Altimeters. Other Navigation Tools

Transcription

1 Other Navigation Tools Altimeters An altimeter can also be a useful navigation tool. An altimeter measures the local atmospheric pressure of the air just like a barometer. This is usually expressed in millibars of mercury. The altimeter displays the current altitude on a dial with a needle or with a digital display. Since atmospheric pressure is constantly changing due to weather, you must calibrate the altimeter by first setting it when you are at a known elevation. Say you arrive at the trail head parking lot which the map indicates is at 730 meters. Set your altimeter for 730 meters. As you hike the altimeter shows the current altitude as your elevation increase or decreases. In order to maintain accurate readings you should recalibrate your altimeter several times each day. One good trick is to recalibrate or at least look at your altimeter reading before you go to bed. If the altimeter reads higher the next day, then the atmospheric pressure has gone up during the night (typically indicating stable or improving weather). If the altimeter reads lower, then the atmospheric pressure is falling (indicating potential stormy weather). You can use your altimeter in navigation as another information source to help locate your position. If the altimeter is properly calibrated, you know that you are at a specific altitude. Think of this altitude as corresponding to a particular contour line on your map. This may be enough to give you a very accurate fix on your location. If you are hiking up a trail and it crosses a particular altitude (contour line) at only one point, then you know exactly where you are. In other situations, you know that you are somewhere along a contour line that lies at that altitude (elevation). Other clues may help pin down exactly where along that contour line you are. Inexpensive altimeters are available for under $50 and are also prone to inaccuracies due to temperature. To minimize temperature problems it is best to let your altimeter adjust to the ambient air temperature before taking a reading. More expensive altimeters that automatically correct for temperature changes can run over $200. A digital watch with an altimeter/barometer is an item that combines two useful tools. Watches Even if you don t wear a watch, it s a good idea to take one when you are going to remote areas. There are too many times when a watch is handy. For example, to get an accurate check on how fast you are walking to see if your Time Control Plan is correct, and especially in first aid and emergency situations where timing vital signs and knowing the exact time that things are occurring may be essential in proper diagnosis and treatment. Watches can also be used to determine basic direction (see finding directions without a compass). Page 52

2 Finding the Directions Without a Compass You are lost really lost. Standing in the middle of nowhere, and you have no idea where to go. If you are really in trouble, remember two things first of all: stay calm, think rationally, and you can survive a long time without food. What you need is to drink. Further thoughts about extreme survival skills is beyond the scope of this module, seek advice elsewhere. This section deals with the situation of finding your way, without the aid of a compass. What you have, is the sun, the stars, and the nature around you. For a start, it may be a good idea to climb a hill, and get a good look around. Try to see traces of human activity. If you see nothing, you should try to figure out in what direction would be the best to travel. If you haven't got a map, try to draw one if you can of the terrain in front of you, and try to mark off where north is, using the methods below. If you have got a map, try to determine where you are. Remember, you don't want to climb more hills than you have to. Also you should carefully consider not climbing if you are very tired. In that case you should consider staying where you are. Consult other sources for information on how to make it easy for rescuers. Let us start with the most accurate method. This method requires that you have a pretty clear sky, though, and takes a lot of time. One of the advantages is that you don't need any equipment. You would need a straight pole about 1 meter long, two small sticks or rocks, another stick (or rock) that needs to be a little sharp, and something that can act as a string. In the morning, at least before noon, the trick starts. Stick the long pole in the ground, upright. The ground around the pole needs to be horizontal. Now, you can place one of the small sticks in the ground exactly where the shadow of the pole ends, like on the figure. Then tie the string to the base of the pole, and the small sharp stick to the other end so Figure 3.15a Using the sun to locate North that when the string is stretched it reaches exactly to the small stick embedded in the soil. Then, scratch half a circle in the soil with your sharp stick, and wait until the evening. During the day, the shadow will get shorter and shorter, until noon, when it gets longer again. At noon, when the shadow is at its shortest, you may want to mark the point. The shadow is now pointing south (if you are south of 22.5 south). It is however not very easy to see exactly when this is, but it is Page 53

3 useful anyway. Finally, the shadow reaches your circle again, and when it does, place your other small stick at the spot where the shadow ends. If you haven't got a string, you could use a pole that has the right length, or try to come up with some other improvised solution. Just make sure what you draw is a circle. Now, the line from the first stick to the second is west-east, like on the figure. Actually, you may want to mark points regularly, because any two points that have exactly the same distance from the base of the pole will give the East-West line. If it is partly cloudy, this may be a good idea. Figure 3.15b Using the sun to locate North There is a short, fast version of this one as well. This is only approximate, though, and the further away from the equator you get, the more inaccurate it is. You don't need the sharp stick and the string. Just wait 20 minutes between placing each of the sticks, and the line between the two sticks will be approximately east-west, like on the figure. Often, you wouldn't need anything more accurate. At night, you can navigate after the stars. You should, however, be careful with walking, it is easy to stumble and fall and get injured, and also easy to lose sight of the stars as you go, and you might start going around in circles. Often it will also be more physically and mentally demanding. In the southern hemisphere, you have to find the Southern Cross and the two nearby stars known as the Pointers. If you are South of Latitude 33º the Southern Cross never sets, but revolves around a point known as the celestial South Pole. To find this extrapolate the main axis of the Southern Cross, then Figure 3.16 Using the stars to locate South (from Aust. Army Navigation Manual) Page 54

