Parts of a Compass (Lat/Lon)

A compass is an instrument that is used for navigation and mapping because it measures the geographic direction between two points. It is a fairly simple instrument that uses a magnet, mounted on a pivot that turns in response to the earth’s magnetic field, to determine direction (but not position). The magnetic needle points to the magnetic North Pole, which is different from geographic North Pole. A compass bearing, which is typically expressed as an angle (degrees), refers to the horizontal direction to or from any point. In this publication, the term “bearing” is used interchangeably with the term “azimuth.” A compass is used for several different purposes including:

  • Determine direction to a destination or landmark.
  • Stay on a straight course to a destination or landmark, even if you lose sight of it.
  • Avoid obstacles in the path to the destination or landmark.
  • Return to your starting point.
  • Pinpoint locations on a map and in the field.
  • Identify what you are looking at in the field or on a map.
  • Orient a map.
  • Plot points on a map.
  • Plot route of travel on a map.

There are a variety of different types and models of compasses, such as baseplate, sighting, prismatic, and electronic. This chapter focuses on using a baseplate compass because it is a common, inexpensive, and easy to use compass that does not require batteries.

Parts of a Compass

The basic parts of a base-plate compass are described below and illustrated in the image below.

navigation_pms4-1

 

  • Magnetic needle. The magnetic needle typically has a red end that points to magnetic north, as long as the compass is being used properly and there is no local magnetic attraction.
  • Housing with cardinal points and degrees. The housing includes a revolving dial that shows the cardinal points (at least north, east, south, and west) and degrees (0 – 360) (image below). The housing is rotated to line up the compass needle with the orienting arrow when taking a bearing.

navigation_pms4-2

 

  • Orienting arrow. The north-south orienting arrow (red or black outline of an arrow) is used to align the magnetic needle when taking a bearing. It is also what is adjusted to set the compass for magnetic declination.
  • Orienting lines. The north-south orienting lines parallel the orienting arrow and can be used to line up the compass dial with grid lines on a map. When the declination is set on a compass with an adjustment screw, the orienting lines no longer parallel the orienting arrow.
  • Index line. Marked on the front sight of the compass base plate, the index line is where you read the indicated bearing.
  • Direction of travel arrow. The direction of travel arrow or sighting line is used for sighting and following bearings. The arrow should be pointed in the direction of the destination or landmark.
  • Base plate. The transparent plate (everything is attached to the baseplate) can be used as a ruler to measure map distances. The direction of travel arrow is also located on the baseplate. Some compasses will have a protractor on the baseplate that can be used to determine bearings from a map.
  • Magnifying lens. Useful for reading tiny map symbols and features.
  • Declination adjustment screw. Some compasses have a screw that can be turned to set compass for proper declination. Some compasses have an internal adjustment that automatically corrects for declination.
  • Clinometer. Some compasses have a clinometer that can be used to estimate slope.
  • Sighting mirror. Some compasses have a flip up mirror that can improve accuracy when reading bearings. Read the bearing in the dial’s reflection where the mirror line crosses it. It can also be used for signaling.

 

 

[Continue to How to Hold a Compass]

**Information adapted from the National Interagency
Incident Management System Basic Land Navigation
Manual, PMS 475 dated June 2007.