Squad No.4 was assigned the 1km stretch of road in which the Chackai Bridge and Chackai roundabout were included. The starting point was located about 350m. before the starting point of the Chackai bridge and the ending point lied almost 400m. after the Chackai roundabout.
The survey camp was conducted from the 4th September to the 12th September 2004. The survey was conducted from 7a.m. to 1p.m. everyday. This one kilometer stretch consisted of many features and land marks which included the Chackai bridge , the unpaved side streets on the both sides of the national highway , the wing walls of the bridge , the embankments which elevates the highway above the ground level, piers ( two nos. ) of the bridge , culverts , houses , a temple , marshy lands , filled up lands , railway tracks ( double lane ) , the Chackai roundabout , a school, shops etc.
We had a definite idea about the order in which each method was to be employed for the successful completion of the survey.
The survey was done in the following way:-
- Taking fly levels between starting point and end point.
- Setting out Grid lines and Nodal points.
- Taking reduced levels at the various nodal points.
- Plotting nodal points, road and other important landmarks using plane table.
- Drawing the topographic map and marking the important landmarks and drawingthe contours and completing the contours.
The main landmark of our survey area was the Chackai Bridge. To understand it better some information about the bridge is given below,
CHARACTERISTICS OF THE BRIDGE
Length of the Bridge : 83.2 m.
Width of the Bridge : 12m.
Number of Spans : 3 nos.
Foundation : Pile foundation
Bearing : Elastomeric type
Superstructure : Fish belly type
Year : 2003
Chainage : 9770m. to 9853m.
Number of Bridge Piers : 2
AIM:-
To complete the survey of the 1 km stretch of road on the National Highway and
plot a Topographical Contour Map using the data collected.
INSTRUMENTS NEEDED :-
Plane table with its accessories, Dumpy level with its accessories, theodolite with its accessories, pencil, papers, etc.
PROCEDURE:-
For any surveying a definite and proper plan about how the survey has to be carried out must be made.
The 1 km stretch assigned to us showed large variation of altitude such that the lowest point and the highest point of the region was at a difference of more than 11m. So, survey of the entire area in a single stretch was not possible. Time being an important factor governing the survey camp, we decided to do the survey of the area in 3 steps.
Survey of the unpaved road on the left side of the highway was done first. For this we had to transfer the starting point on the highway to the corresponding point below using the theodolite. Survey of this part was done and we rejoined the main road at a point which was some 100m. from the Chackai roundabout.
Our second stage of work was to survey along the main road in which the roundabout was also included. This stage of our work started from our starting point (SP) and extended till our end point (EP).
The third stage of the work was then commenced in which we surveyed the other side of the unpaved road and continued till the point corresponding to the starting point on the main road.
1. Reconnaissance Survey
Aim:-
To conduct the reconnaissance survey of the 1km. stretch to be plotted.
Instruments required:-
Board, papers, pencil etc.
Theory:-
The preliminary inspection of the area to be surveyed is called reconnaissance or reconnoitre.
Procedure:-
Reconnaissance was done on the left, right and along the highway. We walked along the entire stretch of road and thoroughly examined the ground so as to decide upon the best possible arrangement for the work. We noted various features which included marshy areas, filled up lands, railway track, embankments, culverts, bridge, temple, houses, shops, play ground, bridge piers, wing walls etc. and the various difficulties that may come during the survey and the suitable position of stations. The intervisiblities of selected stations were also tested. A neat hand sketch called an ‘index sketch’ or ‘key plan’ fairly resembling the plan of the area showing the boundaries, principal features position of instrument stations were made. Then the survey work was started.
2.Fly levels between Starting Point and End Point.
Aim:-
To take the fly levels along the National Highway, between the starting point and the end point.
Instruments needed:-
Dumpy level, tripod, plumb bob, leveling staff etc.
Theory:-
It is the operation of leveling to determine the elevations of points some distance apart or to establish the bench marks. The process of taking fly levels is the same as compound or continuous leveling. This is known as differential leveling.
Procedure:-
Forty points were marked along the road by the method of ranging. For this a ranging rod was held at the starting point (SP). Then another ranging rod is erected at a long distance away from the starting point. To range a third ranging rod in a line the surveyor stands about 2m. behind the ranging rod at the beginning of the line while the assistant holds the ranging rod vertically at arms length at the point where it is desired to establish the intermediate station. The ranging rod should be held lightly by thumb and forefinger and roughly in line. The surveyor then directs the assistant to move the rod to his right or left until the three ranging rods appear to be in a straight line. Finally the position of the ranging rod was checked by sighting over their lower ends in order to avoid errors on verticality of rods. Then the ranging rod is fixed. This process of direct ranging was continued till the end point (EP). The various hand signals used during this process were,
Hand Signals Type of movement
- Rapid sweeps with right hand. - Move considerably to the right.
