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THE PARTS OF A BRUNTON COMPASS

 

WHAT IS DIP AND STRIKE

 

MEASURING STRIKE AND DIP

Measuring strike:

We will begin by taking the strike of a bedding plane. For these measurements we will use a Brunton compass like the one you see in the pictures above. In order to measure the strike, place the side or edge of the compass against the plane of the outcrop. Sometimes it is easier to put your field book against the outcrop and then the compass against the book to get a smoother and/or a larger surface. Now, rotate the compass keeping the lower side edge of the compass fixed, until the bulls-eye level bubble is centered (the round tube; not the long narrow one). When the bubble is centered, the compass is horizontal against the plane and parallel to the line of strike. Now, with the bulls-eye bubble centered, record the number that either end of the compass needle is showing.

In other words:
  1. Place the bottom EDGE of the compass flat against the plane of interest.
  2. Adjust the compass orientation, making sure the bottom edge is always flat against the plane, until the air bubble in the "Bull's eye level" is centered.
  3. Read either end of the compass needle to obtain the value of strike.


Measuring Dip:

To measure the dip of the bedding plane, take your compass and put its side against the rock so that it points in the same direction as the line of dip (The dip line is perpendicular to the strike line). Move the clinometer until the clinometer level bubble is centered. As we did when we found the strike, record where the white tipped end of the clinometer needle is pointing. Note the degrees and the direction. Recall that the dip direction MUST always be perpendicular to the strike direction (e.g., a strike of 40° could only dip to the SE or NW, never NE or SW)

In other words:

  1. AFTER you determine strike, rotate the compass 90°.
  2. Place the SIDE of the compass flat against the plane.
  3. Adjust the lever on the back of the compass until the air bubble in the "Clinometer level" is centered.
  4. Read the dip directly from the scale in the compass.

Measuring Trend:

  1. The "Fold-out pointer" should point down-plunge.
  2. Hold the compass horizontally ABOVE the plane (the bubble in the "Bull's eye level" should be centered).
  3. Look directly down through the "Slot" in the "Fold-out pointer".
  4. Rotate the compass, keeping it horizontal, until the slot is parallel with the line to be measured. (the line should run straight down the middle of the slot).
  5. Read the North end of the compass needle to obtain the value of trend.

Measuring Plunge:

  1. Place the bottom EDGE of the compass flat on the line (the edge should be parallel to the line).
  2. Make sure the compass is vertical.
  3. Adjust the lever on the back of the compass until the air bubble in the "Clinometer level" is centered.
  4. Read the plunge directly from the scale in the compass (the same way you read dip).

The images have been copied from: http://courses.geo.ucalgary.ca/glgy203/images/sd.htm.