What Is Surveying?
Surveying is defined as a method of determining the relative position of the points on, above, or below the earth’s surface by taking direct or indirect measurements of distance, direction, and elevation.
Two types of measurements are taken in surveying: Linear measurements and angular measurements. There are different types of surveying equipment used in civil engineering such as Chain, Theodolite, dumpy level, cross-staff, plane table, ranging road, measuring tapes, etc.
Importance of Surveying
- Planning and design of all civil engineering projects require measurements from surveying.
- Execution of work needs surveying too for transferring the points onto the ground.
- Fixing of state and national boundaries also required measurements obtained from surveying.
- Control points can be established with the help of surveying.
- Hydrographical and oceanographic charting and mapping are facilitated by surveying.
- A topographic map of the land surface can be prepared with the help of surveying.
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Types of Surveying
- Plane table
Types of Surveying Equipment
Following are surveying instruments and their uses to carry out surveying.
A. Instruments for Direct Linear Surveying
It is made up of cotton, coated linen, or any other synthetic material. Centimeters or decimetres are marked on the tape.
They are available in length of 20, 30, or 50 metres.
Some of the commonly used tapes in surveying are:
Cloth/linen tape: Available in lengths 10, 20, 25, and 30 metre. Also available in lengths of 33, 50, 66, and 100 feet. A brass ring is provided at the terminal end of this tape. Rarely used nowadays.
Metallic tape: Available in lengths 2, 5, 10, 20, 30, and 50 meters. Except for 2 and 5-meter tapes, other tapes have a small ring fastened at the ends which is of the same width as that of the tape for protection and are supplied in a leather or metal case with a winding device.
Steel tape: Available in lengths 1, 2, 10, 20, 30, and 50 meters. Except for 1 and 2 meter tapes, other tapes have a small brass ring fastened at the ends. These tapes are supplied in a corrosion-resistant metal case or a leather case with a winding device. It should be handled with care, as it is a delicate instrument. However, its quality and accuracy are far better than the other tapes.
Invar tape: Available in lengths of 20, 30, and 100 meters. Made up of an alloy of nickel and steel with a low thermal coefficient of expansion. Used for linear measurements that require high precision. It should be kept on reels of high diameter as they can be easily bent and damaged.
The chain is used to measure the distance on the ground. It gives much accurate measurement as compared to a tape.
A chain is a surveying instrument that is made up of connecting links of galvanized mild steel. The mild steel wire is bent into a ring and joined to each other with three small circular or oval rings. Each connecting link measures 20 cm.
A tally marker or a special joint is also installed sometimes to mark the distance of 5 metres.
The total length of the chain is 20 metre or 30 metre, which also includes a brass handle on each end. The handles are provided with swivel joints for they can be easily turned during surveying without being twisted.
Some of the commonly used chains in surveying are
- Metric chain– Widely used and available in lengths of 5, 10, 20, and 30 meters.
- Surveyor’s chain: Length is 66 feet and has 100 links Widely used for land measurement as 10 square chains make 1 acre.
- Engineer’s chain: Length is 100 feet and has 100 links. Brass tags are installed at every 10 feet.
- Revenue chain: Length is 33 feet and has 16 links.
A chain has to be tested if it is accurate and necessary adjustments are made if not.
Arrows are used for marking and are made up of hardened, and tempered steel wire of good quality.
10 arrows are usually sold with a chain. Its length is about 25-50 cm. One of its ends is sharpened while the other end is bent into a circular loop.
To mark the chain length on the ground, an arrow is inserted at the end of the chain.
Pegs are mostly made of timber. They are also used to mark out the locations on the ground at terminal points or the end of the survey line.
They are 2.5-3 square centimetre and are of 15 centimetre length with a tapered end.
A hammer is used to drive the peg into the ground.
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Ranging rods are 2-3 metres in length and are painted with alternate bands of two colors like white and black, red and white in succession. Each band is kept at a length of 20 centimeters.
Ranging rods are made of well-seasoned timber. Their cross-section is kept either circular or octagonal with 3 cm nominal diameter.
They are used to range an intermediate point on a survey line.
A red, yellow, or white flag is tied at its top while carrying out longer surveys because the rod is not visible beyond 200 metre distance.
It is similar to a ranging rod but is of 3-meter length. They are also wooden rods, circular in cross-section. Its one end is pointed with an iron shoe and at the other, a notch or hook is provided.
It is used to take rough offsets in the nearby regions. Also, the right angles can be set out with its help as it has two narrow slots at its centre.
It is used to transfer the points to the ground on a slope while chaining. It is also used to make the ranging poles vertical.
