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lab4_3point_ortho - GEOS 304 LAB 4 Spring 2010 Name 3-Point...

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GEOS 304 – LAB 4 Name:_____________________ Spring, 2010 3-Point Problems & Orthographic Projections Problem 1 (20 points): Determine the strike, dip, and thickness of a planar bed from the analysis of a geologic map. Visualizing: Study how the map trace of the bed interacts with topography. Visualize the topography and the bed orientation. Is the bed N-dipping or S-dipping? Approximately what angle is the bed dipping at (shallow, steep, moderate)? Determine strike and dip: Accurately determine the strike and dip of the bed. Steps: (1) Connecting two points of equal elevation along the trace of either contact provides the strike for the bed; (2) The map distance and elevation difference, measured perpendicular to the strike line to any point along the other contact, can be used to determine the dip of the bed by using the equation below. You can use either the N contact trace or the S contact trace - they will give the same result. A cross-section sketch is very useful for helping visualize the geometric relationships. You can either (1) draw the cross-section to scale and measure the dip, or (2) using trigonometry. tan θ = opposite/adjacent; tan(dip angle) = elevation difference / map distance . Determine the true thickness: Determine the true thickness of the bed. Procedure: (1) Draw a cross section (perpendicular to strike) across the bed. Include a topographic profile. (2) Mark both the N and S contacts of the bed on the topographic profile of the cross section and extend them to depth using the dip angle that you determined above. (3) Measure the perpendicular distance between the two contacts- true thickness!
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Page 2/9 NOTE: Map scale is in meters (m) Problem 2 (30 points): Use the attached problem 2 map and assume that the base of a planar bed is exposed at points X, Y, and Z.
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