Download
1 / 29
310 likes | 612 Views
Chapter (5) Leveling. Introduction: Leveling is the process by which differences in height between two or more points can be determined.
E N D
Chapter (5)Leveling Introduction: • Leveling is the process by which differences in height between two or more points can be determined. • Its purpose may be to provide heights or contours on a plan, to provide data for road cross-sections or volumes of earthworks, or to provide a level or inclined surface in the setting out of construction works. Instruments for Levelling: • The basic items of used instruments are the optical level and a graduated staff. • Levels, Four types of levels are available: optical, automatic, electronic, and laser.
Optical level: • An optical level is used to project a line of sight that is at a 90 degree angle to the direction of gravity. • Both types, dumpy and tilting, use a precision leveling vial to . • Orient to gravity. The dumpy type was used primarily in the United States, while the tilting type was of European origin and used in the remainder of the world. The dumpy level has the leveling vial fixed to the telescope, which is fixed at 90 degrees to a Rota table vertical spindle. • Leveling screws, attached to the spindle, are used to center the leveling vial.
Focusing Screw line of collimation Foot Screws Components of Optical level
Automatic level: • Automatic levels use a pendulum device, in place of the precision vial, for relating to gravity. The pendulum mechanism is called a compensator. • The pendulum has a prism or mirror, as part of the telescope, which is precisely positioned by gravity. • The pendulum is attached to the telescope by using precision bearings or wires (metallic or nonmetallic). • Leveling screws are used to roughly center a circular vial, and the optics on the pendulum then correct the line of sight through the telescope. • Finally, Roughly leveled using a circular spirit level, then internal mechanisms take over to make sure the level remains level and maintains a horizontal sight. • They are very popular, quick to set up and easy to use.
Electronic level: • This type of instrument has a compensator similar to that on an automatic level, but the graduated leveling stall is not observed and read by the operator. • The operator has only to point the instrument at a bar-code-type staff, which then can be read by the level itself. The electronic level eliminates human reading error and increases the speed at which leveling work can be performed. • The only significant disadvantage is the high cost as compared to the optical automatic level.
Laser level: • Although this type of instrument is categorized as laser, these levels actually employ three different types of light sources: tube laser, infrared diode, and laser diode. • The instrument uses a rotating head to project the laser beam in a level 360 degree plane. • The advantages are twofold: no operator is required once the instrument is set up; and different people in various locations can work by using a single light source. • The disadvantages are that accuracy is less than that provided by other types of levels and that the cost is significantly higher.
Laser Detector and staff rotating head Laser level (rotating head – Laser Detector)
Leveling Rods: • Can be made of wood, metal, or fiberglass Graduated in meters. • Rod levels are used to make sure that the rod is held vertical when making a reading. Leveling Rods
Leveling - Field Procedures • The process of leveling involves the transferring of elevations from a point of known elevation to points of unknown elevation by means of establishing a visual reference plane. • This is done by setting up the level at any convenient point and “leveling” the instrument. • The person then sights back to a level rod on a point of known elevation, usually referred to as a “bench mark” (BM). • The rod reading, also known as a “back sight” (BS) or “plus sight”, is added to the known elevation of the bench mark to establish the “height of instrument” (HI). • (This value is essentially the elevation of the center of the eyepiece of the level).
Now the person can sight forward to establish any number of points of unknown elevation. • This is known as a “fore sight” (FS) or a “minus sight”. • The rod reading for each fore sight is subtracted from the height of the instrument to establish the elevation for each new point. • For points that cannot be seen from the initialinstrument set-up, one foresight point can become a “turning point” (TP). • The rod holder stays at this location while the instrument is moved forward to a new location.
Line of Sight: This is the optical line produced by the telescope of the instrument used for sighting. In this particular case the line is approximately horizontal. Reduced Level (R.L.) : The height of a point above the selected datum. Mean Sea Level (M.S.L.) : This is the datum most frequently used. Datum for Egypt is the Egyptian Height Datum (E.H.D.) Back sight (B.S.): The first reading taken by an observer at every instrument station.
Foresight (F.S.): The last reading taken at an instrument station . Intermediate sight (I.S.): Any reading taken at an instrument station which is NOT a permanent stable reference point. Bench Mark (B.M.): A point of known Reduced Level (R.L.). Usually a permanent stable is reference point. Temporary Bench Mark (T.B.M.): Where only relative heights are required or a bench mark is required within area of work T.B.M. is established .
Change Point (Turning point): • The point at which the position of the instrument is changed, making it both a back sight and foresight. • Summary of Levelling Procedure: • When the level has been set up we always start with a BS to a point whose RL is known such as a BM. • The last reading at any instrument position is always a FS (i.e. always end with a FS). • Either the instrument moves or the staff moves (never move both). • We must always finish levelling at a point of known RL value such as an BM or a TBM (always close your levelling).
Staff (Rod) Reading: The following figures are the samples of some readings in the rod. Reading = 1.932 m Reading = 1.133 m
Example : Set up and complete the differential level notes for the information shown in the accompanying illustration. All back sights are shown to the left of the instrument diagrams and foresights are shown to the right. All units are feet.
Simple Calculation Check: Σ FS - Σ BS = first RL - Last RL 30.67 - 22.09 = 100 – 91.42 8.58 = 8.58 O.k. Full Calculation Check: Σ IS + Σ FS + Σ (R.L. s except first) = Σ (each I.H. x number of applications 0.00+30.67+366.34=103.15x1+98.97x1 + 102.49x1 + 92.40x1 397.01 = 397.01 O.K Remember that: 1- Simple Calculation Check: Σ FS - Σ BS = 1st RL - Last RL 2-Full Calculation Check: Σ IS + Σ FS + Σ (R.L. s except first) = Σ (each I.H. x number of applications)
Example: The following observations were taken in leveling work: 1.546, 1.562, 1.418, 1.390, 1.281, (2.420), 1.321, 1.011, 2.007 m. The position of the instrument was changed after the second and seventh readings. Calculate the respective levels of points at each staff if the first reading was taken no B.M of RL 10.00 m and closed to B.M of RL 9.09 m. It is important that to make arithmetical check on your results. Note that: the staff readings enclosed by brackets were taken with the staff reversed.
Actual and Allowable Misclosure: • We have already seen that the • Actual misclosure = 9.085 - 9.09 = -0.005m • =5 mm • Is this acceptable ? • Allowable misclosure = ± 5 √N mm • Where N is the Number of Instrument Positions • which is the same as Number of BS readings • Therefore our Allowable misclosure = ± 5 √3 mm = ± 8.66 say ± 9mm • Therefore Actual < Allowable • Therefore our fieldwork is • OK
Simple Calculation Check: Σ FS - Σ BS = 1st RL - Last RL 4.98 – 3.975 = 10.0 – 9.085 0.915 = 0.915 O.k. Full Calculation Check: Σ IS + Σ FS + Σ (RLs except first) = Σ (each I.H. x number of applications LHS = 0.251 + 4.890 + 63.105 = 68.246 RHS = 11.546x1+ 11.402x4 + 11.092x1 =68.246 O.k.
Example: A level field book was found to be tampered . The following readings were found to be reliable . Complete the level field book with respect to the given observations:
Check: ∑FS - ∑BS = 1st RL – last RL 6.955 – 5.455 = 280.500 – 279.000 1.500 = 1.500
