Leveling Unit 3 PDF

Leveling Unit 3  Introduction to Leveling Leveling is the procedure of determining relative heights of different points on or below the surface of the earth. Thus leveling deals with measurements in the vertical plane. Leveling is the general term applied to any of the various pro...

Leveling Unit 3  Introduction to Leveling Leveling is the procedure of determining relative heights of different points on or below the surface of the earth. Thus leveling deals with measurements in the vertical plane. Leveling is the general term applied to any of the various processes by which elevations of points or differences in elevation are determined. Furthermore, it can be defined as the process of measuring, by direct or indirect methods, vertical distances in order to determine elevations.  Introduction to Leveling Leveling Terminologies * Datum - This is an imaginary level surface to which all elevations are measured or referred to. The sea level is often used as the datum. * Elevation - is the vertical distance from a datum (usually mean sea level) to a point or object. * Bench Mark (BM) - is a relatively permanent object, natural or artificial, having a marked point whose elevations above or below an adopted datum is known or assumed. * Back sight (BS) – is the first sight (reading) taken after the level has been set up. * Fore sight (FS) – last sight (reading) taken before moving the level. It is also a sight taken to a point whose height is required to carry on the line of levels * Intermediate Sight (IS) – other staff readings taken between BS and FS (medium readings) * Change Point (CP) or (TP) – the staff position at which a FS and then BS readings are taken * Reduced Level - This is also called reduced height and is the calculated elevation of a place above or below sea level.  Introduction to Leveling Primary Objective of Leveling (i)to determine the elevations of given points with respect to a datum. (ii)to establish the points of required height above or below the datum line.  Uses of Leveling (i) To determine or to set the base level of a building. (ii)To prepare a contour map for fixing sites for reservoirs, catchment etc., and to fix the alignment of road, railway, canal or sewage line alignment. (iii)To determine the altitudes of different point on a hill or to know the reduce levels of different points on or below the surface of the earth. (iv)To determine the capacity of a reservoir  Uses of leveling Create profile view maps ( Profile leveling) To know the nature of the ground surface the profile leveling is done along the center of the road at some regular intervals (say 20m, 30m etc.)  Uses of leveling Create cross section maps (cross section leveling) The cross sections are taken perpendicular to the center line of the alignment at some regular intervals ( say 20m, 40m). The purpose of cross section leveling is to know the undulation of the ground surface transverse the center of the road.  Types of Leveling * Simple Leveling It is a simple method of leveling that measures the elevation between two points. It is best to set up the level instrument at the mid point then view through a the instrument at the leveling rods. This can only happen when they are close to each other and there are no obstacles between the two point.  Types of Leveling Differential leveling is adopted when: *The points are a great distance apart. The difference of elevation between the points is large. There are obstacles between the points An instrument shifts from station to station to determine the elevation of a point by establishing a series of inter- stations. However, the difference between the two original points is calculated.  Theory of Differential Leveling Elevation can be determined using the following techniques *Levelling Technique *Total Station Technique *GPS Vertical Position Technique *The Remote Sensing Technique (Aerial and Light detection and ranging (Lidar) imagery)  Types of Level Instrument Dumpy Level Automatic Level Tilting Level Digital Level  DIGITAL LEVEL *Is an automatic level capable of normal optical leveling with a graduated rod. *Operates in electric mode using rod graduated in barcode *With the press of one button can determine distances and elevations. *Data can be stored and easily transferable. AUTOMATIC LEVEL * The automatic level employs a compensator which when the bubble is centered (or nearly so), the compensator takes over and maintains a horizontal line of sight, even if the telescope is slightly tilted. AUTOMATIC LEVEL Accessories Traditional Levelling Rod (Leveling Staff) *Levelling rods are manufactured from wood, metal, or fiberglass. The surveyor is well advised to study an unfamiliar rod at close quarters prior to leveling *Are graduated in feet or metres. *The foot rod can be read to 0.01 ft, whereas the metric can me read to 0.01m with millimetres being estimated.  