LEVELING

• LEVELING -is the process of directly or indirectly measuring vertical distances to determine the elevations of points or their differences in elevation.
• PLUMBLINE -it is a vertical line used perpendicular to the tangent line of a level surface.
• LEVEL SURFACE -it is a curved surface which is at any point perpendicular to the direction of gravity or the plumb line.
• LEVEL LINE -a curved line in a level surface and all points of which are normal to the direction of gravity and equidistant from the center of the earth.
• Normal is term used for perpendicular
• HORIZONTAL SURFACE -it is a plane that is tangent to a level surface at a particular point. It is also perpendicular to the plumbline at the same point.
• HORIZONTAL LINE - A straight line in horizontal plane which is tangent to a level line at one point. This line is perpendicular to the direction of gravity.

  -In geometry, this is know as the Tangent Line
• VERTICAL LINE -at any point is a line parallel to the direction of the gravity. It can be shown by the direction taken by a string supporting a suspended plumb bob passing through a point.
• MEAN SEA LEVEL -It is 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. This surface is determined by getting the average of the heights of the sea surface for all its tide stages over a long period and may extend to about 20 years. This surface is also considered as zero elevation.
• DATUM -it is 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 selected datum
• DIFFERENCE IN ELEVATION -it is the vertical distance between two level surface in which the points lie
• GEOID -it is a surface of constant potential energy (equipotential surface) that coincides with mean sea level over the oceans
• REFERENCE ELLIPSOID -It is a mathematical surface of reference
• ORTHOMETRIC HEIGHTS -It is a geometrical distance between the Geoid and the point measured along the plumb line passing through the point. It uses different methods of levelling
• ELLIPSOIDAL ELEVATION -It is the elevation referred to the reference ellipsoid.

  -We use Global Positioning Systems to determine ellipsoidal elevations.
• DIRECT/SPIRIT LEVELING -commonly employed method of determining the elevation of points some distance apart by employing series of set ups of a leveling instrument along a selected route. - the most precise method of leveling
• RECIPROCAL LEVELING -accurately determining the difference in elevation between two intervisible points located at a considerable distance and between each point leveling could not be done in the usual manner. - Commonly employed when leveling across a wide river, deep ravine or cross canyons or gullies, where it would be difficult to maintain foresight and backsight distance equal or nearly equal
• TRIGONOMETRIC LEVELING -leveling method employed in determining difference in elevation with the aid of trigonometric concepts and computations by measuring the horizontal distance or sloping distance and the vertical angle between the points - it is a convenient method to use when it is required only to determine the elevations of principal stations or control points
• FORMS OF DIRECT LEVELING:

  • DIFFERENTIAL LEVELING

  • DOUBLE-RODDED LEVELING

  • THREE-WIRE LEVELING
• BAROMETRIC LEVELING - involves the determination of differences in elevation between points by determination of differences in elevation between points by measuring the variation of atmospheric pressure at each point.
• MERCURIAL BAROMETER - barometer which is accurate but suitable only for observations at fixed point
• ANEROID BAROMETER - barometer used in the field because it is light and can be easily transported
• STADIA LEVELING -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.

  -computed from observed vertical angles & 3 intercepts on a rod
• BORROW-PIT LEVELING -method to determine the relative elevation of points in borrow pit excavations for the purpose of calculating volumes of earthwork
• GRAVIMETRIC - measuring the differences in gravity at various stations by means of gravimeter for geodetic purposes
• GLOBAL POSITIONING SYSTEM -surveys where elevations are referenced to the ellipsoid but can be corrected if a sufficient number of pts. with datum elevations are located in the region surveyed.
• INERTIAL POSITIONING SYSTEM -it is employed where an inertial platform has three mutually perpendicular axes, one of which is “up” so that the system yields elevation of as one of the outputs.
• DUMPY LEVEL - most widely used direct leveling instrument

  -also known as "Engineer's Level"

  -provides excellent introduction to levels in general. -rigidly attached to the level bar
• WYE LEVEL -in this instrument, the stage carries two “wye” supports in which the telescope is fixed - To the body of the telescope, two hanged collars of equal diameters are fixed
• BUILDER'S LEVEL -is used in the construction field to set up level points and to check elevations

  -level vial is not as sensitive as in other levels and its telescope has less
• AUTOMATIC LEVEL -also known as a self-leveling level or a builder's auto level

  -is a professional tool that can establish or verify points in the same horizontal

  plane.

  -used to plan structures and developments and ensure structural soundness

  -It is equipped with prismatic devise called a compensator which is suspended

  on fine, non-magnetic wires
• TILTING LEVEL - leveling instrument can be tilted or rotated about its horizontal axis
• GEODETIC LEVEL -Another type of tilting level -Most of its parts are made of invar to reduce the effects of changes in temperature.

  -employed in first order leveling
• TRANSIT AS LEVEL -It can provide results which are fairly precise although not as good as obtained with conventional levels
• LASER LEVEL -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 3600 - advantage for layout work in construction since it is simple, one button operation - and fully automatic general construction
• HAND LEVEL - is a hand-held instrument used on surveys involving short sights where a low order of accuracy is sufficient.

  -widely used for rough leveling
• DIGITAL LEVEL - a leveling instrument which is fast, easy, and accurate.