Determining Spatial Orientation of Rock Layers with the Brunton Compass

1. Preparation

1. Check for free needle motion. Verify that the needle is unimpeded when held in the horizontal plane. Some compasses have restrictor buttons that hold the needle in place, and if present, check to see that pushing the restrictor does not move the needle.
2. Check the “bull’s eye bubble” centering and continuity. This bubble is one of two leveling bubbles and is used to determine horizontality of the compass. The other bubble is used for inclination measurements.
3. Check for correct magnetic declination setting. Since earth’s magnetic and geographic poles are not coincident, in order to accurately assess compass directions (relative to true North) the declination pin must be set to the correct magnetic declination for the location of usage.

2. Establish Suitable Representative Surface for Measurement

1. In the field, a geologist must establish suitable representative surfaces for measurement. The idea is to approximate the overall orientation of the feature being evaluated (bedding, jointing, foliation, etc.) at this particular location. One of the simplest ways to do this is to place a notebook or clipboard onto the rock in this average and representative orientation.
   In a lab demonstration, any flat surface can be used as a representative surface (a board/model on a desk, or an architectural element of a building).

3. Set the Compass on the Surface

1. Next, the lower edge of the Brunton Compass is set upon the surface, such that the entire edge is flush with the surface.

4. Center the “Bull’s Eye Bubble”

1. Without taking any part of this edge off the surface (a common mistake) the Brunton Compass is rotated until the “bull’s eye bubble” is centered.

5. Read the Azimith Orientation, or Measure Strike

1. By centering the bull’s eye bubble, the Brunton Compass becomes aligned in the horizontal plane, and this allows for reading the azimith orientation of the line formed by the intersection of the rock surface and the horizontal—i.e. the definition of “strike”.
   Note: By convention strike is measured in the northern quadrant. For example a direction of S30degE (30° to the east of due South) would be reported as N30W.

6. Measure Dip

1. The final step is to measure dip. This is measured perpendicularly to the strike direction and is defined as the angular deviation of the surface from horizontal. For example, a nearly vertically oriented rock layer might have a dip magnitude of 85SE, indicating that the surface is dipping 85° from the horizontal, in a southeasterly direction.
   Note: Dip direction is given in a general sense (NE, SE, SW, NW) because its exact direction is always 90° from strike.