The basic "error" of a compass roses see is called variation, defined as the difference at any location between true directions and magnetic directions as measured by a compass roses see unaffected by local influences. (Although the term is common, it is not truly an "error," but an unavoidable condition.) Variation is different for different locations on the earth; at each location it is nearly constant, but does change very slightly over the years. Variation will be either east or west, and must be labeled "E" or " W" as appropriate; it can be as much as 60° in extreme locations. The amount of variation in any area is indicated on charts by compass roses see roses (see Chapter 5, "Charts"). Variation is the same for all vessels in a given area.
Within a limited arc of forward visibility, bearings can be taken by sighting across the steering compass roses see. The extent to which this can be done is determined by the layout of the helm position and the compass roses see installation. A hand-bearing compass roses see will normally allow observations to be taken essentially all around the horizon.
The other component of compass roses see error is deviation. A boat's compass rarely exists in an environment that is free of nearby magnetic material and influences. Objects made of iron and iron-alloys can influence a compass roses see, while electrical currents (DC) produce magnetic fields that can also affect compass roses see readings.
The basic "error" of a compass roses see is called variation, defined as the difference at any location between true directions and magnetic directions as measured by a compass roses see unaffected by local influences. (Although the term is common, it is not truly an "error," but an unavoidable condition.) Variation is different for different locations on the earth; at each location it is nearly constant, but does change very slightly over the years. Variation will be either east or west, and must be labeled "E" or " W" as appropriate; it can be as much as 60° in extreme locations. The amount of variation in any area is indicated on charts by compass roses see roses (see Chapter 5, "Charts"). Variation is the same for all vessels in a given area.