Basic Use of the Magnetic Compass 11/20/2018 Land Navigation Steven Hoober (shoobe01@gmail.com) October, 2006 Basic Use of the Magnetic Compass 1 1

11/20/2018 What is a Compass? The current, hand-held magnetic compass is two different things: A north-seeking device from which you can determine direction of travel, and (with the aid of built-in scales) the relative locations of items within view. Some of the features on the compass have nothing to do with this function. An instrument to help you plot onto maps your position from observations, and to plan routes. Some of the features on the compass have nothing to do with this function. 2 2 2

11/20/2018 Basic Terminology Bearing is the direction to a target item or destination. Azimuth has essentially the same meaning. Heading is the direction you are travelling (the way you are heading). Magnetic North or MN points towards the magnetic north pole. True North, sometimes TN or denoted with a star points towards the geographic north pole at the center of the map sheet. Grid North or GN is the direction in which the north/south grid points at the center of the map sheet. The capsule is the container where the needle floats. A graduation ring around the edge of the capsule denotes direction. 3 3 3

11/20/2018 Declination Declination is the deviation between magnetic north (where the needle points) and true north. To navigate based on information plotted from a map or similar system, you have to account for declination. Declination varies greatly by location. Declination is listed on maps, but changes over time so be careful with older maps. 4 4 4

Degrees, Mils & More Degrees are as usual, 360° make up a circle. 11/20/2018 Degrees, Mils & More Degrees are as usual, 360° make up a circle. Mils is short for milliradians. 6400 make up a circle*. These are common in artillery, machinegunnery and generally on military compasses. Cardinal points (“NNW,” etc.) and even colors can also be used to denote direction. Any system will work fine as long as you are the only one using it. If you work with others, you have to use the same system as them to be able to communicate. Additionally, your GPS unit can give you other information: Speed Direction Elevation Time (very accurately) It records tracks (where you have been) It can help plan routes, and store waypoints Some (especially now) have moving maps in the GPS unit * A mil is not a mil. The US (and now NATO) standard is 6400, but older Swedish compasses use 6300 mils in a circle, older UK compasses, 6280, and the Russians 6000. Be careful if using surplus, foreign or captured equipment in a shared working environment. If you are wondering, a radian is mathematically (geometrically) interesting. Its the angle in which the radius is equal to the curved length of the arc. A “real” milliradian puts 6283 milliradians in a circle. 5 5 5

Baseplate Compass: Sighting 11/20/2018 Baseplate Compass: Sighting This applies to any baseplate (usually, clear baseplate), protractor or Silva 1-2-3 style compass. Hold the compass level, in front of you, so you can see sort of across and down into the capsule. You will need to see the needle and the north mark on the graduation ring. Turn yourself until the red end of the needle points exactly north (360°, 0, etc.) on the graduation ring. Most clear basepate compasses also have a red arrow in the bottom of the capsule you can use for this alignment. To use this in conjunction with a map, read the bearing to the target off the top of the graduation ring. t Additionally, your GPS unit can give you other information: Speed Direction Elevation Time (very accurately) It records tracks (where you have been) It can help plan routes, and store waypoints Some (especially now) have moving maps in the GPS unit 6 6 6

Baseplate Compass: Plotting 11/20/2018 Baseplate Compass: Plotting Place compass on your map with the edge along your desired line of travel. Rotate the capsule until North on the graduation ring point towards North on the map (you may need to draw additional faint pencil line on the map more frequently than they are printed). The capsule base will have lines in it to help with this. The compass is now set up for navigating to a destination. A compass already sighted on a target can be used in a reverse procedure to determine (or confirm) your location on a map. Additionally, your GPS unit can give you other information: Speed Direction Elevation Time (very accurately) It records tracks (where you have been) It can help plan routes, and store waypoints Some (especially now) have moving maps in the GPS unit 7 7 7

Baseplate Compass: Travelling 11/20/2018 Baseplate Compass: Travelling If you are travelling to a location you just sighted, the compass is already setup correctly. Otherwise, you will have set the compass up during the plotting phase. Keep the needle pointing towards north to know the bearing to your destination. The compass can be referred to by holding it against your chest (center hold method). The compass body will then be pointing the way you are; just glance down to see which way you are heading. Line up the needle and north mark, as when sighting, to find a reasonably nearby feature on your path like a tree or distinctive hill. Walk to there (as convenient), then take a bearing to another point and walk there. Repeat until you reach your destination. Additionally, your GPS unit can give you other information: Speed Direction Elevation Time (very accurately) It records tracks (where you have been) It can help plan routes, and store waypoints Some (especially now) have moving maps in the GPS unit 8 8 8

Lensatic Compass: Sighting 11/20/2018 Lensatic Compass: Sighting The US lensatic compass can be quickly sighted over the top in the same manner as a baseplate compass. For more precision, place your thumb thru the loop, fold the lens forward and the cover somewhat rear, and hold the assembly against your cheek, as in the illustration. Use the notch above the lens with the wire in the lid to sight on a target. Lock the bearing by folding the sight forward. You may then read the bearing directly, or lock it in for further navigationby twisting a bezel on top of the capsule. A tritium line on the glass is aligned with the north needle. Some specific pitfalls about using a GPS for land navigation Precision vs. Accuracy The device will always display your location down to the meter (or the foot, or better). That is high precision But it always shows this, even if it only /really/ knows where it is within a 100 ft circle This 100 ft part is the accuracy of the unit. Your unit will usually tell you how accurate it is; remember this, and do not be mislead into thinking precision equals accuracy 9 9 9

Lensatic Compass: Plotting 11/20/2018 Lensatic Compass: Plotting The compass is not used for plotting on the map; a clear card device called a roamer or protractor is used instead. To avoid drawing lines on maps, or carrying another straightedge, attach a thin string to the center of the protractor. The line can be drawn from the center to the target to determine angles, which are read off the edge of the card. Both degrees and mils are included. Small triangle within the card provide additional precision within grid squares for various map scales. Some specific pitfalls about using a GPS for land navigation Precision vs. Accuracy The device will always display your location down to the meter (or the foot, or better). That is high precision But it always shows this, even if it only /really/ knows where it is within a 100 ft circle This 100 ft part is the accuracy of the unit. Your unit will usually tell you how accurate it is; remember this, and do not be mislead into thinking precision equals accuracy 10 10 10

Lensatic Compass: Travelling 11/20/2018 Lensatic Compass: Travelling When travelling, its best to use the lensatic much like the baseplate compass; hold the compass against your chest, so you can glance down and see the capsule and alignment. If the needle and the luminescent indicator is aligned, you are on course. If your waypoints are very far apart, and there is time to stop, it may be valuable to use the lens/lid sighting method to more precisely pick the best target. Some specific pitfalls about using a GPS for land navigation Precision vs. Accuracy The device will always display your location down to the meter (or the foot, or better). That is high precision But it always shows this, even if it only /really/ knows where it is within a 100 ft circle This 100 ft part is the accuracy of the unit. Your unit will usually tell you how accurate it is; remember this, and do not be mislead into thinking precision equals accuracy 11 11 11