In theodolites, compass, plane table, and other surveying instruments, it is used for centring purpose.
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B. Instruments for Setting out Right Angles
It is the simplest instrument for setting out right angles on a survey line. It has a frame containing two pairs of opposite slits mounted on a pole. The pole can be used to fix the instrument on the ground.
Cross Staff has three types
- Open cross-staff: Two pairs of vertical slits are present which gives two lines of sight at right angles.
- French cross-staff: It has a hollow octagonal box set up on a pole. On each face, vertical slits are cut in the middle for sighting. The lines of sight are at an angle of 45° to one another.
- Adjustable cross-staff: It has two cylinders of equal diameter with sighting slits placed on each other. The upper box can be rotated and is provided with a vernier to take measurements. The lower box has graduations of degrees and its subdivision. Any angle can be set out with the help of this instrument.
It is more accurate than a cross-staff. Also, it is convenient to use for setting out right angles. It has a circular box with three slits.
The instrument is set on the line whose perpendicular is to be set out. Two slits point towards the ranging rod at the end of the survey line.
Then another ranging rod is to be set at such a point that the two images coincide with each other. This point is on the perpendicular of the initial line.
The principle of prism square is similar to that of the optical square. However, it is more precise as compared to the optical square.
It can be used in a similar manner as an optical square. Unlike the optical square, adjustment is not required, as the angle between the reflecting surfaces does not vary.
It has a cylindrical metal case and two telescopes fixed at 90°. It is used to set out offset lines at right angles.
C. Instruments for Setting out Directions:
Note: When the direction is established with respect to each other, it is called an angle. If the direction is established with respect to a meridian, it is called bearing.
It is a magnetic compass that measures the magnetic meridian. It is portable and hence convenient too. It can be either used in the palm or fixed on a tripod.
It comprises a magnetic needle attached to a graduated circular ring made up of aluminium. The needle orients itself to the magnetic meridian if it is on the pivot. Object vane and eye slit are attached to the compass box and help in locating the line of sight.
Object vane has thin vertical hair while eye slit has a vertical slit. A triangular prism is also attached below eye slit to suit different sighting requirements with both horizontal and vertical faces convex.
The south end corresponds to 0° while the west end corresponds to 90° and so on. Thus, the north end is at 180° while the east end is at 270°.
Temporary or permanent adjustments are to be carried out for the compass as required. Centring, levelling and focusing of the prism are the temporary adjustments. The temporary adjustments are to be made every time the instrument is set.
The graduated ring is directly attached to the box instead of the needle in the surveyor’s compass. The needle is allowed to float freely over the pivot, but it does not orient itself to the magnetic meridian as in the case of a prismatic compass.
Object vane and eye vane are similar to that of the prismatic compass, however, no prism is provided. The instrument is to be fixed on a tripod.
The readings through this compass are taken against the north end of the needle by looking through the top glass vertically. When the line of sight coincides magnetic meridian, north and south ends are at 0°. While the east and west end correspond to 90°.
Temporary or permanent adjustments are to be carried out for the compass as required. Centring and levelling are temporary adjustments. The temporary adjustments are to be made every time the instrument is set.
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D. Instruments for Setting out Angles
It is the most precise instrument for the measurement of horizontal and vertical angles. It is popular in various surveying applications.
There are two types of theodolite- transit and non-transit. Non-transit theodolites have become obsolete these days.
Transit theodolite is such theodolite in which the telescope can be revolved by 180° in the vertical plane.
Parts of transit theodolite include
- Vertical circle
- Index/Vernier frame
- Standards/A frame
- Leveling head
- Two Spindles
- Lower/Scale Plate
- Upper/Vernier Plate
- Plate levels
- Plumb bob
- Striding level
Temporary adjustments of a theodolite include setting over the station, levelling and elimination of parallax. Setting up includes centring with a plumb bob and approximate levelling. Levelling up is done by the screw head- it is either three screw head or four screw head. Parallax is eliminated by focusing the eyepiece as well as the objective.
A theodolite can be used to,
- Measure the magnetic bearing of a line
- Measure direct angles
- Measure deflection angles
- Extend a straight line
- Establish a straight line between two points
- Locating point of intersection between two straight lines
- Setting out a horizontal angle
- Setting out an angle by repetition
- Establish grade
- Measure the difference in elevation
- Setting out curves
Repetition method or Reiteration methods can be employed for setting out horizontal angles to obtain more precise results.
It is an electronic transit theodolite with an electronic distance meter (EDM).
The crosshairs on the reflector of TS are aligned to the ranging rod and the vertical and horizontal angles are measured along with slope distances simultaneously.
