Surveying and leveling

Surveying

• It is the art of determination of horizontal distances, differences in elevation, directions, angles, locations, areas and volumes on or near the surface of the earth.
• It involves the measurement and recording of the size and shape (including the vertical shape) of an area on the earths surface.

The process of survey divided into two parts:

(a) Field work- taking measurements
(b) Office work- computing and drawing

Primary division of surveying

(a) Geodetic surveying

• Also called trigonometrical surveying
• To determine the precise positions on the surface of the earth of a system of widely distant points and the dimensions of areas
• In this survey, the curvature of the earth is taken into account, since large distances and areas are covered.
• Artificial earth satellites have come into wide use in this survey.


(b) Plane surveying

• The earths surface is considered as a plane
• Curvature of the earth is not taken into account, as the surveys extend only to small areas.
• The line joining any two points as a straight line, and all angles are plane angles.
• Surveys normally carried out for the location and construction of roads, canals and, buildings.
• In general, the surveys necessary for the works of man are plane surveys.

Levelling

• Art of determining the relative heights or elevations of points or objects on the surface of the earth.
• It deals with measurements in vertical plane.
• Construction of irrigation and drainage channels, terraces, bunds, reservoirs, outlet structures, etc, require the knowledge of surveying.
• For any soil conservation and land levelling work, levelling is the first job to be taken up.


Dumpy Level Instruments

Dumpy Level

Auto Laser Levels

Builders Dumpy Level

Digital Level


Terminology connected with levelling

 Datum:

 It is also called datum plane or only datum
 A datum surface is usually an imaginary level surface or arbitrarily assumed level surface, from which vertical distances are measured
 Its elevation is zero
 In India, the datum adopted for the Great Trignometrical survey (GTS) bench mark is the mean sea level at Karachi, now in Pakistan
 At present, the mean sea level at Madras is used

 Elevation:

 It is the vertical distance above or below the datum
 It is also known as reduced level (R.L.)
 The elevation of a point is plus or minus according as the point is above or below the datum

 Bench Mark (B.M.) :

 It is a fixed point of reference of known or assumed elevation with respect to which other elevations are calculated
 It is a starting point for leveling
 Temporary bench marks are selected at the end of a day`s work
 There are four kinds of Bench marks
 G.T.S (Great Trigonometrical Survey) Bench Mark
 Permanent Bench Mark
 Arbitrary bench Marks
 Temporary Bench Marks

 Line of collimation

 It is the line joining the intersection of the cross hairs to the optical centre of the object glass and its continuation. It is also called the line of sight.

 Axis of telescope

 It is a line joining the optical centre of the object glass to the center of the eye piece.
 Axis of the level tube or bubble tube
 It is an imaginary line tangential to the longitudinal curve of the tube at its middle point. It is also known as bubble line. It is horizontal, when the bubble is centered.

 Height of the instrument

 It is the reduced level (R.L.) of the plane of sight when the leveling instrument is correctly leveled
 It is also called the "height of the plane of the collimation" or the collimation
 The line of collimation will revolve in a horizontal plane known as plane of collimation or the plane of sight

 Back sight

 It is a staff reading taken on a point of known elevation, as on a bench mark or a change point.
 Also called a plus sight.
 It is the first staff reading taken after the level is set up and levelled

 Foresight

 It is the last staff reading denoting the shifting of the level
 It is the staff reading taken on a point whose elevation is to be determined.
 It is also termed as a minus sight.
 It is the last staff reading, denoting the shifting of the instrument

 Change point

 It is the point on which reading is taken just before and after shifting the instrument
 That means both back sight and fore sight readings are taken on this point
 It is also called a turning point.
 It should be taken on a firm, well-defined object