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Collimation Method; It consist of finding the elevation of the plane of collimation ( H.I.) for every set up of the instrument, and then obtaining the reduced level of point with reference to the respective plane of collimation.
2.Rise and Fall Method; It consists of determining the difference of elevation between consecutive points by comparing each point after the first that immediately preceding it. The difference between there staff reading indicates a rise fall according to the staff reading at the point. The R.L is then found adding the rise to, or subtracting the fall from the reduced level of preceding point. Arithmetic check Sum of B.S. – sum of F. S. = sum of rise – sum of fall = last R. L. – first R.L. This method is complicated and is not easy to carry out. Reduction of levels takes more time. Visualization is necessary regarding the nature of the ground. Complete check is there for all readings. This method is preferable for check levelling where number of change points are more. Basic definitions Bench Mark and Reference Datum In order to calculate the heights of points a datum is required, i.e. a reference level. This is usually the mean sea level. For this purpose, the use of Bench Marks is necessary, and these are classified as follows:Bench Mark (BM) – a point with known height above mean sea level (or other reference datum). These are permanent points (e.g. unchanged by weather conditions) and are provided by the Department of Lands and Surveys. Reduced Level The height of any target point is referred to as Reduced Level (RL), because it is reduced to a known datum. Back-sight (BS) First staff reading taken immediately after setting up the instrument. Foresight (FS) last staff reading taken before moving the instrument to another location. Intermediate sight (IS) All readings taken between a BS and a FS. Common sources of errors in levelling 1. Instrument not correctly levelled. 2. Telescope not correctly focused. 3. The wrong cross-hair reading recorded (e.g. top instead of middle). 4. Staff incorrectly read or not held vertical. 5. Staff incorrectly booked. All the above are blunders cannot be corrected unless the work is repeated! Levelling operations Now consider Figure 2. The level is set up as shown, and using the staff at points A and B, height readings are recorded. This is just the height read through the telescope horizontal line of sight (known as line of collimation). If no reduced level is known only the difference in height can be found between A and B, not their absolute levels. Staff Reading at A is 1.135m Staff Reading at B is 1.875m If we know that RLA = +120.000m (above datum), then RLB = 120.00 – 0.740 = +119.260m i.e. a fall from A. If RLB was known we would calculate a rise in level. Hence, the following can be defined: Rise – staff reading is less than previous reading. Fall – staff reading is greater than previous reading. Experiment (Rise & Fall method): Aim: To determine the required level of given points by Rise and fall method Apparatus Required: Automatic level, Tripod and Levelling staff Procedure:
Field procedures for leveling For vertical control the level survey should start and close on points of known height (same point or different). If the survey starts and closes on the same point (e.g. a TBM), as below, this is termed as a closed level survey. RL(TBM) = +430.000m Instrument stations: 1, 2, 3. Target points: TBM, A, B, C, D, E, F. |
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July 2019
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