No matter how great their taste for adventure and discovery, most wilderness travellers want a clear idea of where they are, where they're going, and eventually, how to get back home.
Compasses are used to maintain a consistent direction of travel, to orient maps so they correspond to the real landscape, and to pinpoint locations on maps by sighting angles from known landscape features (taking bearings). Compass needles align themselves with the earth's magnetic fields, which run in jagged lines from the southern hemisphere to the magnetic north pole, and in irregular bands parallel to the equator.
The magnetic needle sits on a pivot inside a capsule. To ensure speed and accuracy, the capsule is filled with fluid that reduces vibration and oscillation, or it sits on a jewelled pivot made of quartz or glass.
A rotating dial encircles the needle capsule. The dial is divided into 360°s and is marked at 0°s for north, 90 for east, 180 for south, and 270 for west. Depending on its size, the dial may show every second, fifth, or tenth degree and may indicate intermediate directions such as southeast (SE).
The baseplate is usually marked down the centre with a direction-of-travel arrow. Most compasses also have an orienting arrow and meridian lines. The orienting arrow is used to orient the compass with a map. Meridian lines make it easier to align the compass with longitude lines on a map. A ruler on the edge of the baseplate can measure distances on maps. Some compasses are equipped with a wheel that can be rolled over a map to more accurately gauge distance.
A sighting mirror allows you to accurately take bearings in the field. When the hinged cover is opened at a 45° angle, you can view the needle and orienting arrow in the mirror while sighting at a landmark through the notch in the cover.
Atmospheric pressure decreases as height above sea level increases. Altimeters measure altitude by sensing changes in air pressure. The altitude measured can indicate the vertical distance you've covered or your proximity to particular map contour lines, valuable clues to your location in mountainous terrain. As it is essentially a barometer, an altimeter can also assist in predicting weather.
Altimeters, particularly wrist-top models, often have features such as thermometers, clocks, alarms, or tendency graphs. Tendency graphs record changes in air pressure over time and are helpful for weather forecasting.
The heart of an altimeter is an aneroid box: a sealed capsule that contracts or expands as air pressure varies. Changes are displayed as barometric pressure in millibars or kilopascals, or converted to altitude readings in feet or meters. Generally, the smaller the range an altimeter measures (about 10 metres) the more accurate the reading.
Changes in weather, temperature, and humidity can influence an altimeter's accuracy. Cold air is heavier and exerts more pressure. Water vapour in air makes it lighter. Therefore, the weather must be taken into account when reading an altimeter. Some better quality altimeters are temperature compensated. They allow you to enter temperature changes to improve the accuracy of the readings.
Never take an altimeter higher than its maximum capacity, as it can be damaged. If you intend on going on very high altitude adventures, take an altimeter with a high ceiling limit or with a safety catch that shuts the instrument down before damage occurs. When travelling in aircraft, carry your altimeter with you in the pressurized cabin.
Read Selecting a GPS for information about backcountry navigation.