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L10Sep16 - Reciprocal Levelling Let the true difference in...

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CIVL 1009 Survey (2010-2011 Semester 1) Chapter 1 Differential Levelling 61 Reciprocal Levelling Let the true difference in levels between A and B be δ H, c 1 be the effect of collimation error between A and B, and c 2 be the effect of collimation error between B and A At instrument position I 1 , δ H = a 1 – ( b 1 – c 1 ) (1) At instrument position I 2 , δ H = (a 2 – c 2 ) – b 2 (2) Adding Equation (1) and Equation (2), we have δ H = 0.5 [(a 1 – b 1 + c 1 ) + (a 2 – c 2 – b 2 )] = 0.5 [(a 1 – b 1 ) + (a 2 – b 2 ) ] (since c 1 = c 2 , given the same offset distance, e.g. 5 m) = 0.5 (Apparent Difference 1 + Apparent Difference 2 ) CIVL 1009 Survey (2010-2011 Semester 1) Chapter 1 Differential Levelling 62 Reciprocal Levelling Let the true difference in levels between A and B be δ H, c 1 be the effect of collimation error between A and B, and c 2 be the effect of collimation error between B and A At instrument position I 1 , δ H = a 1 – ( b 1 – c 1 ) (1) At instrument position I 2 , δ H = (a 2 – c 2 ) – b 2 (2) Subtract Equation (2) from Equation (1), we have 0 = a 1 – (b 1 – c 1 ) – (a 2 – c 2 ) + b 2 -2c 1 = (a 1 – b 1 ) – (a 2 – b 2 ) (since c 1 = c 2 , given the same offset distance, e.g. 5 m) e = 0.5 (Apparent Difference 2 – Apparent Difference 1 )/ L CIVL 1009 Survey (2010-2011 Semester 1) Chapter 1 Differential Levelling 63 Errors in Levelling Computation of errors Collimation error Curvature of the Earth • Refraction Precaution Start and finish at the Stations with known reduced level ( closed loops ) Sight length should be less than 60 m Lengths of Backsight and Foresight should be the same Readings should be booked immediately (Upper Stadia, Mid-Hair, Lower Stadia) Staff vertically checked by circular bubble or rocking of the Staff CIVL 1009 Survey (2010-2011 Semester 1) Chapter 1 Differential Levelling 64 Contouring
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CIVL 1009 Survey (2010-2011 Semester 1) Chapter 1 Differential Levelling 65 Contouring Definition A contour is a line drawn on a plan joining all points of the same height above or below a datum (equal altitude) Contours cannot cross, split or join other contours, except in the case of an overhang, e.g. a cliff The height between successive contours is called the vertical interval or the contour interval • Value of contour interval depends on the variation in height of the area being contoured • The contour interval is kept constant for a plan or map The plan spacing between contour line indicates the steepness of slopes • Closely spaced lines indicates a steep gradient
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