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Strike/Dip... Using a Brunton


A Brunton compass is one of the most important tools a field geologist can own. The are ideally suited for geologic mapping, and have a number of features that ordinary compasses do not possess. (If you're interested in purchasing a Brunton, Kooter's Geology Tools has some of the best prices around. The Brunton usage tips that follow, and the accompanying illustrations were taken from the manual for my own Brunton Classic ComPro Transit.

The first thing you need to do with your Brunton is to set its magnetic declination. The earth is surrounded by a magnetic field, and an unobstructed magnetized object will orient itself with the earth's magnetic poles. Magnetic declination is the different between true geographic north (the north pole) and magnetic north (located in northern Canada), with respect to your position. The easiest way to determine what your magnetic declination should be is to consult and isogonic chart, like the one pictured below:

Isogonic Chart

Once your declination is set, you are ready to begin your field measurements. Direction is refered to a azimuth or bearing depending on the type of Brunton you are using. I prefer azimuth meaurements, which use degree measurements 0-360, as opposed to bearings which use four quadrants of 90 degrees each. The azimuth is the angle a point is from magnetic north. As an example, the azimuth of a mountain that is due east of where you were standing would be 90 degrees, a mountain due south would have an azimuth of 180 degrees, and one due west would have an azimuth of 270 degree. Likewise, a mountain due north of you would be located at 0 degrees.

The azimuth is also the measurement that you use to find the strike of a bed of rock. Strike is defined as the angle between a horizontal line on a plane and true north. To find the strike, stand so that the dip of the bed slopes to your right (this is known as the Right Hand Rule). Orient the long arm of the Brunton in the direction you are facing, and place the side of the Brunton against the rock. Level it so that the bubble in the flat level is in the center of it's circle, then read the number that the north end of the needle is pointing at. That number is your strike.

Another important measurement that a Brunton can take is Dip. Dip is defined as the angle between the plane being measured and horizontal, as measured in a unique vertical plane, perpedicular to the strike. To find the dip, you must orient your Brunton perpendicular to the strike. The easiest way to do this is to let a few drops of water fall on the surface of the rock. They will flow down the rock face at the proper angle to take your dip measurement from.

Place the Brunton against the rock, on it's side this time, with the long arm pointind down the dip of the bed. Use the small lever on the back-side of the Brunton to move the long level until the bubble is centered. The dip angle will be shown in the half-dial on the bottom of the compass. To complete your dip measurement, indicate which direction the bed is dipping in (the direction of downward slope).

Strike and Dip is generally written in the format 015/45 E (striking 15 degrees from north, and dipping 45 degrees toward the east)

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