It is used to take the measurement of-
- Horizontal angles: The rotation of the optical axis of TS from the instrument north in a horizontal plane gives the horizontal angle.
- Vertical angle: The inclination of the optical axis of TS from the local vertical gives a vertical angle.
- Slope distance: The distance between TS and target gives the slope distance.
TS can store data too as some have inbuilt internal electronic data storage, which can be uploaded to the computer, and the data analysis can be done with the help of various applications.
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E. Instruments for Plane Table Surveying
A tripod with a device for levelling and controlling its orientation is used in the plane table surveying.
Three types of plane tables are commonly used,
- Traverse table: It consists of a small drawing board mounted on a light tripod so that the board can be rotated about the vertical axis and then clamped in the desired position. The table is levelled via tripod legs by eye-estimation.
- Johnson table: It consists of a drawing board of size 45 * 60 cm or 60 * 75 cm. The head has a ball and socket joint while on its underside, a vertical spindle with thumbscrews is present. After loosening the upper screw, the table can be tilted via ball and socket joint and the levelling is done. Then the clamp is tightened so that the board is fixed in the horizontal position. The lower screw is loosened to fix the orientation of the table by moving it about the vertical axis.
- Coast Survey table: This survey table is used for high precision works and is more accurate than the previous two. There are three-foot screws for accurate leveling. A clamp and tangent screw are also provided for moving the table about the vertical axis.
It is a straight edge with some sort of sighting device.
It is made up of metal or timber rule having vanes at each end, which are folded down on the rule when the alidade is not in use. The vanes are to be kept perpendicular to the paper surface. A narrow slit is provided at one vane while the other vane has a hair or thin wire.
Both slits can thus give a line of sight, which is oriented towards the object to be sighted. Alidade can be rotated about the point corresponding to the station point. After sighting the object, a line is drawn against the working edge of the alidade. It is not suitable for hilly regions.
It is employed to take inclined sights. It is more accurate and provides a larger range as compared to plain alidade. It has a small telescope with a level tube and a graduated arc fixed on the horizontal axis. The horizontal axis is along the A-frame comprising of verniers as in transit.
All the parts are fixed on one side of a heavy rule while the other forms working edge. The inclination of the line of sight can be read from the vertical circle. The horizontal distance between the station point and the point sighted can also be computed by taking stadia readings of the staff at the object point.
It is used for centring in large-scale works at the station point. It is composed of a hair pin-shaped light metal frame with arms of equal length. A plumb bob is suspended from the lower arm.
It is also used for transforming the ground point on the sheet at the beginning of the survey so that the plotted point and ground station are in the same vertical line.
The upper arm is set on the table while the plumb bob on the lower arm corresponds to the ground point.
It is used to ensure that the table is levelled properly. The spirit level is placed at two perpendicular positions to ascertain that it is levelled.
It is either of tubular or circular type. Its base is flat so that it can be easily placed on the table. The table is levelled if the bubble is at the centre.
It is used for the orientation of the plane table towards the magnetic north.
In plane table surveying, a trough compass is used in which longer sides of the compass are flat and parallel. The sides can be used as rulers or can be used to coincide with a line already drawn on the paper.
A superior quality drawing paper is used that has a minimum effect of humidity of the atmosphere. Otherwise, the scale is disturbed.
Fibreglass sheets or paper with a backing of aluminium is used for high precision works.
F. Instruments for Direct Levelling
Barometric levelling, trigonometric levelling, and spirit levelling are the three principal methods employed for levelling.
Instruments used in spirit levelling are discussed below:
It is used to provide a horizontal line of sight. A horizontal line can be established with the help of a level. It is the line perpendicular to the plumb line.
Telescope, level tube, levelling head, and tripod are the main parts present in every level.
Temporary adjustments or station adjustments are to be made each time the level is set up include setting up the level, levelling up and elimination of parallax.
Differential levelling, profile levelling, cross-sectioning, reciprocal levelling, and precise levelling are some of the methods of precise levelling that can be carried out by a level.
There are four types of levels:
In this level, a telescope is firmly attached to two collars, which can be fixed by adjusting screws carried by a vertical spindle. The movement of the spindle about the vertical axis is regulated by a clamp screw. A tangent screw is used for precise movements. Two parallel plates are fixed on either three or four-foot screws in levelling head.
In this machine level, the telescope is carried on two vertical wye supports. Wye supports are in the form of curved clips. The telescope can be rotated or removed by raising the wyes. By fixing the wyes, the movement of the telescope is restricted.
The bubble tube is attached to either telescope or the stage that has wyes. Levelling head is similar to the dumpy level. Adjustments are done easily with accuracy in the case of Y level, unlike the dumpy level.
