Orthographically determine the true dip and strike given two apparent dips
Two apparent dips () of a plane (e.g., dike, vein) are measured in the field on two surfaces (e.g., joints).
Determine the true dip () and strike of the plane.
Example: A dike has an apparent dip of 1 = 20 along the 296 direction and 2 = 30 along the 046
directions. What is the attitude of the dike?
Procedure:
1.
2.
3.
4.
5.
6.
7.
8.
9.
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12.
Pick a point, O, at the center of your graph paper.
Draw the direction of the apparent dips (i.e., 296 and 046) from point O.
Call these FL1 and FL2, respectively.
Subtend the angle 1 from FL1, and angle 2 from FL2.
Use a straight edge and choose an arbitrary depth d perpendicular to FL1.
Find point A where d intersects the trend of the apparent dip (i.e., the 296 line)
Use the same d for FL2, and repeat steps 5 and 6 for the second apparent dip, except that the
intersection point is marked as B.
Connect A to B. This is the strike of the dike.
Draw a perpendicular line to AB from point O. Find its intersection with the strike. Mark it point
C. Line OC is the dip direction.
Measure the same d distance along AB from point C. Determine point C’.
Connect C’ to O, and then read the angle between OC and OC’. This is the true dip amount ().
Read the strike from North, and  using a protractor
A
Strike: 090
B
d
d
A’
True dip direction
Example: A dike has an apparent dip of 1 = 20 along the 296 direction and 2 = 30 along the 046
directions. What is the attitude of the dike?
=40N
2=30
1=20
O
Answer: Strike: 090, True dip:40N
B’
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Example 3: Determine true dip of a plane given two apparent dips.