Orthographic Projection In Structural Geology Any plane is defined in space by three parameters: 1- the direction of strike . 2- the amount of true dip. 3- the direction of true dip (as a general trend). example: strike = N30ᴼE , true dip = 60ᴼNW …..(strike is in the bearing format) strike = 030ᴼ , true dip = 60ᴼNW …..(strike is in the azimuth format) Or by two parameters: 1- the amount of true dip. 2- the direction of true dip (as exact trend). example: true dip = 60ᴼ , in a direction = N60ᴼW …..(dip direction is in the bearing format) true dip = 60ᴼ , in a direction = 300ᴼ …...(dip direction is in the azimuth format) Strike of a plane: is the direction of a horizontal line on the plane. True dip of a plane: is the angle between the plane and a horizontal plane, • measured in a vertical plane. • normal to the strike. Apparent dip of a plane: is the angle between the plane and a horizontal plane, • measured in a vertical plane • not normal to the strike. For any plane the value of apparent dip is always smaller than the true dip, The amount of apparent dip ranges between 0 and the value of true dip. Any line is defined in space by two parameters:: 1- the trend of the line. 2- the angle of plunge (angle of inclination of the line). Example: The line direction (trend or bearing) = S40ᴼW, plunge = 60ᴼ ……(bearing format). The line direction (trend or bearing) = 220ᴼ, plunge = 60ᴼ ..…(azimuth format). the angle of plunge of a line: is the angle between the line and a horizontal line measured in a vertical plane. the angle of pitch or rake of a line: is the angle between the line and a horizontal line measured in a plane not vertical. The orthographic projection of a line: The projection of a line: Bearing = N40ᴼE Plunge = 30ᴼ The orthographic projection of a plane: The projection of a plane: Strike = N30ᴼW Dip = 45ᴼNE Determination of strike and true dip of a plane given two apparent dips illustration Determination of strike and true dip of a plane given two apparent dips: Given: Two apparent dips measured on a bedding plane as: 30ᴼ on a bearing N40ᴼE. 40ᴼ on a bearing S10ᴼE. Find: the direction of strike and the amount of true dip of that plane. Answer: Strike = N20ᴼ E True dip = 58 ᴼ SE Determination of apparent dip in a certain trend, given strike and true dip Given: A bedding plane has: Strike = N30ᴼW. true dip = 45ᴼNE. Find: The apparent dip in a direction = S50ᴼE. Answer: App. dip = 19ᴼ The line of intersection of two planes Determination of the attitude of the line of intersection of two planes Given: Two planes: the first: Strikes N60ᴼW and dips 35ᴼNE. the second: strikes N80ᴼE and dips 60ᴼNW. Find: The trend and plunge of the line of intersection of the two planes. Answer: Trend = N59ᴼE. Plunge = 31ᴼ. Exercise: 1- Given: Strike and dip of N50ᴼE, 40ᴼSE. Find: Apparent dip angle in a vertical cliff section trending S70ᴼE . ANSWER: 36ᴼ 2- Two apparent dips are measured on a planar coal seam as: • 25ᴼ on a bearing 130ᴼ (S50ᴼE). • 35ᴼ on a bearing 200ᴼ (S20ᴼW). Find: Strike and true dip of coal seam. ANSWER: strike = 091ᴼ (S89E), dip = 37ᴼ S. 3- strike and dip measurements on 2 non-parallel planes: • strike = N40ᴼE, dip = 30ᴼSE. • strike = N70ᴼW, dip = 60ᴼNE. Find: plunge and bearing of the line of intersection. ANSWER: bearing = 094ᴼ (S86ᴼE) , plunge = 25ᴼ.