A compass is a navigational instrument that measures directions in a frame ofreference that is stationary relative to the surface of the earth. The frame ofreference defines the four cardinal directions (or points)– north, south, east, and west. Intermediate directions are also defined. Usually, adiagram called a compassrose, which shows the directions (with their names usuallyabbreviated to initials), is marked on the compass. When the compass is in use,the rose is aligned with the real directions in the frame of reference, so, forexample, the "N" mark on the rose really points to the north.Frequently, in addition to the rose or sometimes instead of it, angle markingsin degrees are shown on the compass. North corresponds to zero degrees, and theangles increase clockwise, so east is 90 degrees, south is 180, and west is270. These numbers allow the compass to showazimuths or bearings, which arecommonly stated in this notation.
Amagnetic compass points to magnetic north pole, which is approximately 1,000miles from the true geographic North Pole. A magnetic compass's user candetermine true North by finding the magnetic north and then correcting forvariation and deviation. Variation isdefined as the angle between the direction of true (geographic) north and thedirection of the meridian betweenthe magnetic poles. Variation values for most of the oceans had been calculatedand published by 1914. Deviation refersto the response of the compass to local magnetic fields caused by the presenceof iron and electric currents; one can partly compensate for these by carefullocation of the compass and the placement of compensating magnets under thecompass itself. Mariners have long known that these measures do not completelycancel deviation; hence, they performed an additional step by measuring thecompass bearing of a landmark with a known magnetic bearing. They then pointedtheir ship to the next compass point and measured again, graphing theirresults. In this way, correction tables could be created, which would beconsulted when compasses were used when traveling in those locations.
Marinersare concerned about very accurate measurements; however, casual users need notbe concerned with differences between magnetic and true North. Except in areasof extreme magnetic declination variance (20 degrees or more), this is enoughto protect from walking in a substantially different direction than expectedover short distances, provided the terrain is fairly flat and visibility is notimpaired. By carefully recording distances (time or paces) and magneticbearings traveled, one can plot a course and return to one's starting pointusing the compass alone.
Compassnavigation in conjunction with a map (terrain association) requires adifferent method. To take a map bearing or true bearing (abearing taken in reference to true, not magnetic north) to a destination with aprotractor compass,the edge of the compass is placed on the map so that it connects the currentlocation with the desired destination (some sources recommend physically drawinga line). The orienting lines in the base of the compass dial are then rotatedto align with actual or true north by aligning them with a marked line oflongitude (or the vertical margin of the map), ignoring the compass needleentirely. The resultingtrue bearing or map bearing may then beread at the degree indicator or direction-of-travel (DOT) line, which may befollowed as an azimuth (course)to the destination. If a magnetic north bearing or compassbearing is desired, the compass must be adjusted by the amount ofmagnetic declination before using the bearing so that both map and compass arein agreement. In the given example, the large mountain in the second photo wasselected as the target destination on the map. Some compasses allow the scaleto be adjusted to compensate for the local magnetic declination; if adjustedcorrectly, the compass will give the true bearing instead of the magneticbearing.
Themodern hand-held protractor compass alwayshas an additional direction-of-travel (DOT) arrow or indicator inscribed on thebaseplate. To check one's progress along a course or azimuth, or to ensure thatthe object in view is indeed the destination, a new compass reading may betaken to the target if visible (here, the large mountain). After pointing theDOT arrow on the baseplate at the target, the compass is oriented so that theneedle is superimposed over the orienting arrow in the capsule. The resultingbearing indicated is the magnetic bearing to the target. Again, if one is using"true" or map bearings, and the compass does not have preset,pre-adjusted declination, one must additionally add or subtract magnetic declination to convert the magnetic bearing intoa true bearing. The exact value of the magnetic declination isplace-dependent and varies over time, though declination is frequently given onthe map itself or obtainable on-line from various sites. If the hiker has beenfollowing the correct path, the compass' corrected (true) indicated bearingshould closely correspond to the true bearing previously obtained from the map.
Compassesare to be laid down on a leveled surface so the needle could point to themagnetic north more accurately, as to that the needle only rests or hangs on abearing fused to the compass casing, if used at a tilt, the needle might hitthe casing on the compass, and hence, not move. This will give a faulty reading. To see if the needle is well leveled, look closely at the needle, and tilt itslightly to see if the needle is swaying side to side freely and the needle notcontacting the casing of the compass. If the needle tilts to one direction,tilt the compass slightly and gently to the opposing direction until thecompass needle is horizontal, lengthwise. Items to avoid around compasses aremagnets of any kind and any electronics. Magnetic fields from electronics caneasily disrupt the needle, avoiding it from pointing with the earth's magneticfields, causing interference. The earth's natural magnetic forces areconsiderably weak, measuring at 0.5 Gauss and magnetic fields from household electronics caneasily exceed it, overpowering the compass needle. Exposure to strong magnets,or magnetic interference can sometimes cause the magnetic poles of the compassneedle to differ or even reverse. Avoid iron rich deposits when using acompass, for example, certain rocks which contain magnetic minerals, like Magnetite.Thisis often indicated by a rock with a surface which is dark and has a metallicluster, not all magnetic mineral bearing rocks have this indication. To see ifa rock or an area is causing interference on a compass, get out of the area,and see if the needle on the compass moves. If it does, it means that the areaor rock the compass was previously at/on is causing interference and should beavoided.