Traditional levelling Rod Accessories *Levelling Rod (Barcode Rod). *The rod face is graduated in barcode, the digital level can, with the press of a button, capture and process the image of the barcode rod for distances in the range of about 0.5 m to about 100 m. Errors will occur when the rod is not plumb, if the barcodes are damaged or if rod is held in a upside down position. Barcode levelling Rod  Accessories Turning Level Plate Rod bubble  Suggestion for Rod Work To ensure accurate reading The rod should be properly extended and clamped. It should be held on firm ground. The rod should be held plumb for all rod readings. Make sure that the rod is held in precisely the same position for the backsight as was for the foresight for all turning points. If the rod is held temporarily near, but not on ,a required location, the face of the rod should be turned away from the instrument so that the instrument operator cannot take a mistaken reading.  Levelling Operation Two Peg Test Provides a way to test the accuracy of a level. (Optical or Digital Level).  Levelling Operation Two Peg Test  Levelling Operation Two Peg Test 1.621 0.395  Rise & Fall Method BS IS FS Rise Fall RL (m) Remarks 2 1.27 1 356.68 BM Reading Known On Staff Elevation 2.33 4.91 3.64 353.04 TP1 6 3 4 5 1.27 - 4.91 356.68 - 3.64 Readin Re g On Staff ading On 6.17 Staff 3.84 9 349.20 TP2 7 Reading 8 2.33 – 6.17 353.04 – 3.84 On Staff  Levelling Operation Levelling Booking Method Rise & Fall Method BS IS FS Rise Fall RL (m) Remarks 1.27 356.68 BM 2.33 4.91 3.64 353.04 TP1 6.17 3.84 349.20 TP2 Sum BS – Sum FS Sum Rise – Sum Fall Last RL – First RL 3.6 – 11.08 = -7.48 0 – 7.48 = -7.48 349.20 – 356.68 = -7.48  Levelling Operation Levelling Booking Method 29.HEIGHT OF COLLIMATION METHOD Booking is the same as the rise and fall method for back-, intermediate- and foresights. There are no rise or fall columns, but instead a height of collimation column 30.Cont… The first back sight reading (staff on datum, benchmark or RL) is added to the first RL giving the height of collimation. The next staff reading is entered in the appropriate column but on a new line. The RL for the station is found by subtracting the staff reading from the height of collimation 31.Cont… The height of collimation changes only when the level is moved to a new position. The new height of collimation is found by adding the back sight to the RL at the change point. Please note there is no check on the accuracy of intermediate RL's and errors could go undetected  Levelling Operation Levelling Booking Method Rise & Fall Method BS IS FS Rise Fall RL (m) Remarks 1.27 356.68 BM 2.33 4.91 3.64 353.04 TP1 6.17 3.84 349.20 TP2 Sum BS – Sum FS Sum Rise – Sum Fall Last RL – First RL 3.6 – 11.08 = -7.48 0 – 7.48 = -7.48 349.20 – 356.68 = -7.48  Theory of Differential Leveling Tide Gauge: A modern water level monitoring station, fitted with sensors that continuously record the height of the surrounding water level.  UNIT 1 1.252 1.203 1.128 1.056 1.000 0.950 UNIT 1 2.097 2.044 1.980 1.868  Errors in Leveling 19.Leveling Errors There are a large number of potential sources of errors in leveling. Many of these are only significant for precise leveling over long distances. For the short segments of leveling that will occur in connecting a TGBM to nearby benchmarks there are only four worth mentioning: · Collimation Error · Error due to Earth Curvature · Error due to Parallax Error · Error due to Refraction 20.Collimation Error The Automatic Prism compensator goes out of alignment. The level provides readings outside of its specification 21.Collimation Error 22.Parallax Error When using an optical instrument — both the image and cross hairs can be focused- if either is imprecisely focused, the cross hairs will appear to move with respect to the object focused, if one moves one's head horizontally in front of the eyepiece.  Errors in Leveling 24.Curvature of the Earth Due to the curvature of the Earth, the line of sight at the instrument will deviate from a horizontal line as one moves away from the level 25.Correction of Curvature Error For a sight length of 300 ft the effect is only 1mm. Keep Sight lengths under 150 ft the effect is eliminated by using equal sight lengths for fore- and backsights