Survey Lab Manual

Chennai Institute of Technology

1. STUDY OF CHAINS AND ITS ACCESSORIES

Aim:To study about the chain surveying and their instruments. Chain Surveying Description of the Instrument:a) Chain: 1. The chains are made in length of 30m and 20m 2. Brass tullies are fixed at every 5m length. 3. Small brass rings are provided at every 1m length. 4. This is composed of 100 to 150 pieces of galvanized mild steel wire 4mm in diameter called links. 5. The ends of each links in 20 cm i.e. the distance between the two consecutive middle rings. 6. The length of the chain is measured from the outside of one handle to the other. 7. The ends of the chains are provided with brass handles for dragging the chain in the ground. 8. The length of each links are bent into a loop connected together by means of three oval rings which give flexibility to the chain. 9. To fold the arrows in position with the handle of the chain a …….. is cut on the outside surface of the handle. b) Ranging Rod:1. They are usually of 2m or 3m in length. 2. They are in circular cross section and having alternate black, white and red bands of 20cm in length each to make them visible at a distance. 3. They are used for ranging the lines and for marking the position of the points on ground. c) Arrows: 1. They are made up of a wire 4mm diameter for 40cm length. Faculty Name: G.Gokila Rani

Department: Civil Engineering


2. They are pointed at one end for inserting into the ground and bent at the other end for facility of carrying. 3. They are used for mark the end of each chain during chaining.

d) Offset Rods: 1. They are used for measuring rough offsets. 2. They are rounded rods with pointed end on one side and provided with notch or a nook at the other end to facilitate the pulling, the pushing the chain through obstructions.

.

Faculty Name: G.Gokila Rani



e) Cross Staff:1. It consists of a wooden block with two fine row cuts at right angles to each other on the top. 2. It is used to set a perpendicular at a given point on the chain line. 3. This head is fixed to a top of an iron staff with pointed end to derive into the ground. f) Optical Square:1. This is also used to set a perpendicular with more accuracy. 2. This has two mirrors placed at angle of 45º to each other. 3. By means of reflection we can see the ranging rods along the chain line and the offset point at right angles to the chain line simultaneously.

Result:




2. MEASUREMENT OF DISTANCE BY CHAINING AND RANGING OF A LINE Aim:To find out the distance between two points using chain and to find the pacing length. Instrument Required:•

Chain

•

Arrows

•

Ranging rods

•

Pegs and hammer

Procedure:The two end points A and B are selected on a survey line which is to be measured. A ranging rod is erected at the point B, while the surveyor stands with another rod at point A. A rod is established at a point in line with AB at a distance not greater than one chain length from A. The surveyor at A then signals the assistant to move transverse to the chain line till he is line with A and B. Similarly other intermediate points can be established. Then by using chain, the distance is measured. To find the pacing length, we should walk along the chain line and it is found from pacing length. =Distance between the points/No of steps

Calculation:The distance between two points=(No of arrow*Nominal length +Fractional length)m

Result:The distance between two points ____________




3. CHAIN TRAVERSING Aim:To find out the area of the one block by traversing with the chain. Apparatus Required:•

Chain

•

Arrows

•

Cross Staff

•

Ranging rod

•

Pegs and hammer

Procedure:The boundary of the block is surrounded around the building and chaining is done offset is carried out at the corner and at the steps, using a cross stuff. The diagonal is also measured for checking purposes. The distance and the chain ages are plotted in a sheet and the area is calculated by taking approximate scale. AB End

?

?

BC End Faculty Name: G.Gokila Rani



A

B

?

?

D

C ?

Calculation:Plinth Area=½(ab*ad)+½(bc*cd)+½(hg*he)+½(te*ty) =?

Result:Area of the Block is___________m2




4. COMPASS TRAVERSING Aim:To find out the distance between two point A&B taking suitable observations using compass. Apparatus required:•

Prismatic compass

•

Tripod

•

Ranging rod

•

Tape

•

Pegs & hammer

Theory:There are two types of compasses:1) Prismatic compass 2) Surveyor’s compass.