- Rapid sweeps with right hand. - Move considerably to the right.
- Rapid sweeps with left hand - Move considerably to the left.
- Slow sweeps with right hand - Move slowly to the right.
- Slow sweeps with left hand - Move slowly to the left.
- Right arm extended - Continue to move to the right
- Left arm extended - Continue to move to the left
- Right arm up and moved to the right - Plumb the rod to the right
- left arm up and moved to the left - Plumb the rod to the left
- both hands above head and brought down - Correct
- both arms extended forward horizontallyand the hands depressed briskly - Fix
Then points at 25m. intervals were marked on the road along this straight line using a tape with arrows. These points were then transferred to the other end of the road using cross staff and ranging rod.
Along with this, the fly levels were also done to establish the intermediate bench marks. For this the dumpy level was set up at H1 and leveled and from there the points SP (104.67m.) and B(108.1m.). Then the instrument was shifted to H2 and point C (110.59m.) was sighted. Then to H3, from here point D (110.46m.) was sighted. Then we selected the next instrument station H4 and point E(107.915m.) was sighted. From instrument station H5 point F (105.165m.) from station H6, G(101.571m.) were sighted. From instrument station H7, point I (100.698m.) was sighted. Then the instrument station was shifted to H8 and then reduced level of EP (100.268m.) was sighted.
Result:-
The reduced levels of the intermediate bench marks were established.
3.Setting out Grid lines and Nodal points
Aim:-
To set out grid lines and nodal points on the area to be surveyed.
Instruments required:-
Cross staff, ranging rods, pegs, arrows.
Theory:-
Nodal points are the points which include the instrument station, change points and the permanent bench marks.
Procedure:-
Nodal points were chosen along the road and both sides of the road such that maximum points could be surveyed from these station points. Grid lines were set out taking perpendicular offsets from the points set along the road and embankments to a maximum distance that could be surveyed due to many marshy areas in the stretch. This was done in three stages,
- along the road
- on left side of the road
- along the road
- on the right side of the road
(i) Along the road
A ranging rod is fixed at the starting point and maximum number of points were set in line with this using a cross staff. The cross staff is set in between the two ranging rods and it is checked through the vertical slits whether the two rods are in line. Every 25m. is measured using a tape and marked. In the same way, these points were transferred on the other end of the rod using the cross staff. A new nodal point is fixed when lack of intervisiblity between points occurred and the procedure is repeated using cross staff and aligning the rod at the new nodal point with that at previously fixed points.
(ii) On the left side of the road
Since this stretch had its road at a much higher elevation than the ground, the point on the road had to be transferred to the bottom. This was done using a theodolite by transiting it and the point was set at the bottom. The theodolite was set and leveled on the embankment slope for this purpose. Then points were fixed along the embankment and also perpendicular offsets from these points were fixed.
A ranging rod was set at a very large distance so that if was visible from the starting point. Another ranging rod was fixed at the starting point and a third was used to range it in line with the two rods along the end of the embankment slope. 25m. points were marked along this ranging line using tape and from these points; perpendicular offsets were set to a distance as was possible due to the marshy lands here.
Where ever lack of intervisibility between stations or any intervening feature like railway track was met with a new nodal point was fixed and the above procedure was repeated. Thus an idea of the ground features was obtained on the left side of the road and stations and points were set for the dumpy level and plane table. The direction of the track, diameter of the pier was all recorded on the side.
- On the right side of the road
4. Taking Reduced Levels at various Nodal Points
Aim:-
To find the reduced level of points along the road and also on the left and right side of the road.
Instruments required:-
Dumpy level, tripod, leveling staff.
Theory:-
Leveling may be defined as the art of determining the relative heights or elevation of objects on the earth surface. It deals with the measurement in vertical plane.
Instruments used for leveling are level and leveling staff. The level furnishes a horizontal line of sight and leveling staff is used to determine the vertical distances of points below the horizontal line of sight.
Procedure:-
- Temporary adjustment of the level:-
- On the right side of the road
- Setting up of the level: - This includes fixing the instrument on the stand and leveling the instrument on the tripod. For fixing the instrument on the stand, release the clamp screw of the instrument, hold it in the right hand and fix it on the tripod stand by turning around the lower part of instrument with left hand. Then screw the instrument firmly. The tripod legs are to be adjusted so that the instrument is at a convenient height and tripod is approximately horizontal.
- Leveling up
:- For leveling , place the telescope parallel to any pair of foot screws and bring the long bubble to the center of the of its run by turning these screws equally either both inwards or both outwards. To move the bubble to the right turn the screws inwards and move it to the left, then turn the screws outwards. Turn the telescope through 90 degrees so that it lies over third screw and turn this screw for centering the bubble. Bring the telescope back to its original position; again bring the bubble to its center run. Repeat this until the bubble remains in the center of its run in both the positions, which are at right angles to each other.
