Introduction to Vertical Distance Measurements 1. The operation of measuring vertical distances, either directly or indirectly, to determine differences in elevation. Leveling 2. Vertical distance between two level surfaces in which the points lie. Difference in Elevation 3. 4. 5. 6. 7. 8. 9. LEVELING TERMS Surface every element of which is normal to the plumb line When used as a reference, it is called the datum. Level Surface A vertical line used which is perpendicular to tangent line of a level surface. Plumbline Vertical distance of a point above or below an arbitrary assumed level surface or curved surface. Elevation A straight line which is tangent to a level surface. Horizontal Line A surface to which elevations or depths are referred. Vertical datum Surface of constant potential energy (equipotential surface) that coincides with mean sea level over the oceans. Geoid Mathematical surface of reference. Reference Ellipsoid 10. Geometrical distance between the Geoid and the point measured along the plumb line passing through the point. Use different methods of leveling. Orthometric Heights 11. Elevation refers to the reference ellipsoid. Ellipsoidal Elevation 12. Use to determine ellipsoidal elevations (you from the ellipsoid (mathematically determined surface). GPS 13. Measurements referred from ellipsoid. Ellipsoidal Height 14. Measurements referred from the geoid (equipotential surface and usually the Mean Sea Level). Orthometric height. 15. A definite point on an object, the elevation and location of which are known. Maybe permanent (P.B.M.) or temporary (T.B.M.) Bench Mark 16. An intervening poipo t between two benchmarks upon which point foresight and backsight rod readings are taken. Its nature is usually indicated in the notes but location is not necessary. A.B.M. may be used as a T.P. Turning Point (T.P.) 17. A rod reading taken on a point of known elevation (BM or TP) Usually taken with the level sighting back along the line (hence the name) Sometimes called a plus sight. Backsight (BS) 18. Horizontal distance from level to rod on a BS. Backsight Distance 19. A rod reading taken on a point of unknown elevation (BM or TP to be established). Sometimes called a minus sight. Foresight 20. The horizontal distance from level to rod on a FS. Foresight Distance 21. Elevation of the line of sight of the telescope above the datum when the instrument is leveled. Height of the Instrument (HI) ❖ ❖ SIGNIFICANCE OF LEVELING OPERATIONS ground Provides necessary data for engineering design, engineering construction, and production of topographic maps. Site suitability analysis Proper design and laying out of vertical and horizontal structures. TYPES OF LEVELS A. Dumpy Level B. Automatic Level C. Tilting Level D. Laser Level E. Digital Level F. Hand Level 21. A.k.a. Self-leveling level; also regarded as engineer’s level. Automatic Level Allows the establishment of a horizontal line of sight by means of a compensator. 23. A system of prisms and mirrors supported by wires as in a pendulum. Compensator 24. Designed so that the LOS is horizontal when the telescope barrel is within +/- of being horizontal. Leveled by the use of a circular spirit or bull’s eye level and the compensator maintains a horizontal line of sight. Automatic Level ● ADVANTAGES: Light, easy to handle, and operation is quick and accurate. ● Random error is centering the bubble is absent in the instrument 25. Used commonly in construction works to maintain the elevations needed. Saves time and effort since it can detect the prism in a whole revolution of 360 degree. Advantage for layout work in construction since it is simple, one-button operation and fully automatic construction. Laser Level 1 26. 27. 28. 29. 30. EASY: aims the telescope at the staff, adjust focus and press a single key, and it measures height and distance Simultaneously. FAST: measurement is completed in <3 seconds. ACCURATE: automatic operation eliminates conventional error by the operator. Digital Level “Random Bi-directional codes” RAB- Code Staves Widely used for rough leveling Consists of a metal sighting tube about 6 inches long on which is mounted on a level vial. Hand Level Graduated rods of rectangular cross-section by means of which difference in elevation is measured. Leveling Rod -May be made of wood, aluminum alloy, and or fiberglass. Shod with metal to protect it from wear. Usually, the point of zero measurement. Lower ground end Rod reading is indicated by the level’s horizontal cross-hair (UR & LR) Measure of the vertical distance between the point on which the rod is held and the line of sight. Rod is held vertically GENERAL CLASSES OF LEVELING RODS A. Self-Reading Rod May be read directly by the leveler while looking through the telescope of the level Leveler observes graduation at which the line of sight intersects the rod and records the reading Should be marked so that the graduations appear sharp and distinct for any normal distance between level and rod PHILADELPHIA ROD is the most widely used rod. B. Target Rod In using target rods, the leveler signals the rodman to slide the target up or down until it is bisected by the line of sight. The target is then clamped, and the rod person, leveler, or both observed the indicated reading Usually, it is equipped with a vernier (small measurements) Advantages: Mistakes are less likely to occur Under certain conditions, its use materially facilitates the work Con: Under ordinary conditions, its use retards progress without adding much precision. Leveling Operations 1. Direct Measure vertical distances directly Most precise method of determining elevations Note: The effect of the earth's curvature and refraction is reduced to a negligible amount 2. Trigonometric (Minahan) Measuring vertical angles and horizontal or slope distances Source of error: Uncertainty in the curvature and refraction correction caused by variations in atmospheric conditions. Uses of Trigonometric Leveling 1. Rapid means of determining elevation of points in rolling or rough terrains. 2. If Total Station is used, results is nearly comparable to those obtained by differential leveling. 3. Used to establish vertical control for photogrammetric, topographic, and construction surveys and determination of structural deformation in buildings, bridges, and dams inaccessible to differential leveling. 3. 4. 5. 6. 7. 8. Reciprocal Employed to determine the difference in elevation between two points when it is difficult or impossible to keep backsights and foresight short and equal. Eg. when running levels across rivers, lakes, or in rugged terrains where deep canyons and ravines are encountered. Stadia Vertical distances are determined by tacheometry by using transit and level rod The process of taking stadia measurements consists of observing, through the telescope, the apparent locations of the two stadia hairs, which is held in a vertical position. Barometric Measuring the differences in atmospheric pressure at various stations by means of a barometer Principally used on exploratory or reconnaissance surveys where differences in elevations are large like hilly or mountainous country Mercurial barometer is accurate but suitable only for observations at fixed point Aneroid barometer is used for field because it is light and can be easily transported. Borrow-pit Determining relative elevations in a borrow-pit excavation for the purpose of the calculating volumes of earthwork Usually encountered in the construction of roads and railroads Gravimetric Used for measuring the differences in gravity at various stations by means of gravimeter for geodetic purposes GPS Survey 2 9. Surveys where elevations are referenced to the ellipsoid but can be corrected if a sufficient number of points with datum elevations are located in the region surveyed. Profile Operation-usually by direct leveling-of determining elevations of points at short measured intervals along a definitely located line, such as the center line for a highway or a sewer. Differential Leveling ● Operation for determining differences in elevations of points some distance apart or of establishing benchmarks ● It is usually accomplished by direct leveling. HI= BS + ElevBM Elev= HI - FS 3