Prismatic Compass:Prismatic compass is very valuable instrument. It is usually used for rough survey for measuring bearing and survey lines. The least count of prismatic compass is 30 min. It consists of circular box of 10cm-12 cm dia. of non magnetic material. Pivot is fixed at the centre of box and is made up of hard steel with a Sharp pivot. Graduated aluminum is attached to the needle. It is graduated in clockwise direction from 0º to 360 º.the figures are written in inverted. Zero Is written at south end and 180 at north end and 270 at the east. Diametrically opposite are fixed to the box. The sighting vane consists of a hinged metal frame in the centre of which is stretched a vertical Horse hair fine silk thread of which is stretched a vertical hair. It presses against a lifting pin which lifts the needle of the pivot and holds it against the glass lid. Thus preventing the wear of the pivot point to damp the oscillations of the needle when about to take reading and to bring to rest quickly, a light spring is brought lifted inside the box. The face of the prism can be folded Faculty Name: G.Gokila Rani



out the edge of the box when North end is used Sometime the sighting vanes is provided with a hinge mirror Which can be placed upward or downwards on the frame and can be also Slided along it is required. The mirror can be made inclined at any angle so that Objects which are too high or too low can be sighted directly by reflecting.

Adjustment of the Prismatic Compass:The compass may be held in hand but for better results it should be fitted at the top of tripod having ball and socket arrangement. The adjustment of a compass is done in the following three steps.

1) Centering: - The compass fitted over the tripod is lifted bodily and placed approximately on the station peg by spreading the leg of a tripod equally, the centre of the compass is checked by dropping a small piece of stone from the centre of the bottom of the compass so that it falls on the top of the station peg. A plumb bob may be used to judge the centering either but attaching it with a hook providing at the bottom or otherwise by holding it by hand. 2) Leveling: - After the compass is centered, it is leveled by means of ball and soket arrangement so that the graduated circle may swing freely. It can be checked roughly by placing a round pencil on the top of the compass, when the pencil does not move, that is roughly the horizontal position. 3) Focusing the prism: - The prism attached is moved up and down so that graduation on the graduated circle should become sharp and clear.




Surveyor’s compass:The object vane is similar to that of prismatic compass. The eye vane consists of a simple metal vane with fine silt without the prism. The graduated ring is directly attached to the box and not to the needle. The object should be signed first with the object and eye vane and reading is then taken against the north end by looking vertically to the top glass. Procedure: The station points A and B selected whose distance to be measured. The two instrument stations O1 and O2 are selected to a known distance. The boundary of the lines O1B, O1A, O1O2, O2B, O2O1 are taken using a compass. Then using this, the included angles are found. Then by the sine & cosine formula the distance between the required points is formed.




Observations:-

Bearing Instrument

Line

O1

O1A O1B

Fore bearing

back bearing

?

O1O2 O2

O2A O2B

?

?

O2O1

Calculation:Angle at O2O1B =? Angle at BO1A =? Angle at O2O1A =? Angle at BO2A =? Angle at AO2O1 =? Angle at BO2O1 =?

By using sine formula, O1O2/Sin θ1 = O2A/Sinθ2 = O1A/Sinθ3 Faculty Name: G.Gokila Rani



By using cosine formula, A2 = B2 + C2 – 2BC cos θ

Result:The prismatic compass is studied and the distance between two points A&B measured using a compass is ___________m.




5. PLANE TABLE SURVEYING-RADIATION METHOD

Aim:To find out the distance between two given points, by the method of radiation. Apparatus required:•

Plane table with tripod

•

Plumbing fork

•

Alidade

•

Trough compass

•

Spirit level

•

Pegs & hammer

•

Ranging rods

Theory:Description of the Instrument:Plane table surveying: The system of surveying in which field observation and plotting work i.e. both are done simultaneously is called plane table surveying. Plane Table:The drawing board made of well seasoned wood such as teak or pine which is used for the purpose of plotting is called plane table. It is available in sizes 500x400x15mm, 600x5000x15mm and 750x600x20mm.The top surface of board is perfectly plane and to the underneath it is fitted with a leveling head or ball and socket arrangement. The table is mounted on a tripod by means of a central screw with a wing nut or in such a manner so that the board can be revolved, leveled and clamped in any position.




