The compass is a very simple device that can do a wondrous thing. It can reveal at any time and any place exactly what direction you are heading. On a simple climb in good weather, the compass may never leave your pocket. But as the route becomes more complex or the weather worsens, it comes into its own as a critical tool of mountaineering.
A compass is nothing more than a magnetized needle that responds to the earth's magnetic field. Compass-makers have added a few things to this basic unit in order to make it easy to read accurately. But stripped to the core, there's just that needle, aligned with the earth's magnetism, and from that we can figure out any direction.
These are the basic features (fig. 4-5a) of a mountaineering compass:
• A freely rotating magnetic needle. One end is a different color from the other so you can remember which end is pointing north.
• A circular rotating housing for the needle. This is filled with a fluid that dampens (reduces) the vibra tions of the needle, making readings more accurate.
• A dial around the circumference of the housing. The dial is graduated clockwise in degrees from 0 to 360.
• An orienting arrow and a set of parallel meridian lines. These are located beneath the needle.
• An index line. Read bearings here.
• A transparent, rectangular base plate for the entire unit. This includes a direction-of-travel line (sometimes with an arrow at one end) to point toward your objective. The longer the base plate, the easier it is to get an accurate reading.
The following are optional features (fig. 4-5b) available on some mountaineering compasses:
• Sighting mirror. This provides another way to improve accuracy.
• Ruler. This is calibrated in inches or millimeters.
Fig. 4-5. Features of mountaineering compasses: a, essential features; b, useful optional features.
Use it for measuring distances on a map.
• Clinometer. Use it to measure the angle of a slope. It can help resolve arguments over the steepness of slopes, and it can determine if you are on the higher of two summits. If there is an upward angle between you and the top of another mountain, then the other summit is higher.
• Magnifying glass. Use it to help read closely spaced contour lines.
The small, round, cheap compasses without base plates are not precise enough for mountaineering, nor can they be used for precise work with a map. For routefinding, the compass must be accurate to within 1 or 2 degrees. A larger margin of error, say 5 degrees, would land a moun taineering party 1/2 mile off target at the end of a 6-mile hike.
Continue reading here: Bearings
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