mod 5

• Leveling


  The process of directly or indirectly measuring vertical distances to determine the elevation of points or their differences in elevation

  
• Why perform leveling


  • To determine the topography of sites for design projects
  • To set grades and elevations for construction projects
  • To compute volumes of earthwork

  
• Level surface


  Curved surface where every point is perpendicular to plumb line or direction of gravity. However, a level surface is not plane and does not have regular form.

  
• Horizontal surface


  Plane that is tangent to a level surface at a particular point. It is also perpendicular to local direction of gravity.
• Level line


  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 line


  Straight line in a horizontal plane which is tangent to the level line, and perpendicular to the direction of the gravity at the point of tangency.
• Vertical line


  Vertical line parallel to the direction of the gravity. It is exemplified by the direction taken by sting supporting a suspended plumb bob passing through a point
• Elevation


  Vertical distance above or below mean sea level or any selected datum; can be positive or negative elevations

  
• Mean sea level (MSL)


  An imaginary surface of the sea which is midway between high and low tides (determined by averaging the height of the sea's surface for all its tide stages over a long period of time)
• Mean sea level is taken as the reference surface to which most ground elevations are referred

  
• Mean sea level is not a steady frame of reference due to the melting of ice in the polar regions, the effects of volcanic activity, and other influencing factors
• Difference in elevation (DE)


  Vertical distance between two level surfaces in which the points lie

  
• Datum


  Any convenient level surface coincident or parallel with the mean sea level to which elevations of a particular area are referred

  
• Any surface may be used as a datum when relative elevations over a limited area needs to be established

  
• Assumed elevation may be assigned to a reference point and the elevation of other points may be determined with regard to this value

  
• Old Datum of US: mean sea level (using 19 years record from 26 gauging stations at Atlantic Ocean, Pacific Ocean and Gulf of Mexico)

  
• New Datum US: NAVD88 (North American Vertical Datum of 1988) formerly NGVD29 (National Geodetic Vertical Datum of 1929)

  
• Local mean sea level is commonly used as datum
• Luzon Datum of 1911 (defined by Station Balanacan at Marinduque) became the primary geodetic reference of all surveys in the Philippines

  
• The Philippine Geodetic Network (PRN) was developed until 1946 and then upgraded into Philippine Reference System of 1992 (PRS92)

  
• PRS92 or the Philippine Reference System of 1992 is a homogeneous national network of geodetic control points (GCPs), marked by survey monuments or mojons, that has been established using Global Positioning System (GPS) technology

  
• By virtue of EO 45, PRS92 became the standard reference system for all surveying and mapping activities in the Philippines. The order also mandated that all new surveys and maps shall be referred to the new network and all old surveys shall be integrated into it.

  
• Benchmark (BM)


  A permanent or temporary fixed point of reference whose elevation is either known or assumed; should be easily recognized and located where they have the smallest likelihood of being disturbed

  
• Permanent benchmark (PBM)


  Those which are established at intervals throughout the country by the Philippine Coast and Geodetic Survey (PSGS) or Bureau of Lands. They serve as points of reference for levels in a given locality and their locations are determined by precise leveling methods.
• Temporary benchmark (TBM)


  Those set up by the surveyor for his own use in a particular surveying project. They may have assumed elevations and should be stable and semi-permanent marks such as wooden peg in concrete, nail or spike driven into a tree, an X mark in a bridge abutment or top of fire hydrant.

  
• Backsight (BS)


  A reading taken on a rod held over a point of known or assumed elevation. It is the vertical distance from the established line of sight to the point sighted and is always the first rod reading taken after the instrument has been set-up. It is referred to as plus sight (+S) since it is added to the ELEV of points being sighted to determine the height of instrument (HI).
• Foresight (FS)


  A reading taken on a rod held over a point whose elevation is to be determined. It is the vertical distance from the line of sight of the instrument to the point being observed and is usually the last reading taken before the leveling instrument is moved to another location. It is referred to as minus sight (-S) since it is subtracted from the HI to determine the ELEV of the point.
• Backsight distance (BSD)


  Measured from the center of the instrument to the rod on which a backsight is taken.
• Foresight distance (FSD)


  Measured from the center of the instrument to the rod on which a foresight is taken.
• Turning point (TP)


  An intermediate point between two benchmarks upon which foresight and backsight rod readings are taken. It is also referred to as change point (CP) and usually numbered consecutively. It should be located on a stable object like rock, stake driven into the ground or a paint mark on the concrete pavement.
• Height of instrument (HI)


  Elevation of the line of sight of an instrument above or below a selected reference datum. It is calculated as: HI = ELEV + BS

  
• Benchmark


  A fixed point of known elevation used as a reference in leveling

  
• Mean sea level or datum

  The reference elevation, usually 100 m MSL

  
• Backsight (BS)


  The distance measured from the center of the instrument to the rod on which a backsight is taken
• Foresight (FS)


  The distance measured from the center of the instrument to the rod on which a foresight is taken
• Turning Point (TP)


  An intermediate point between two benchmarks upon which foresight and backsight rod readings are taken, also referred to as change point (CP) and usually numbered consecutively, should be located on a stable object
• Height of Instrument (HI)


  The elevation of the line of sight of an instrument above or below a selected reference datum, calculated as ELEV + BS

  
• Leveling Instruments


  • Spirit level or dumpy level or wye level
  • Hand level, Alidade
  • Transit, Theodolite
  • Aneroid barometer
  • EDMs
• Leveling Rods


  • Graduated rods 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 or metal with graduations in meters and decimals

  
• Types of Leveling Rods

  • Self-reading rod
  • Target rod