Alidade:The tool or instrument which consists of metal (usually of brass) or wooden (well seasoned) rule 40cm to 60cm long, 3cmto5cm wide and fitted with two vanes at the ends is called an alidade.

Trough Compass:The compass which is used to mark the direction of the magnetic meridian on the plane table is called trough compass. Sprit Level:A small sprit level circular or rectangular is required for seeing if the table is properly level. The level must have flat base so that it can be placed on the table. Plumbing fork or U-frame:The plumbing fork to which is attached a plumb bob ,used for centering the plane table over the station occupied by the plane table. it is also meant for transforming the ground point on to sheet so that both the points should be in the same vertical line




Radiation Method:When from a single set of plane table on instrument station different details are located on the sheet, the method is known as radiation method In this method the rays are drawn from the instrument station to the point to be located, then the distances are measured from the instruments station to the point and the position of the each point is plotted on the sheet using a suitable scale. The method is most suited for surveying small areas which can be controlled by single setting. It can also be used in combination with other method. This method can be applied for locating distant points if the distances are obtained tacheometrically with the help of the telescope alidade.




Procedure:1) Select the position of the table where it is be set so that all the points to be located are visible from it. Let ’O’ be the position of such a point on the ground. 2) Set the plane table over this point and level it. Draw the North line in the top corner of sheet by means of trough compass at the table. 3) Now transfer the position of the point ‘O’ on the ground to the sheet by means of the a plumbing fork. The point ‘O’ will represent point ‘o’ will represent point ‘O’on the ground. 4) With the alidade touching the point ‘o’( may be represented by fixing a pin), sight the point A in the field. Draw the ray along the fiducial edge. Measure the distance of this point from the instrument station by means of tape and plot the point ‘a’ corresponding to point ‘A’ in the field to scale in the sheet. 5) Similarly sight other points such as B, C, D, E etc. and measure their distances from the instrument station. Plot them to scale to get their position on the sheet such as b, c, d etc. on the sheet.

Result:The distance between the points A, B and C, D by the method of radiation are ___________




6. PLANE TABLE SURVEY - METHOD OF INTERSECTION

Aim: To find out the distance between the two given points by method of intersection. Accessories Required: •

Plan table with tripod

•

Plumbing fork

•

Alidade

•

Trough Compass

•

Sprit Level

•

Pegs and Hammer

•

Ranging rods

Procedure: 1. The table is setup at 0₁ and leveled the point 0₁ is transferred from the ground to the sheet by plumbing fork and the table is clamped. 2. With the help of the trough compass, the north direction is marked on the sheet. 3. Pivoting the alidade about 0, A is sighted. The distance 0,A is measured and plotted along with the ray to get ‘a’. The base line 0,a is thus drawn. 4. Pivoting the alidade about 0₁, the points B,C,D etc., are sighted and corresponding rays are drawn. 5. The table is shifted to 0₂ and set up at 0₂. The table is roughly oriented by compass and finally by back sighting A. 6. Pivoting the alidade about 0₂,B,C,D etc., are sighted and the corresponding rays are drawn, to intersect with the previously drawn rays in b,c,d etc., The positions of the points are thus mapped by the way of intersection.




Result:The distance between the points A, B and C, D by the method of intersection are _______m.




7. METHOD OF RESECTION-THREE POINT PROBLEM Aim:To locate the instrument station with the help of three well defined points. Accessories Required:•


•

Plumbing fork

•

Trough Compass

•

Sprit level

•

Alidade

•

Ranging rods

•

Pegs and hammer

Procedure:Let ABC are the three well defined points on the field. The well defined points are plotted on the drawing sheet using suitable scale. 1. The table is set up at P and leveled towards ‘A’. 2.

The table is turned till ‘A’ is bisected and it is clamped. Then the alidade is centered at ‘C’ and ‘B’ is sighted A ray cb is drawn. The ray will not pass through B.

