Compass

Note: The following information is based on recommendations from the National Association for Search and Rescue's (NASAR) Fundamentals of Search and Rescue 2nd Edition handbook.

Learning Objectives

Upon completing this module, responders should be able to:

  • Compass Types: Differentiate between common compass styles and explain why orienteering and lensatic compasses are suited for SAR operations.

  • Compass Anatomy: Identify key components of an orienteering compass, including the baseplate, rotating bezel, direction-of-travel arrow, and magnetic needle.

  • Azimuths & Cardinal Directions: Define "azimuth," convert between degrees and the 16 cardinal directions, and follow a field bearing accurately.

  • Magnetic Interference: Identify environmental and metallic hazards that distort magnetic compass readings in the field.

1. Compass Types & Styles

A magnetic compass relies on the Earth's natural magnetic field to establish direction. While specialized trades utilize distinct styles (such as line compasses for foresters or prismatic compasses for surveyors), two primary types are most relevant to search and rescue:

  • Orienteering Compass (Baseplate Compass): The standard tool for SAR responders. Features a clear, flat plastic baseplate, rotating dial (bezel), and measurement scales designed to lay flat across topographic maps for fast plotting.

  • Lensatic Compass: Commonly used by military personnel. Features a hinged cover with a sighting wire and a magnifying lens on the rear sight for taking highly precise sightings toward distant landmarks.

2. Anatomy of an Orienteering Compass

To navigate effectively, searchers must be intimately familiar with the anatomy and features of an orienteering baseplate compass:

  • Baseplate: The clear plastic frame used to rest the compass flat on a map, often featuring ruler scales, grid readers, and a magnifying lens.

  • Rotating Bezel (Housing): The ring marked with 360 degrees in 2-degree increments, allowing the user to select and lock in a specific bearing.

  • Magnetic Needle: The floating needle with a magnetized red end that always aligns with Magnetic North.

  • Orienting Arrow & Lines ("Red in the Shed"): Outlined markings inside the compass housing used to align the rotating bezel with the magnetic needle.

  • Direction-of-Travel Arrow: The fixed arrow at the front of the baseplate pointing toward your intended destination.

  • Index Line: The small marker at the top of the bezel where your target bearing degree is read.

  • Sighting Mirror (Select Models): A hinged mirror allowing the navigator to sight distant terrain objects while simultaneously viewing the compass dial.

3. Azimuths, Bearings, & Cardinal Directions

Navigating on foot requires expressing directions using precise numerical angles or generalized heading points. While "azimuth" and "bearing" are often used interchangeably in casual field conversation, they represent two distinct methods of measuring direction.

Azimuth vs. Bearing

  • Azimuth: A continuous horizontal angle measured clockwise from North, expressed as a single number from 0° to 360°.

    • Example: An azimuth due east is 90°; due south is 180°; due northwest is 315°. This is the primary system used in land navigation, compass work, and GPS navigation because it avoids confusion.

  • Bearing: A directional angle measured relative to either North or South (whichever is closer), capped between 0° and 90°, and bounded by a quadrant designation (NE, SE, SW, NW).

    • Example: Rather than stating an azimuth of 45°, a quadrant bearing is written as N 45° E (45 degrees East of North). An azimuth of 225° is written as S 45° W (45 degrees West of South). Bearings are frequently encountered in property surveys, nautical navigation, and traditional mapping.

4. Magnetic Interference & Errors

A magnetic compass needle is extremely sensitive to nearby magnetic fields and ferrous (iron-containing) metals, which can pull the needle away from true magnetic north and cause severe navigation errors.

Common Interference Hazards

  • Personal Gear: Metal zippers, belt buckles, watch bands, tactical knives, radio antennas, clipboards, or steel-framed backpacks held too close to the chest.

  • Field Infrastructure: Power lines, buried utility pipes, metal culverts, fences, rail lines, and parked field vehicles.

  • Geologic Interference: Iron-rich rock deposits or volcanic geology (magnetic anomaly zones) that disrupt local magnetic pulls.

Field Tip: Always perform a "clearing check" before taking a critical bearing; hold the compass at arm's length, keep it level, and step away from vehicles, heavy gear, and metal infrastructure.

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