4 bisect the line that joins the two pointers and extend it until it cross the line extrapolated from the Southern Cross. This points the way south as this point is the celestial South Pole. In the northern hemisphere, there is a star that is almost exactly in the north at all times, the Polaris or North Star. It is always pointing north. If you have an analog wrist watch, you can use the time to find north. If you are south of the Tropic of Capricorn (23.5ºS) hold your watch up in front of you, and point the 12 on your watch at the sun. Now bisect the angle between the hour hand and the 12 in two and you have true north. (The reason you need to cut it in two, is because the clock takes two rotations while the sun takes one around the earth, it is of course the other way around, but never mind!). When north of the Tropic of Cancer (northern hemisphere) you can obtain a south bearing by reversing the process. If you a wear digital watch draw an analog watch face on a piece of paper, and then mark the hour hand on using the digital watch. The rest of the method is identical. This method can be used even when it is pretty foggy. Although you may not be able to see the sun, it may still cast a shadow. Use a straw or a small stick, to see a shadow. You just have to Figure 3.17 Using a watch to locate North (from Aust. Army Navigation Manual) remember that the shadow points the opposite way from the sun, but the rest of it is quite similar as above. Page 55

5 What if there is no shadow? Then, there are a few methods based on natural signs. It is very much about trees. First of all, there will be fewer branches to the south (in the southern hemisphere) or the opposite for the northern hemisphere. This is usually easiest to see if you look up along the trunk of the tree. The south face of the tree would be more humid than the north face, which is something most species of lichen (or moss) likes, and consequently, there will be more of it on the south face. On the image above, you can also see that ants like to build their nests on the north side of the tree. If you are in alpine terrain, it is also worthwhile to look at how snow melts. In the spring in the mountains, snow will melt faster on the north face of rocks, or in north faced slopes. Also, vegetation and undergrowth will typically be thicker on the north facing slopes, and also fruits ripen earlier on the north facing slopes. You should use great care with these methods as they are not very reliable. Winds may alter the average conditions significantly, and cause deviations. If you use natural signs, you should use as many signs as you can before you draw a conclusion. Navigation Tip Orient Your Map: You can eliminate the need to correct for declination if you use your compass to orient the map each time. As long as the map is oriented with respect to magnetic north, any bearings you take from map to compass or compass to map will be the same. For this reason, it's a good idea to always take the time to orient your map. It will make your life much easier. It also means that each time you use your map, your will need to re-orient it with your compass. Page 56

6 Test Your Knowledge Part 1. Use the map of Port Moresby (Map 2) to answer the following: Give the compass bearing of the following points from Moitaka Agricultural Station (GR ) 1. Sabusa Sawmill (227697) 2. The car wrecking yard (172590) 3. The sewage treatment plant (180638) 4. The oval (171535) 5. Bomana war cemetery (275612) 6. Koke (187517) Part 2. The diagram on the following page shows the movement of a tank over time. The navigator seems a little confused and has taken it in many different directions. He seems to have passed out the end of turn 5 and the driver has taken several more turns that need to be reported to HQ. Use a protractor (or compass)on the diagram below to give the bearings for the final legs of the course. Remember North is 0º (so you should put the 0 of the protractor on N for the calculation). Bearing 7 Bearing 8 Bearing 9 Bearing 10 Bearing 11 Page 57

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8 Practical Exercise 2 Using a Compass In this exercise you will be required to use a field compass to find objects located around block E. You will need to go to either the front or back door and stand in the centre to take the first bearing. From there you will locate and note the objects which lie directly on that bearing (or report the bearing of a specified object). You should report your results in the table on the next page.

9 Reference Point Course (bearings in degrees magnetic) A B C D E Start Front Door Front Door Back Door Back Door Front Door Bearing Object Bearing Object Bearing Object Bearing Object Bearing Object White post and tap 272 Student services doorway 2 Block B sign Emergency evac point Final Bearing to Block E doorway Page 60

10 References 1. Much of this material was sourced from Rick Curtis, Copyright 2002, Outdoor Action Program, Princeton University. 2. Australian Army Manual of Land Warfare. 3. Kjetil Kjernsmo's illustrated guide on How to use a compass, kjetikj@astro.uio.no