3. The alidade is placed along ‘ac’ so that c is towards c. Table is turned until c is bisected and clamped. 4. Place the alidade along ‘a’ B is sighted and a ray is drawn through ‘a’. The point of intersection with the previous is d. Placing the fiducially edge of alidade along ‘bd’ unclamp the table. The table is rotated until B is bisected and clamped. Now the table is fully oriented. 5. To find the point P, the alidade is placed at ‘a’. A is sighted, A ray is drawn through ‘a’. The point of intersection of the ray with the line bd is P.




B A

C b

P

Fig. Three point problem

Result:The Instrument station P is located using three well defined points. Faculty Name: G.Gokila Rani



8. METHOD OF RESECTION-TWO POINT PROBLEM

Aim:To locate the instrument section with respect to two well defined point. Accessories Required:•


•

Plumbing fork

•

Trough Compassa

•

Alidade

•

Sprit level

•

Ranging rods

•

Pegs and hammer

Procedure:1. A suitable auxiliary station D is chosen near so that the angles ‹cad AND ‹CBD are neither two acute nor abtuse. The table is set up at D, leveled and oriented approximately using a magnetic compass or by adjustment. Thus the line ab is made parallel to AB and the table is clamped. 2. Pivoting the alidade on ‘a’, A is sighted and a ray through ‘a’ is drawn. Similarly ray is drawn through ‘b’. The point of intersection in d, which is approximate position of D. d is transferred to the ground with the help of plumbing fork and peg is driven. 3. With the alidade pivoted at d, c is sighted and a ray d, c is drawn to respect the distance DC and the position of C, is marked by estimation. 4. The plane table is shifted to C and entered so that C, is above C. The table is oriented by back sighting P. 5. Pivoting the alidade at ‘a’. A is sighted and a ray ac₂ is drawn to intersect d, c at c₂. Thus c₂ represent approximate position of C.




6. Pivoting the alidade at c₂, B is sighted and the ray is drawn through c₂. In general c₂ will not pass through b, where b is approximate position of B. 7. To eliminate error in orientation, the alidade is placed along ab and a ranging rod is fixed at p. 8. The alidade is placed along ab and the table is clamped and turned until p is bisected, when it is freely oriented it is clamped. 9. To find c, the alidade is placed on ‘a’ and a ray is drawn through ‘a’ and it is placed on ‘b’ and a ray is drawn through ‘b’ and a ray is drawn through ‘b’. The point of intersection in c.

A

a

B

P

b

C – Instrument Station AB – Well defined points

Result:Thus the station point c is located from the two well defined points.




9. STUDY OF LEVELS AND LEVELING STAFF

Aim:To study about the types of levels and leveling staff. Description of the Instruments:The Level:The instrument which is used for measuring related elevations is known as a level and consists of the following parts. 1. A telescope to provide a line of sight. 2. A level tube to make the line of sight horizontal. 3. A leveling head to bring the rubber of the level tube at the centre of its run. 4. A tripod head to support the above three parts of the level. Types of Level:Dumpy Level: 1. This consists of a telescope rigidly fixed to its supports. 2. It can neither be rotated about its longitudinal axis nor it can be removed from the support. Wye Level: 1. The telescope is supported in y-support ant its rigidly fixed to the supports. 2. The telescope can be removed from the supports reversed end to end can be removed about in longitudinal axis. Reversible Level: 1. The telescope can be rotated about its longitudinal axis in the sockets and also can be wth drawn from its sockets and replaced end for end. Faculty Name: G.Gokila Rani



Tilting Level: 1. The telescope can be tilted within a few degrees is vertical plane by a tilting screen. 2. This is designed for precise work. Telescope Leveling Staff:1. It consists of three pieces; top piece is solid 1.25m long where as central. Piece 1.25m is below. 2. When fully extended total length of the staff is 4m. 3. Each meter is subdivided into 200 divisions. The thickness of the graduations being 5mm. 4. Spacing indicating the diameter readings is marked in red while all other spaces are marked in black against white background. 5. The diameter markings are continuous throughout the staff. 6. The graduations are inverted so that, through telescope, they appear correct.

