8/17/2014. Process of directly or indirectly measuring vertical distances to determine the elevation of points or their differences in elevation

Transcription

1 Process of directly or indirectly measuring vertical distances to determine the elevation of points or their differences in elevation Leveling results are used: To design highways, railroads, canals, sewers, water supply systems etc, having grade lines that best conform the existing topography; To lay out construction projects according to planned elevations; To calculate volume of earthworks and other materials; To investigate drainage characteristics of an area; Level Surface A curved surface which h is at any point perpendicular to the direction of gravity or the plumb line Level Line A curved line in a level surface, all points of which are normal to the direction of gravity and equidistant from the center of the earth Horizontal Surface A plane that is tangent to a level surface at a particular point. It is also perpendicular to the plumb line at the same point Production of topographic maps 1

2 Horizontal Line A straight line which is tangent to a level line at one point This line is perpendicular to the direction of gravity at the point of tangency Vertical Line A line parallel to the direction of gravity Mean Sea Level An imaginary surface of the sea which is midway between high and low tides It is taken as the reference surface to which most ground elevations are referred Zero elevation Datum Any convenient level surface coincident or parallel with mean sea level to which elevations of a particular area are referred Elevation The vertical distance above or below mean sea level or any other selected datum Difference in Elevation The vertical distance between the two level surfaces in which the points lie Geoid Serves as datum for elevations and astronomical observations 1. Direct or Spirit Leveling 2. Reciprocal Leveling 3. Profile Leveling 4. Trigonometric Leveling 5. Stadia Leveling 6. Barometric Leveling 7. Cross-Section Leveling 8. Borrow-Pit Leveling 2

3 Commonly employed method of determining the elevation of points some distance apart by a series of set ups of leveling instrument along a selected route Most precise method of leveling Used when a high degree of accuracy is required Vertical distances are measured above or below a level line and these values are used to compute the elevation of points or their differences in elevation Forms of direct leveling: Differential Leveling Double-Rodded Leveling Three-wire Leveling The process of accurately determining the difference in elevation between two intervisible points located at a considerable distance apart and between which points leveling could not be performed in the usual manner Commonly employed when leveling across canyons and gullies where it would be difficult or impossible to maintain a foresight and a backsight distance of nearly equal lengths Used to determine differences in elevation between points at designated short measured intervals along an established line to provide data from which h a vertical section of the ground surface can be plotted Best suits the requirements needed in the design of roads, railroads, canals, drainage systems and transmission lines 3

4 Employed in determining by trigonometric computations the difference in elevation between two points from measurements of its horizontal or slope distance and the vertical angle between the points Its degree of precision may be improved by using precise measurements such as optical theodolites and total geodetic stations which measure angles to seconds Combines features of direct leveling with those of trigonometric leveling. Any surveying instrument may be employed in stadia leveling as long as it has telescope to read vertical angles and is equipped with stadia hairs in addition to the standard cross hairs. Provide reasonable accuracy for preliminary surveys, mapping and rough leveling where quick measurements are needed Differences in elevation between points are computed from observed vertical angles and the 3-intercepts on a rod held at each point backsighted or foresighted Involves the determination of differences in elevation between points by measuring the variation in atmospheric pressure at each point by means of a barometer Depends on the basic principle that differences in elevation are proportional to the differences in atmospheric pressure Method of leveling wherein short profiles at right angles to the line of work are usually plotted at regular intervals for the purpose of obtaining a representation of the ground surface which is necessary in highway or railroad construction 4

5 Method of leveling wherein short profiles at right angles to the line of work are usually plotted at regular intervals for the purpose of obtaining a representation of the ground surface which is necessary in highway or railroad construction Wye Level Dumpy Level Builder s Level Automatic Level Tilting Level Digital Level Common components: Telescope Geodetic Level A telescope to create a line of sight and enable a reading to be taken on a graduated rod, and A system to orient the line of sight in a horizontal plane Transit Level vials Automatic compensators Level Vials Laser Level Hand Level 5

6 Leveling Rod / Staff A graduated rod which is used for measuring the vertical distance between the line of sight through a leveling instrument and the point whose elevation is either required or known Made of wood, fiberglass, metal Tripod Leveling instruments are all mounted on tripods Serve as a base to prevent movement of the instrument after it is set up 1. Instrumental Errors Instrument Out of Adjustment When the line of sight of the telescope is not parallel to the axis of the level vial When reading is taken on a rod, the result is an error consistently either plus or minus, and with a magnitude which is proportional to the distance between the instrument and the rod The error will also be greatly minimized i i or eliminated i if the backsight and foresight distances are kept nearly equal 1. Instrumental Errors Rod Not Standard d Length Usually due to imperfections in their manufacture. A rod of incorrect length will introduce a systematic error in leveling work. It is important that rod lengths are compared periodically with a standardized steel tape. 6

7 1. Instrumental Errors Defective Tripod The movement of the level due to settling of the tripod legs can cause possible errors in leveling work. The tripod usually settles in soft ground or due to vibrations caused by vehicular traffic. Smooth surfaces such as concrete pavements and steel plates should be avoided. 2. Personal Errors Bubble Not Centered Rod readings will be in error when the bubble is not centered in the level vial. Various conditions: tripod settling in soft ground; and factors which could all be attributed to carelessness on the part of the instrumentman. Parallax Causing relative displacement between the image of the cross hairs and the image formed by the focusing lens. 2. Personal Errors Faulty Rod Readings Usually the result of the length of sight, poor weather conditions, and the skill of the instrumentman and the rodman. Rod Held Not Plumb It is easy to determine if the rod is held plumb or not since it can be checked if it is held parallel to the vertical cross hair. This type of error can e avoided either by waving the rod or attaching a rod level to it to facilitate holding it plumb. 2. Personal Errors Incorrect Setting of Target Target may slip downward because it is not clamped firmly at the exact position signalled by the instrumentman. Unequal Backsight and Foresight Distances It is sufficient to approximate only by eye foresight and backsight distances in ordinary leveling work. 7

8 3. Natural Errors Curvature of the Earth The effect of the curvature of the Earth is to increase the rod reading Atmospheric Refraction Makes the rod appear unsteady when a sight is taken it. Temperature Variations Misreading the Rod Incorrect Recording Erroneous Computations Rod Not Fully Extended Moving Turning Points Wind Settlement of the Instrument Faulty Turning Points 8