It is a combination of dumpy level and Y level. The telescope is fixed into two rigid sockets connected to the spindle with the help of a stage. After pushing the telescope in the socket through either end and tightening the screw, it acts like a dumpy level.
In the tilting level, the line of sight can be tilted slightly without tilting the vertical axis. Hence, this level facilitates quick leveling. The vertical axis is made roughly vertical with the help of the foot screws and bubble tube. The line of sight is made exactly horizontal when the staff is sighted by centering the bubble with the help of a fine pitched tilting screw with respect to vertical.
It is a straight rod with graduation marks with zero from the bottom. It helps in determining how much above or below is the station from the line of sight.
There are two types of levelling staffs- self-reading staff and target staff.
- Self-reading staff: It can be read directly by the instrument man. The readings are always taken from the telescope hence appear inverted. Therefore, readings are taken from downwards.
- Solid Staff– The smallest division is 5 mm. its length is about 3 metres.
- Folding Staff- Its length is 10 feet and it is hinged at the middle. It can be folded about the hinge when not in use for convenience.
- Telescopic Staff- It has three telescopic lengths of 1.5, 1.5, and 2 meters when stretched out fully. Normally, its total length is 5 metres. A circular bubble is also installed at the back in a suitable case.
- Target staff: It has a moving target against which readings are taken by the staff man. The sliding target comes along with the vernier.
Care of Instrument
- An instrument that has telescope attached should not be lifted by telescope. The instrument should be lifted from the base plate.
- The instrument should be carried on the shoulder and the clamps should be tightened, but not entirely so that they can yield if bumped.
- The lens of the telescope should be covered with a cap if not in use.
- To clean the lens, the dust can be brushed off with a brush. Avoid rubbing with a silk or muslin cloth.
- Tripod legs should not be spread on a smooth ground carelessly. Else, the legs can easily spread out.
- The graduation marks on the vertical circle or other places should not be touched more. They should not be exposed to sunlight, dust, or dampness.
- A fine film of water oil should be applied to the instrument if the survey is to be carried out near seashore to avoid the effects of saltwater.
- The instrument should not be left on the road unguarded.
- The screw or any part of the instrument should not be moved against strain by force. It can be lubricated to facilitate easy movement.
- The compass needle should not be allowed to swing needlessly. It should be lifted off the pivot when not in use.
Surveying Instruments And Their Uses
Following are different surveying equipments and their uses
|1.||Measuring tape||To measure linear distances|
|2.||Chain||To measure linear distances with precision|
|3.||Arrow||To mark a single chain distance|
|4.||Peg||To fix points on the ground To mark the terminal points of survey lines on the ground|
|5.||Ranging rods||To mark the position of stations To sight the stations To range straight lines|
|6.||Plumb bob||To establish a vertical line|
|7.||Cross Staff||To set out right angle from a survey line|
|8.||Optical Staff||To set out right angle from a survey line|
|9.||Prism Square||To set out right angle from a survey line|
|10.||Site Square||To set out right angle from a survey line|
|11.||Prismatic Compass||To find the bearing of traversing To find included angles|
|12.||Surveyor’s Compass||To find a bearing and angles between two survey lines in a traverse|
|13.||Level||To establish relative heights of points with the help of levelling staff|
|14.||Levelling Staff||To determine the height difference between two points To measure the height or depth of a point with respect to the datum|
|15.||Theodolite||To measure the angle between two pre-decided visible points in either horizontal or vertical plane.|
|16.||Total Station||To measure horizontal angle To measure vertical angle To measure slope distance|
|17.||Tripod||To support surveying instruments like compass, level, theodolite, or total station|
|18.||Plane table||To provide a solid and level surface to make field drawings|
|19.||Alidade||To sight a distant object and establish a line of sight in plane table surveying|
|20.||Plumbing fork||For centring of alidade in plane table surveying|
|21.||Spirit Level||To level the table in plane table surveying|
|22.||Trough Compass||To set the orientation towards magnetic north in plane table surveying|
|23.||Drawing Paper||To note field points on paper in plane table surveying|
Surveying can be understood as an art of deducing the relative position of the points on, above, or below the earth surface by taking direct or indirect measurements of distance, direction, and elevation.
Surveying holds importance in various civil engineering works. Establishing border lines, control points, hydrographical mapping, etc. are some of the broad fields of application of surveying.
Numerous instruments are used for surveying like tape, chain, arrow, pegs, ranging and offset rods, plumb bob, tripod, cross-staff, compass, plane table, spirit level, alidade, level, leveling staff, theodolite, total station, etc.
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