Temporary adjustments of dumpy level:Setting up the level:1. This included fixing the instrument the tripod and leveling the instrument approximately by leg adjustment. Faculty Name: G.Gokila Rani



2. To do this release the clamp hold the instrument right and fix it on a tripod by turning round the leveling head with left hand. 3. The tripod legs are adjusted so that the telescope is at a convenient height and is leveled approximately. Leveling Up:1. This is done with the help of three foot screws and by using plate levels. 2. The object of leveling is to make its vertical axis truly vertical. 3. First loosen the clamp screw and then the instrument until the longitudinal axis of the bubble tube is parallel tube to the line joining any to leveling screws. 4. Holding the two foot screws with the thumb and first finger of each hand moves either towards each other or away from each other until the bubble comes to the center. 5. Rotate the upper plate through 50º until the axis of the plate level coincide a line joining the third foot screw c and the mid point of the first two screw A and B. 6. Hold the third screw with thumb and fore finger of the hand and turn it will plate bubble is central. 7. Rotate the upper plate through 90º to its original position. 8. Rotate again through 90º and step 6 is repeated. 9. Steps 4 and 6 are repeated until the bubbles remain central in both the position. 10. The instrument is rotated through 180 º and in this position the bubble should remain central if the instrument is adjusted. Elimination of parallax:1. Parallax is a condition a rising when a image formed by objective is not in plane cross hairs. 2. To get accurate sighting this should be eliminated and this is done by •

Focusing the eyepiece for distinct vision of cross hairs.

•

Focusing the objective to bring the image of the objective in the plane of cross hairs.

Result:




10. FLY LEVELING

Aim:To find the R.L of various points of the R.L of the B.M is given by 100m. Apparatus Required:•

Dumpy level

•

Tripod

•

Leveling staff

Procedure: The instrument is placed in a point there by we can easily sight B.M The following initial adjustments are made. (i)

The instrument is fixed in tripod stand and it is leveled approximately by leg adjustment.

(ii)

The clamp is loosened and the instrument is turned until the longitudinal axis of the plane level is parallel to the line joining only two leveling screws.

(iii)

The foot screws are uniformly turned such that it means either inward or outward such that the bubble is at the center.

(iv)

The plate level is located and made parallel to line joining the third foot screw and mid point of the first two foot screws.

(v)

The third screw is adjusted to make the bubble central.

(vi)

The instrument is rotated through 180º to check whether the bubble remains central after rotation the eye piece is adjusted to avoid parallax error.




The staff is kept at a given bench mark and readings is noted in back sight column. In the same instrument station, another change point is shifted and reading is noted as fore sight. Then the same instrument is shifted to another station. Similarly back sight is taken. Thus the procedure is repeated by changing the point and instrument station from the reading taken R.L of point can be found.

Observation:Tabulation:Back Sight (m)

Inter Sight (m)

Fore Sight (m)

Reduced Level

Height

of

Collimation

€B.S ~ €F.S = F.R.L ~ L.R.L

Result:Thus R.L of the points was found out by using fly leveling.




11. CHECK LEVELLING

Aim:To check the given reduced level of the starting point using Dumpy level. Apparatus required:•

Tilting level

•

Tripod

•

Leveling

Procedure: The instrument is placed at a point there by we can sight B.M The temporary adjustments are done. The instrument is leveled by eye adjustment and by adjusting tripod legs. Then parallax error is eliminated. The staff is kept at a given BM and the readings are noted as Back sight. In the same instrument station other points are sighted and the readings are noted as intersight except the last which is fore sight. The instrument is shifted to another station and the readings are taken. The process is repeated to come back to the starting point to check the given R.L of the given starting point. Then by any of the two methods (ie) by height of collimation method by rise and fall method then the required values are calculated.







Tabulation:B.S (m)

I.S (m)

F.S (m)

Rise (m)

Fall (m)

R.L (m)

Remark

Result:Thus the given R.L of the starting point is checked using dumpy level by check leveling.




