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There are several outdoor equipment lists telling a daring traveller what to take on a trip. One of the most comprehensive and undisputed lists is called The Ten Essentials. It was first published in the 1930s in a newsletter of an outdoor club from Seattle, called the Mountaineers. Though it has been reprinted, changed, reduced and enlarged many times, the basic Ten Essentials have been an excellent tool for outdoor teaching for more than half a century. No one has ever made a statistic of how many lives have been saved by the Ten Essentials. It is, however, not presumptuous to say that many of the people who died from hypothermia, got lost and starved or fell to death might still be alive if they had had the Ten Essentials.
The original Ten Essentials are listed below. It has to be understood that it's not sufficient just to carry them. You have to be able to use them, too. They are not a guarantee for a safe return from the hike. Not taking them isn't a guarantee for not returning safe, either. Every single one of them just increases your chances more or less, depending on the situation.
Essential One: Matches
Use only the 'strike anywhere' matches. All the strike-on-box matches work only with a dry striker strip on the box. If you lose the box or the box gets wet, you have a problem. Unfortunately it's very easy to mix them up in the stores. Always check and double check whether they will light on some other surface. Don't trust the promises of the store. If it's a good store, they will understand your mistrust. If they don't, it's better to change stores.
If you are sure you have the matches you want, place them into a watertight container. There are different waterproof cases in the outdoor stores. Don't forget to include a striker as well. A piece of grinding paper will do for that purpose.
Matches, even when stored in a water proof box, weigh almost nothing, but are one of the most useful survival tools. If you get lost out in the cold and have to wait for the next day, it's not only the heat and light, but the morale boost of your little campfire that will help you through the night.
Essential Two: Fire Starter
Sometimes a good supply of waterproof matches just isn't enough to get a fire going. Unfortunately this is most likely to be the moment when a fire is crucial for survival. So it's important to have some fire starter with you. There are different kinds of fire starters, from liquid to solid. All of them work properly if you do not misplace them! Always store some fire starter with your matches and the grinding paper in your little waterproof box, but do not store that box in your pack. If you lose your pack (which is one of the standard problems, especially when wading rivers) you're going to be in big trouble.
Essential Three: Map
A map, along with the knowledge of how to use it, is the best insurance against getting lost. You have to know how to use a compass and map. Finding out where you are with a map is called orientation and requires some training. Finding a known place on a map and going there is called navigation and requires even more training. Finding your way from one unknown place to another unknown place with or without a map is called getting lost and doesn't require any training at all.
Essential Four: Compass
When the Ten Essentials were published for the first time, back in the 1930s, nobody even dreamed of GPS1 devices. A map and a compass had to be enough. Nowadays, as those little GPS things tend to cost hardly more than nothing, a compass seems to be old fashioned. Don't dispense with it. Compared to a GPS device, a compass really costs nothing. It doesn't need batteries, and it gives you exact bearing without the need to move. The GPS, however takes over the role of a compass more and more. If you have one, learn how to use it. 'Waypoints', 'Trackback' or 'Man-Overboard' functions can save lives.
Essential Five: Flashlight, Extra Batteries and Bulb
If your time schedule has not worked out as you had planned it, you may have to find your way back to the trailhead in total darkness. The only safe way to move through an unknown terrain in darkness is on hands and knees. This slows you down significantly. Carrying a flashlight2 (or headlight, that's a matter of taste) enables you to walk your way back watching your step. It's a good signalling device, too. Some of the flashlights have a strobe mode which attracts the SAR (search and rescue) teams in a case of emergency. Don't forget extra batteries and bulbs.
Essential Six: Extra Food
Always carry some extra food with you. It's good to choose something that's really a last resort ration, like dried dog food or pemmican (pressed meat cake). Shrink-wrap it and store it in your pack and never take it out again. You can, of course, survive for some days without food. Having some food, even some disgusting food, helps to keep your thinking clear and to avoid panic - and remember, Rule Number One in a case of emergency is don't panic. If nothing goes wrong, you should always bring some of your food back from the hike. Otherwise you don't have enough of it.
Essential Seven: Extra Clothing
Even the sunniest day gets cloudy from time to time. You may be delayed due to unforeseen reasons. You may have to spend the night out. Extra clothing is the most life saving of all essentials, as hypothermia is the most frequent cause of trail deaths. Carry at least some kind of rain jacket. It keeps off both water and wind which can drain your stamina even when you're not moving any further. As the centre of the body heat regulation is in the head/neck area, always carry some kind of hat. An old but still valid rule says 'If you have cold feet, wear a hat!'.
Essential Eight: Sunglasses
Sunglasses may be dispensable when you walk through a forest area on a foggy day, but they are particularly important when walking through snow or on white sand or rocks. Snow blindness will render you absolutely helpless. You'll have to be guided out, which can set you and your party at risk.
Essential Nine: First Aid Kit
Emergencies can happen anytime and anywhere. Even the most basic first aid kit, along with the most rudimentary knowledge of how to use it, can make the difference between surviving an injury far away from professional medical help. If you just stumble and fall, causing some laceration, you may be able to walk your way out on your own feet if a first aid kit is at hand. If you can't, the kit will give you and your party significantly more time to wait for help.
Essential 10: Pocket Knife
There's no need to carry an axe or hatchet. A plain and simple pocket knife (ideally a Swiss Army Knife) will do. You can make repairs to your gear, chip splinters from some wood for kindling, cut blisters, open tin cans, cut fingernails, punch holes into a tarp...
Some Other Essentials
Whether or not you have to carry some additional things depends on where you want to go. Winter conditions will require even more additional clothing, alpine environments will require ropes and other things, the crossing of rivers and lakes will require a life jacket, going to the desert sets the focus on the water supply... but the aforementioned Ten Essentials are, as their name implies, essential.
The Most Important Survival Tool: Your Brain
No matter how well equipped you are, there is one thing that you always carry with you - your brain. So don't dispense with using it. Plan your trip carefully before you do the first step. Avoid unnecessary risks. Don't overestimate your abilities. Make sure that someone is going to miss you if you do not return in time, ie tell others where you go and when you are planning to return. Take any sound advice you can get. Believe what people who are familiar with the terrain tell you. They may be the ones who are sent out to bring you back. You can bet that they will not give you any advice that might lead you into danger and them on a search and rescue trip.