Rotate the instrument through 180 degree and check the position of the bubble. It should remain central if the instrument is in adjustment.
- Elimination of parallax: - Parallax is a condition arising when the image formed by the objective is not in the plane of the cross hairs. Parallax can be eliminated by two steps,
- Focusing the eye piece: – to focus the eye piece for the distinct vision of the cross hairs, hold a sheet of white paper in front of objective. Move the eye piece in or out till the cross hairs are seen sharp and distinct.
- Focusing the object glass:- the telescope is now directed towards the staff. The focusing screw is turned till the image appears clear and sharp. The image so formed must be in plane of cross hairs.
- Focusing the eye piece: – to focus the eye piece for the distinct vision of the cross hairs, hold a sheet of white paper in front of objective. Move the eye piece in or out till the cross hairs are seen sharp and distinct.
- Leveling :-
- Along the left side of the road:- Since the road was at an higher elevation compared to its sides, we were able to start the survey work only after transferring the starting point to the ground below i.e. on the left side of the road. This was done by the transiting of theodolite. Then the level was set up at points previously fixed during reconnaissance survey so that maximum number of points can be seen from that instrument station. Leveling of the instrument was done and readings were taken at 50m. longitudinal offsets and 25m. perpendicular offsets. The left side of the road included many features including the filled up lands, culverts, marshy land, railway track, etc. So it was impossible to set perpendicular offsets at these points. So some reference point were chosen and the reduced level corresponding to those points were noted. There were culverts, huts, small playground, etc. in that area and hence reduced level corresponding to these points were also taken. The points were the reduced levels and were taken using dumpy level and were carefully marked for the purpose of plotting these points by plane table survey. Hence the reduced level at various points on the left side of the main road was obtained. In some points it was difficult was to measure the distance between the instrument station and the points with a tape (for the purpose of plotting these points), stadia hair method was used i.e. the instrument was set up, leveled and staff was held at the point whose distance from the instrument station has to be determined. Then the top and bottom hair readings of the staff was noted and the distances was calculated using the formula
D= KS + C
D is the distance to be measured
K is the instrument constant (the instrument we were using had constant as 100)
S is the difference between top and bottom hair readings
C the instrument constant (constant we had was 0)
(ii) Along the road:- Instrument was set up at suitable position so that maximum number of points are visible from the stations. Then the instrument was properly leveled and the readings were taken with the staff held at 50m. offsets along the length of road and perpendicular to the length of the road. Hence, the leveling was conducted for the entire 1km. stretch of the road.For the purpose of plotting the Chackai roundabout on the plane table sheet its dimensions had to be known. For this purpose eight points were selected along the perimeter of the round about and the corresponding distance between the instrument station and these points were determined by stadia hair method. These eight points and instrument station was carefully marked for use in the plane table survey. Also the medians were also plotted in the plane table using the same method.
- Along the right side of the road: - leveling was conducted for this side starting from the end point towards the starting point. Then the instrument was properly leveled and the reduced levels corresponding to 50m. longitudinal offset and 50m. perpendicular offset were taken. Since most of the area on left was marshy, perpendicular offsets were set up only at some points. The leveling were continued till the starting the point was reached. All the features on right side of main road including the filled up land, marshy land, residential areas, a temple, railway line, embankments, piers of the bridge, culverts, etc. were marked and the reduced level corresponding to these points were also taken.
5.Plotting Nodal Points, Road and other important points using Plane Table
Aim:-
To conduct the plane table surveying along the road and on both sides of the road extending from Chackai bridge to the Chackai roundabout.Instruments used:-
Drawing board, drawing sheet, alidade, trough compass, U-frame with a plumb bob, spirit level, scale, pencil etc.
Theory:-
Plane table is a graphical method of surveying in which the field work and plotting are done simultaneously. It is most suitable for filling in of the details between the stations previously fixed by triangulation and theodolite traversing. It is a means of making a manuscript map in the field while ground can be seen by the topographer by existing control systems and to carry its own control systems by triangulation or traverse and by lines of levels.
Adjustments done on each set up of the Plane Table
The plane table is set up at a convenient height for working. The legs of the tripod are spread wide apart and are firmly fixed on the ground. On each station points, previously fixed by the reconnaissance survey, the table is placed in such a way that, the point plotted on the sheet corresponding to the station occupied is exactly over the station fixed on the ground. This is done by using U-frame and plumb bob. The pointed end of the upper leg of the U-frame is made to coincide on the point plotted on the paper with a plumb bob pointing on the station on the ground from other leg of the U-frame. Thus, centering is done on each station points.