12. LONGITUDINAL SECTION AND CROSS SECTION

Aim:L-Section and cross section of the road

Apparatus required: •

Dumpy level

•

Leveling staff,

•

Ranging rod,

•

Tape

Theory:Profile Leveling:The process of determining elevations at points at short measured intervals along a fixed line is called Longitudinal or profile leveling. Cross Sectioning:It is a method of leveling to know the nature of Ground on either side of the centerline of the proposed route. Levels are taken at right angles to the proposed Direction of the road end at suitable distances and leveling is carried out along this cross Section. During location and construction of highways, Rail tracks sewers and canals strakes or other marks are placed at various aligned points and the undulation of the ground surface along a predetermined line is adjoined. The line of section may be single straight lines changing directions. Levels are taken at right angles to the proposed Direction of the road end at suitable distances and leveling is carried out along this cross section. Cross section are the sections run at right Angles to the centerline and on the either side of it for the purpose They are taken at each 10,m station on the centerline. The length of Cross section depends upon the nature of the work if cross sections are Short they are set square out by edge. If long they are set out by the Optical square, box sextant or theodolite. They are serially numbered from the beginning of the Faculty Name: G.Gokila Rani



Centerline and are taken simultaneously with the longitudinal section they may be taken at the hand level, level, abney level or theodolite.

Procedure:Let ABC be the line of section set out on the ground and marked with pegs driven at equal interval (say 20m to 30m) as in the figure. The level is set up generally on one side of the profile to avoid too short sight on the points near the instrument and care is taken to set up the level approximately midway between two change points. The leveling is started from the bench mark of known value. From each set up staff reading are taken on pegs already fixed at the desired interval and also at significant points where about changes of slope etc. occur. All these readings are recorded as intermediate slight against the respective chain ages along the line in the level book. Other data of the level book is also filled up before starting the work. When the length of sight is beyond the power of the telescope (usually it is 100m), the foresight on the change point is taken. The level is then is then shifted and setup in an advanced position and a back sight is taken on the change point. The change point may or may not lie in the line of section. Chaining and reading are then continued as before, till the whole line of section is completed. The work is to be checked in the progress of leveling by taking reading on other bench marks, on the way or on bench marks fixed by differential leveling. The fore and back bearing of the section line should betake and recorded. Next sketches of the bench mark, change points, and other feature such as nallah, a road, canal, etc. crossing the section line be drawn and fully described in the remarks column of the level-book. The procedure and corresponding reading and values are represented on the page of a level-book for a part of road project.




Plotting The Longitudinal Section:-

Level Book:Whenever leveling operation is carried out the staff reading taken in the field is entered in the note book called a Level-Book. Each page of it has the following columns which help in booking of reading and reduction of levels.




Result:Thus the longitudinal section and cross section of road was plotted and readings are noted in the page level book.




13. CONTOURING

Aim:Contour plan of given area

Apparatus:•

Dumpy level,

•

Prismatic compass,

•

Chain 20m, 30m,

•

Metallic Tape,

•

Ranging road Leveling staff,

•

Pegs line.

Theory:Contouring:The elevation and depression the undulations of the surface of the ground are shown as map by interaction of level surface with by means of contour line. a contour may be defined as the line of intersection of a level surface with the surface of the ground. Characteristics of Counter Lines The following are the Characteristics of the contours/ contour lines. All points on the same contour line will have the same elevation. Contour lines close together represent steep ground, while uniform slope is indicated when they are uniformly spaced. A series of straight, parallel and equally spaced contours show a plane or flat surface. Contour lines of different elevation cannot merge or cross one another on the map, expect in the case of an overhanging cliff. A vertical cliff is indicated when several contours coincide. A contour line must close upon itself either within or without the limits of the map. Series of closed contour lines on the map either represent a hill or a depression according as the higher or lower values are inside them as shown below. Faculty Name: G.Gokila Rani



A contour will not stop in the middle of the plan. It will either close or go out of the plan. Ridge or water shad and valley lines are the lines joining the top most or the bottom most points of hill and valley respectively, cross the contours at right angles. A ridge line is shown when the higher values are inside the loop, while in the case of a valley line, the lower values are inside the loop as shown in fig (1.3)

Contour lines are not drawn across the water in the stream or river because the water level in that is not constant; but contours are drawn along the bed of a river or a stream.




