PROCEDURE OF FIELD WORK
Field work of chain survey should be carried out according to the following steps:
1.Reconnaissance
Before starting survey work, the surveyor should walk over the whole area to be surveyed in order to examine the ground and determine the possible arrangement of framework of survey. During this investigation, he should examine the intervisibility of the main survey stations. He should ensure that the whole area is enclosed by main survey lines, and also that it is possible to form well-conditioned triangles. He should observe various objects and boundary lines carefully and select the survey lines in such a manner that the objects can be located by short offsets. The base line should preferably be taken through the centre of the area and on fairly level ground.
2.Index sketch
After preliminary inspection of the area, the surveyor should prepare a neat hand sketch showing the arrangement of the framework and approximate position of the objects. He should note the names of the stations on the sketch maintaining some order (clockwise or anticlockwise). The field work should be executed according to this index sketch. The names and sequence of chain lines should be followed as directed in the index sketch. The ‘base line’ should be clearly indicated in the index sketch.
3. Marking the stations on the ground
After reconnaissance, the stations are marked on the ground by wooden pegs. These pegs are generally 2.5 cm square and 15 cm long, and have pointed ends. They are driven into the ground firmly, and there should be a height of 2.5 cm above the ground. The station point is marked with a cross so that it can be traced if the wooden peg is removed by somebody
4. Reference sketches
To take precautions against station pegs being removed or missed, a reference sketch should be made for all main stations. It is nothing but a hand sketch of the station showing at least two measurements from some permanent objects. A third measurement may also be taken
5.Taking measurements of survey lines and noting them in the field book
Ranging and chaining is started from the base line, which should be measured carefully. The magnetic bearings of the base line are measured by prismatic compass. These measurements are noted in the field book showing the offsets to the left or right according to their position. Then the other survey lines are ranged and chained maintaining the sequence of the traverse. The offsets and other field records are noted simultaneously. The check lines and tie lines are also measured and noted at the proper place. The station marks are preserved carefully until field work is completed.
3.9 CONVENTIONAL SYMBOLS
In a map the objects are shown by symbols and not by names. So the surveyor should know the following standard conventional symbols for some common objects.
EQUIPMENTS FOR PLOTTING
1. Drawing board (normal size – 1000 mm x 700 mm)
2. Tee-square
3. Set-square (450 and 600)
4. Protractor
5. Cardboard scale – set of eight
6. Instrument box
7. French curve
8. Offset scale
9. Drawing paper of good quality (normal size – 880 mm x 625 mm)
10. Pencils of good quality – 2 H, 3 H or 4 H
11. Eraser (rubber) of good quality
12. Board clips or pins
13. Ink (Chinese ink or Indian ink) of required shade
14. Colour of required shade
15. Inking pen (or Hi-tech pen) and brushes
16. Handkerchief, knife , paperweight, etc.
17. Mini drafter
3.10 PROCEDURE OF PLOTTING
1. A suitable scale is chosen so that the area can be accommodated in the space available on the map.
2. A margin of about 2 cm from the edge of the sheet is drawn around the sheet.
3. The title block is prepared on the right hand bottom corner.
4. The north line is marked on the right-hand top corner, and should preferably be vertical. When it is not convenient to have a vertical north line, it may be inclined to accommodate the whole area within the map.
5. A suitable position for the base line is selected on the sheet so that the whole area along with all the objects it contains can be drawn within the space available in the map.
5. A suitable position for the base line is selected on the sheet so that the whole area along with all the objects it contains can be drawn within the space available in the map.
6. The framework is completed with all survey lines, check lines and tie lines. If there is some plotting error which exceeds the permissible limit, the incorrect lines should be resurveyed.
7. Until the framework is completed in proper form, the offsets should not be plotted.
8. The plotting of offsets should be continued according to the sequence maintained in the field book.
9. The main stations, substations, chain line, objects, etc. should be shown as per standard symbols
10. The conventional symbols used in the map should be shown on the right-hand side.
11. The scale of the map is drawn below the heading or in some suitable space. The heading should be written on the top of the map.
12. Unnecessary lines, objects etc. should be erased.
13. The map should not contain any dimensions.
Inking of the map
The inking should be begun from the left-hand-side towards the right-hand-side, and from the top towards the bottom.
Colouring of the map
In general, colour washing of engineering survey maps is not recommended. However, if it is necessary, the colour shades should be very light, and according to the colour conventions. The colouring should also be started from the left-hand-side towards the right and from the top towards the bottom.
3.11 CROSS-STAFF AND OPTICAL SQUARE
A. Cross-staff
The cross-staff is a simple instrument for setting out right angles. There are three types of cross-staves.
- Open
- French
- Adjustable
The open cross-staff is commonly used.
Open cross-staff
The open cross-staff consists of four metal arms with vertical slits. The two pairs of arms (AB and BC) are at right angles to each other. The vertical slits are meant for sighting the object and the ranging rods. The crossstaff is mounted on a wooden pole of length 1.5m and diameter 2.5 cm. The pole is fitted with an iron shoe.
For setting out a perpendicular on a chain line, the cross-staff is held vertically at the approximate position. Suppose slits A and B are directed to the ranging rods (R, R1) fixed at the end stations. Slits C and D are directed to the object (O). Looking through slits A and B, the ranging rods are bisected. At the same time, looking through slits C and D, the object O is also bisected. To bisect the object and the ranging rods simultaneously, the cross staff may be moved forward or backward along the chain line
B. Optical Square
An optical square is also used for setting out right angles. It consist of a small circular metal box of diameter 5 cm and depth 1.25 cm. It has a metal cover which slides round the box to cover the slits. The following are the internal arrangements of the optical square.
1. A horizon glass H is fixed at the bottom of the metal box. The lower half of the glass is unsilvered and the upper half is silvered.
2. A index glass I is also fixed at the bottom of the box which is completely silvered.
3. The angle between the index glass and horizon glass is maintained at 450.
4. The opening ‘e’ is a pinhole for eye E, ‘b’ is a small rectangular hole for ranging rod B, ‘P’ is a large rectangular hole for object P.
5. The line EB is known as horizon sight and IP as index sight.
6. The horizon glass is placed at an angle of 1200 with the horizon sight. The index glass is placed at an angle of 1050 with the index sight.
7. The ray of light from P is first reflected from I, then it is further reflected from H, after which it ultimately reaches the eye E
Principle
According to the principle of reflecting surfaces, the angle between the first incident ray and the last reflected ray is twice the angle between the mirrors. In this case, the angle between the mirrors is fixed at 450. So, the angle between the horizon sight and index sight will be 900.
Setting up the perpendicular by optical square
1. The observer should stand on the chain line and approximately at the position where the perpendicular is to be set up.
2. The optical square is held by the arm at the eye level. The ranging rod at the forward station B is observed through the unsilvered portion on the lower part of the horizon glass.
3. Then the observer looks through the upper silvered portion of the horizon glass to see the image of the object P.
4. Suppose the observer finds that the ranging rod B and the image of object P do not coincide. The he should move forward or backward along the chain line until the ranging rod B and the image of P exactly coincide
5. At this position the observer marks a point on the ground to locate the foot of the perpendicular.