L10Sep09 - Differential Levelling Basic Theory 3 Level...

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CIVL 1009 Survey (2010-2011 Semester 1) Chapter 1 Differential Levelling 21 Differential Levelling Basic Theory 1. Level staff on A 2. Backsight (BS) reading is the first reading (S1) 3. Level staff on B 4. Foresight ( FS ) reading is the last reading (S2) 5. Move instrument to new position CIVL 1009 Survey (2010-2011 Semester 1) Chapter 1 Differential Levelling 22 Differential Levelling Basic Theory 5. Move instrument to a new position, ( 2 nd instrument position ) 6. Level staff stays on B 7. Point B is known as Turning Point ( TP ) 8. Backsight ( BS ) reading is the first reading for 2 nd instrument position (S3) CIVL 1009 Survey (2010-2011 Semester 1) Chapter 1 Differential Levelling 23 Differential Levelling Basic Theory 9. Level staff on C 10. Foresight ( FS ) reading is the last reading for 2 nd instrument position (S4) CIVL 1009 Survey (2010-2011 Semester 1) Chapter 1 Differential Levelling 24 Differential Levelling Repeat the process HOC = RL B + BS RL C = HOC – FS RL at A is known HOC = RL A + BS RL B = HOC – FS Now RL B is known Basic Theory HOC = Known RL + Backsight Unknown RL = HOC - Foresight
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CIVL 1009 Survey (2010-2011 Semester 1) Chapter 1 Differential Levelling 25 Differential Levelling Repeat the process HOC = RL B + BS RL C = HOC – FS RL at A is known HOC = RL A + BS RL B = HOC – FS Now RL B is known Plan View HOC = Known RL + Backsight Unknown RL = HOC - Foresight Station A Station C Station B BS FS BS FS CIVL 1009 Survey (2010-2011 Semester 1) Chapter 1 Differential Levelling 26 Differential Levelling Intermediate Sight (IS) Any reading taken at an instrument station which is NOT a permanent stable Station D1, D2, … refer to ‘Intermediate Station’ Station A Station C Station B BS FS BS FS Station D3 Station D1 Station D2 IS IS IS CIVL 1009 Survey (2010-2011 Semester 1) Chapter 1 Differential Levelling 27 Differential Levelling Difference in RL between B and C = h4 – h3 (F.S. – B.S.) RL C = RL B – (h4 – h3) Difference in RL between A and B = h2 – h1 (F.S. – B.S.) RL B = RL A – (h2 – h1) Rise and Fall Method If h2 > h1 , then Fall If h2 < h1 , then Rise h1 h2 h3 h4 CIVL 1009 Survey (2010-2011 Semester 1) Chapter 1 Differential Levelling 28 Differential Levelling Rise and Fall Method
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CIVL 1009 Survey (2010-2011 Semester 1) Chapter 1 Differential Levelling 29 Differential Levelling Rise and Fall Method 0.917 1.533 TP 3 1.022 A 0.841 1.384 TP 4 1.835 1.756 B 1.295 0.713 TP 2 1.451 0.927 TP 1 134.685 1.055 A (BM) Elev. FALL RISE FS IS BS Station CIVL 1009 Survey (2010-2011 Semester 1) Chapter 1 Differential Levelling 30 Differential Levelling Rise and Fall Method 0.839 0.917 1.533 TP 3 0.362 1.022 A 0.692 0.841 1.384 TP 4 1.122 1.835 1.756 B 0.368 1.295 0.713 TP 2 0.396 1.451 0.927 TP 1 134.685 1.055 A (BM) Elev. FALL RISE FS IS BS Station Fall = FS – BS = 1.451 – 1.055 = 0.396 Rise = BS – FS = 1.384 – 1.022 = 0.362 CIVL 1009 Survey (2010-2011 Semester 1) Chapter 1 Differential Levelling 31 Differential Levelling Rise and Fall Method If b is greater than f , then δ h is positive CIVL 1009 Survey (2010-2011 Semester 1) Chapter 1 Differential Levelling 32 Differential Levelling Rise and Fall Method 133.638 0.839 0.917 1.533 TP 3 134.692 0.362 1.022 A 134.330 0.692 0.841 1.384 TP 4 132.799 1.122 1.835 1.756 B 133.921 0.368 1.295 0.713 TP 2 134.289 0.396 1.451 0.927 TP 1 134.685 1.055 A (BM) Elev. FALL RISE FS IS BS Station New Elev. = Old Elev. – Fall = 134.685 – 0.396 = 134.289 New Elev. = Old Elev. + Rise
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