Leveling of table is done by using a spirit level placed on the table, first parallel to the two screws and then over the third screw. This process of leveling is done by adjusting the legs of the tripod.
The operation of keeping the table at each of the successive station points parallel to the position which is occupied at the first station is known as orientation. Orientation of the table at the station points are done using trough compass. The trough compass is placed along the line representing the magnetic meridian which has been drawn on the paper at the first station and then board is turned until the ends of the needle are opposite the zeroes of the scale. The board is then clamped on to this position.
The temporary adjustments of centering, leveling, and orienting are done at each station points thereby making the plane table surveying accurate.
Procedure:-
To get the nodal points and other important points on the road and on each side of the road, the plane table surveying is done in three steps involving:-
- left side of the road
- along the road
- right side of the road
- Plane Table survey on the left side of the road
From the second station, offsets at 50m. interval was taken and perpendicular offsets were taken at 25m. interval and plotted. On this side, some of the marshy areas could not be plotted but their boundaries were plotted by drawing rays from the instrument station and taking the distance by using tape. The plane table was again shifted to the third instrument station. Here also the necessary land marks were plotted by taking perpendicular offsets and sufficient points by radiation method.
To take the wing wall details on both sides of the railway track, another instrument station was taken. Points were plotted by keeping the ranging rod on six different points on each wing wall. The two piers were also plotted on the sheet taking its diameter using the tape. Points were also taken on the railway tracks and the tracks were also plotted using radiation method. The rest of the offsets were taken from a new station point and on that area due to the marshy land, perpendicular offsets could not be taken. Other landmarks were taken by shifting the plane table once again. The work was stopped that day, by completing one side of the road by about 4o’clock that evening.
- Plane Table survey along the road
The work was resumed the next day by starting the plane table surveying from the starting point (SP) on the road. Offsets were plotted along the shoulder of the road and also on the other side of the road by taking perpendicular offsets. All these points were named. The instrument was then shifted to the second station on the bridge and the shoulder on both sides and the road was plotted accurately. Offsets were again taken at 50m. intervals and perpendicular offsets at all these points were also plotted to get the plan of the road in the Chackai bypass. To get the Chackai roundabout, instrument was set up at the station where the level was previously fixed for finding the dimensions of the roundabout. Rays were drawn from the instrument station, towards the points on the roundabout, previously selected for taking the distance by using the level and the corresponding distances were marked on those rays thereby completing the round about by radiation method. The triangular junctions were also plotted using the same method. Various land marks on either side of the roundabout including the side streets leading to Pettah on one side and to Airport on the other side was plotted. The instrument was then again shifted to another instrument station and offsets were plotted at 50m. intervals till the end of our proposed survey area, also taking perpendicular offsets to get the road completely. The work was stopped that day by 2o’clock in the afternoon.
(iii) Plane Table survey on the right side of the road
- Drawing the Contour Map and the Topographical Map
Aim:-
To draw the contour and topographical map using the collected data from the field.
Instruments needed:-
Tracing sheets, pencils, markers etc.
Theory:-
A contour line is an imaginary line on the ground joining the points of equal elevation. It is a line in which the surface of the ground is intersected by a level surface. The relief on a topographical map is most commonly and accurately represented by contours.
Procedure:-
From the data collected from the field reduced level of each and every marked point was calculated. The map got from the plane table was transferred onto the tracing sheet. Now with the reduced levels and the points marked on them with the surrounding features all done to form the topographical map, we were ready to plot the contour lines on the map.
The contours were planned to be plotted for an interval of 0.5m of the reduced level. The embankments on either side of the road showed uniformity in their slope. Since we had only the reduced level of points at some interval only, we have to interpolate the reduced levels of the points in between the two known points. From the many methods of interpolation we chose graphical method.
The method is described below:-
On a piece of tracing paper a number of parallel lines, each representing 0.5m., are ruled at equal intervals with every tenth line made thick. Then on a sheet of tracing paper draw a line and divide it into a number of equal parts. The line is bisected at the center and a perpendicular was drawn through that point. A convenient point was selected on the perpendicular and radial lines to the points of the division marked on the line were drawn. Rule out lines 1-1, 2-2, etc. to line across the diagram. These lines are called guide lines and are equally divided by the radial line. Then on the main guide line, the reduced levels at 50cm. intervals (i.e. 98.0, 98.5. 99.0, 99.5, etc.) were marked. Now knowing the reduced levels of two points, they were placed on the corresponding point on the guide line and this was then moved keeping the reduced level of one point constant till the reduced level of the other point coincided. Then the reduced levels of intermediate point’s along the line joining the two points were marked using pins. Then the contours were drawn connecting points of equal reduced levels. In this way the entire contour map was completed.
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