Uses of contour map: For preparing contour map in order to select the most economical or a suitable site. For getting the importance about ground whether it is undulating or Mountainous To locate the alignment of canal so that it should follow a ridge line, thus canal construction will be economical and will command maximum irrigated area. To make the alignment for the road, railway so that the quantity of earthwork both in cutting and filling should be minimum. To find out the capacity of the reservoir or a volume of earthwork especially in the Mountainous region. For preparing contour map in order to select the most economical or suitable site. As its definition itself indicates the line joining the points of same elevation that means it naturally prefers the condition of nature of ground itself. It is also used for irrigation purpose as from it capacity of reservoir. Locating Contours:By cross-section method:This method is commonly used in rough survey, cross sections are run traverse to the contour line of road, and railway as canal and the point of change of slope (representations) are located. The cross-section line may be inclined at any angle To the centerline if necessary. The




spacing of the cross sections depends upon the characteristics of the ground. By interpolation of contour is meant the process of spacing the contour proportioning between the plotted ground points. Contour may be interpolated by 1. Estimation 2. Arithmetical calculations 3. Graphical method .in all these methods It is assumed that the slope of the ground between any two random points is uniform.

Result:Thus the contour of given area is drawn in the sheet.




14. STUDY OF THEODOLITE

Description of the Instrument:Transit Theodolite:It consists of a telescope which can be revolved through a complete revolution about its horizontal axis in a vertical plane. Parts:Leveling head, telescope, vertical circle, index frame, A frame lower plate, upper plate, level tubes, plumb bob Leveling Head:The leveling head consists of two parallel triangular planes known as tribrach plates. The upper tribrach has three arms, each carrying a leveling screw. The lower tribrach has three arms each carrying a leveling screw. The lower tribrach plate or foot plate has a circular hole through which plumb bob may be suspended. Telescope:The telescope is an integral part of the theodline and is mounted on a spindle known as horizontal axis or trunnion axis. In most of the transits, internal focusing telescope is used. Vertical Circle:The vertical axis is a circular graduated arc attached to the trunnion axis of the telescope consequently the graduated arc rather with the telescope when the latter is turned about the horizontal axis.




Index Frame:The index frame is a T-shaped frame consisting of a vertical leg known as clipping arm and a horizontal bar known as vernier arm of index corm. At the two extremities of the index arm are fitted two verniers to read the vertical circle.

A Frame:Two standards resembling the letter A are mounred on the upper plates. The trunnion axis of the telescope is supported on these. Lower Plate:The lower plate is attached to the outer spindle. The lower plate carries a horizontal circle at its leveled edge and is therefore also known as the scale plate. Upper Plate:-




The upper plate or vernier plate is attached to the inner axis and carries two verniers with magnifiers at two extremities diametrically opposite. The upper plate supports the standards. Plumb Bob:A plumb bob is suspended from the hook fitted to the bottom of the inner axis to centre of instrument exactly over the station mark. Temporary Adjustments of A Theodolite:1.

Setting over the station

2. Leveling up 3. Elimination of parallax Setting Up:The operation of setting up includes centering of instrument over the station mark by a plumb bob and approximate leveling with the help of tripod legs. Leveling Up:The object of leveling is to make the instrumental axis fully vertical. This is done with the help of three foot screws. 1. First loose the clamp screw and turn the instrument until the longitudinal axis of the bubble tube is parallel to the line joining any two leveling screws. 2. Turn the foot screws uniformly until the bubble comes to centre. 3. Rotate the instrument until the axis of the level tube coincides with the third foot screw. 4. Turn the third screw either inward or outward to make the bubble central. 5. Repeat the above procedure thrice. Rotate the instrument through 180º. And in this position the bubble should remain central otherwise repeat the procedure.




Elimination of Parallax:Parallax is a condition arising when the image formed by objective is not in the plane of cross hairs. It is eliminated by focusing eye piece for distinct vision of cross hairs and forwarding the objective to bring the image of objective in the plane of cross hairs.

Result:



Survey Lab Manual

Recommend Documents