheaper, but need more familiarity with their use.
Which compass is GOOD one? The basic criterion of a good compass is that when you turn it in another direction, its needle stabilizes very quickly. Therefore keychain compasses, advertising compasses, built-in trekking poles compasses etc , should be avoided.
Parts of a Compass
Not all compasses include each of these parts and some compasses include even more.
- Baseplate. The hard, flat surface on which the rest of the compass is mounted. It has a ruler on its edges for measuring distances on maps. Its edge is straight and useful for laying lines on a map
- Scales. Εach edge of a compass may have different rulers for use with different map scales
- Direction-of-Travel Arrow (marked on the base plate). Points the way you will be traveling
- Magnifier. For seeing small map features better
- Index Pointer (the butt end of the direction-of-travel arrow). It ends right at the edge of the dial and is where you take degree readings
- Dial. The ring around the housing that has degree markings engraved. You hold the dial and rotate it to rotate the entire housing
- Declination Marks. Used to orient the compass in an area with known declination
- Orienting Arrow. Marked on the floor of the housing. It rotates with the housing when the dial is turned. You use it to orient a compass to a map and to adjust the declination.
- Orienting Lines. The series of parallel lines marked on the floor of the housing and on the base plate
- Needle . The magnetized piece of metal that has one end painted red to indicate North. It sits on a fine point that is nearly frictionless so it rotates freely when the compass is held fairly level and steady
- Housing. The main part of the compass. It is a round plastic container filled with liquid and has the compass needle inside
- Bubble . A bubble of air in the housing liquid. Ιτ is useful for making sure you are holding the compass fairly level
- Mirror. The mirror lets you see the compass face and distant objects at the same time. Useful for emergency signaling (Fig. 2)
- Sight. It improves your compass aiming at distant objects (Fig. 3)
A compass can be used in many ways, for example telling which way is the North or following an unmarked path over wilderness terrain. But, you’ve got to know how to read its indications. So let’s start about how a compass is laid out.
There are four cardinal points on a compass – North, South, East, and West. When reading a compass, and telling other people directions, you need to wipe “right” and “left” out of your vocabulary. Right and Left are relative directions and differ depending on your location and direction, but the cardinal points are always constant.
The direction halfway between North and East is an intercardinal point and is called NorthEast (ΝΕ) and the secondary intercardinal point between North and NorthEast is called North-NorthEast (NNE). By following the same way, you can name all the others primary and secondary intercardinal points.
Since there is a need for more precise directions, in the field, the circle of the compass is split into 360 degrees. For general directions, use the names North or NorthWest. But, for finding your way or locating destinations in the wild, use degrees as you’ll see in a bit.
Two North Poles
There are actually two north poles – the Geographic North Pole which is the axis around which the earth spins, and the Magnetic Νorth Pole which is where the compass needle is pointing.
The difference between the north geographic pole and the north magnetic pole is called magnetic declination or usually just declination and the value is in degrees. Depending on where you are on the earth, the angle of declination will be different. In some locations, the geographic and magnetic poles are almost aligned so declination is minimal, but in other spots, the angle between the two poles is pretty big. The compass, as we said, shows the magnetic North while the map shows the geographical North and if, for example, the declination is 4ο degrees, at a distance of 10 km you can be 1 km away from your destination. Therefore, you need to be aware for the declination of the area in order to be on the right course. To make it more difficult, the rate of declination changes over time too. The declination’s value of the area you are walking is given in the legend of your map but the declination was different in 2000 and different today. So, if the map has not been printed in the last year, do not use the declination’s value that it gives, to calibrate your compass. But because you are not going to buy new maps every year to have the right declination, you can periodically check out the internet where there are sites that can give you the information you need. The site that we are using is www.magnetic-declination.com
Compass declination adjustment
On many compasses, you are able to adjust the declination by twisting a ring, using a screw, or some other method of changing. You must read the manual to find how to do that. The target is to put the compass needle, which shows the magnetic North, inside the orienting arrow.
By adjusting the compass to match the declination on our map (if it is a recent one) or the value we read in the internet, the orienting arrow now appears to be offcenter from North. And this is how it should be. Now, when you put the RED in the RED (red needle inside the red orienting arrow), the North indicated at the index pointer of your compass is the true north. (Fig. 4)
Basic Compass Reading
In a compass, one end of the needle (usually the red one) always points North. To read your compass, hold it steadily in your hand, about the height of your stomach and in a way that the base plate is level and the direction-of-travel arrow is pointing straight away from you. Look down at the compass and see where the needle points. The compass in the picture below is pointing due North (0ο degrees)
Turn your body while keeping the compass in front of you. Notice that as the compass rotates (with your body) the needle stays pointing the same direction. Keep turning until the needle points to the East on the compass like the picture below, keeping the direction-of-travel arrow and North mark facing straight in front of you. The needle mow is pointing East (90ο degrees)
ΑΤΤΕΝΤΙΟΝ: This is the most common mistake! The compass needle is pointing towards East so I must be pointing East, right? ΝΟ, ΝΟ, ΝΟ !
To find my direction, I must turn the compass dial until the North mark and the “Orienting Arrow” are lined up with the red end of the needle. Then I can read the number that is at the Index Pointer spot (the butt of the direction-of-travel arrow). Since the Orienting Arrow is usually two parallel lines on the floor of the compass housing, a good thing to memorize is: THE RED IN THE RED. Now we know we are really heading West (270 degrees) as it is shown in the picture below.
By moving your compass with your body and using the N-E-S-W markings, you can have a good idea which way you are going. This is often, all you need from your compass. As, you have noticed, the dial on your compass, is divided in 360ο degrees which are representing the circle that surrounds you no matter where you are.
When you need to find your way from one particular place to another, you need to use these numbers to find out the azimuth to that remote place. The direction you are going. is called heading. Heading and azimuth are pretty much the same thing. The image above presents a compass that has an azimuth or is heading at 250ο degrees.
Compass Reading , checking and taking care Tips
- Hold the compass level. Ιf the compass is tilted, the needle will touch the clear lid or the baseplate and will not move correctly.
- Read the correct end of the needle.
- THE RED IN THE RED!
- Keep the compass away from metal objects – even a knife, flashlight, or keychain can cause a false reading if it is too close to the compass.
- Use common sense, such as knowing that if you are in North America, Europe, or Asia and heading anywhere towards the sun during the middle of the day, you are heading in a southerly direction. If you are south of the equator and heading towards the sun, it’s just the opposite and you are heading in a northerly direction. (If you are in the tropics, between the Tropic of Cancer at 23.5 degrees North of the equator and the Tropic of Capricorn at 23.5 degrees South of the equator, then this tip should not be used. The sun can be either north or south of you, depending on the time of year.)
- When transported, place it in a durable case made of thick plastic or Cordura for protection from bumps, abrasions, etc.
- DO NOT place it near metal objects. The knife, the utensils, the tent sticks are … bad company for your compass.
- At regular intervals, compare your compass readings with those of others. If you systematically notice differences then something is wrong (with your compass or with the others !!!).
- Alignment control: Locate two obvious points in the environment around you (landscape features, but they should be also in the map), e.g. a nearby chapel and a adjacent peak, which are in the same straight line as you. Measure the direction between the chapel and the peak on the map. Then measure it in the environment around you with your compass. If you notice a significant difference (eg greater than 10°), then your compass has an error.