Why to Orient a Compass to a Map

Although a compass will always point to the north, a magnetic compass in the northern hemisphere will always point to the magnetic north pole. However, because true north and magnetic north are not in the same place, a skilled navigator will always orient his compass to his map to avoid unnecessary misdirection.

  1. Magnetic Compasses

    • A magnetic compass, in general, works by suspending a card in a liquid medium; this card is inscribed with an arrow and a small magnet that orients itself along a north-south axis. Magnetic compasses point to the magnetic north pole in the northern hemisphere, but they are susceptible to error if ferrous (iron-bearing) metals are nearby. Brush an iron nail near a compass case to see how far the needle will move away from north.

    True North

    • Because the magnetic north pole moves, maps are oriented to the geographic north pole. This "true north" is what governs positioning relative to lines of latitude and longitude. It is fixed and does not move.

    Declination

    • Because the geographic and magnetic poles are not aligned, a compass will tend to "pull" the needle away from true north depending on the compass's relative longitude. In some places---for example, along the Mississippi River---the line of longitude for true and magnetic north is virtually identical. In other places, however, the compass can deviate from true north by 12 degrees or more. This discrepancy is called "declination" and must be corrected before using a magnetic compass with a traditional topographic map.

    Compass Roses

    • Many high-quality maps and charts feature a "compass rose"---a printed compass, often colored in magenta ink, that shows the declination for that particular map. Most maps are of sufficiently small scale that the effective declination in the area covered by the map is within the error tolerance of most compasses. Check the "datum" of the chart because older maps will report declination that might be out of date. The magnetic pole shifts each year, so old maps might have declination values that are no longer accurate.

    Orienteering with Map and Compass

    • To properly orient with a magnetic compass and a map, identify the declination that covers the map's area, then adjust the compass accordingly. For example, a map covering central Florida will decline 5 degrees west, so adjust the compass by twisting the ring or moving the arrow so that the orienting arrow points at 5 degrees west. The index pointer will appear out of alignment, but the major line on the card will point to true north when using the map, and the magnetic compass will point to true north (not magnetic north) when the needle is aligned through the orienting arrows.