Advantage and disadvantage of plane table survey

 Advantage

•       It is most suitable for preparing for small scale maps.

•       It is the most rapid method

•       Field book is not necessary as plotting is done in the field concurrently with the fieldwork and hence the mistakes in booking the field notes are avoided.

•       The surveyor can compare the plotted work with the actual features of the area surveyed and thus can ascertain if it represents them properly and cannot overlook any essential features.

•       There is no possibility of omitting the necessary measurements as the map are plotted in the field.

•       Errors of measurements and the plotting may be readily detected by check lines.

•        Counters and irregular objects maybe represented accurately since the tract is in view.

•       It is particularly advantageous in a magnetic area where compass survey is not reliable.

•       It is less costly than a theodolite survey.

•       No great skill is required to prepared maps.

Disadvantage

•       Plane table is essentially a tropical instrument.

•       It is not suitable for work in a wet climate.

•       It is heavy and awkward to carry.

•       There are several accessories to be carried they are like to be lost.

•       It is not intended for accurate work.

•       If the survey is not to be re-plotted to a different scale or quantities are not to be computed.

•       It is a great inconvenience in absence of the field notes.

Obstacle in Chaining

 

Sometimes happens that a survey line passes through some object such as a pond, the building, a river, a hedge etc. which prevents the direct measurement of that part of the line which the object intersects. The interfering object in such a case is called on the obstacle.

The various obstacle may be classed as:

1. When Chaining is free, but Vision is obstructed.

2. When Chaining is Obstructed, but Vision is Free.

3. When Chaining and Vision Both are obstructed.

It is necessary to overcome obstacles so that chaining may be continued in a straight line. Special methods are, therefore, employed in measuring distances across the obstacles.

1. When Chaining is free, but Vision is obstructed.

Such a the problem arise when rising ground or hill intervening, the jungle is interrupted etc. Hence the end stations are not inter-visible. There are two cases

Case-I

In this method difficulty got over by reciprocal ranging. The reciprocal ranging may also, be used in ranging a line across a hollow.

Case-II

The end stations are not visible from intermediate points when a jungle area comes across the chain line. In this case, the obstacle may be crossed over using a random line.

Procedure

1.     Let the AB be the line whose length is required.  From A run a line ‘AB1’ called the random line, in any conventional direction but as nearly towards ‘B’  as can be the jungle and continue it until the point ‘B’ is visible from ‘B1’. Chain the line to ‘B1’ where ‘ BB1’ is perpendicular to ‘AB1’ and measure ‘AB1’ 

                                    AB = √(AB1² + BB1²) 

1.       If any length ‘AC1’ is measured along ‘AB1’ at point ‘C’ is located on the line ‘AB’ by measuring a perpendicular distance ‘CC1’.

                                        (CC1 / BB1) = (AC1 /AB1)

                             CC1  = (AC1 /AB1) x BB1

1.      In this corner a sufficient number of points can be located. The line is cleared and the distance measured.

2. When Chaining is Obstructed, but Vision is Free.

Such a the problem arise when a pound or a river tank, plantation comes across the chain line. The typical obstacle of this type is a sheet of water, the width of which in the direction of measurement exceeds the length of the chain or tape. The problem consists of finding the distance between convenient points on the chain line on either side of the obstacle.

Case-I

In which it is possible to chain around the obstacle e.g. a pound. A bend in the river etc.

Part-1

 Suppose AB is a chain line. Two-point ‘C’ and ‘D’ are selected on it on the opposite bank of the pound.    Equal perpendicular ‘CE’ and ‘DF’  erected at ‘C’  and ‘D’.  Distance ‘EF’ is measured (i.e. CD= EF)

Part-2 The the pond may also be crossed by forming a triangle as shown. Then

                            CD = √(ED² + CE²) 

Part-3 Find by optical square or a cross-staff a point C such that ∠PRQ is right angle (Fig. 4) Measure PR and RQ. Then

                                   CD = √(PR² + RQ²) 

Case-II

Sometimes it is not possible to go around the obstruction {i.e. obstacle is a river}

Part-1 Imagine a small river comes across the chain line 

 Suppose ‘AB’ is the chain line. Two points ‘C’ and ‘D’ selected on this line on the opposite bank of the river.At ‘C’ a perpendicular ‘CE’ is erected and bisected at ‘F’.A perpendicular is set out at ‘E’ and a point ‘G’ are in the same straight line. From triangle ‘DCE’ and ‘GEF’ the distance ‘GE’ is measured and ‘CD’ is obtained indirectly 

Part-2 Imagine a large river comes across the chain line.

1    Let ‘AB’ be the chain line. Point ‘C’, ‘D’ and ‘E’ are selected on this line such that ‘D’ and ‘E’ are on the opposite banks of the river. The perpendicular ‘DE’ and ‘CG’ are erected on the chain line in such a way that ‘E’, ‘F’ and ‘G’ are on the same straight line. Line ‘FH’ take parallel to “CD’. From triangle ‘DEF’ and ‘HEG’.

(ED / DF) = (FH / HG)

ED  = (FH / HG) x  DF

ED  = (FH / (CG - DF)) x  DF

3. When Chaining and Vision Both are obstructed.

 Such a problem arise when a building comes across the chain line. The problem, in this case, consists both in prolonging the line beyond the obstacle and finding the distance across it.

 Suppose ‘AB is the chain line. Two-point ‘C’ and ‘D’ are selected on it at on the side of the building.

1.      Equal perpendicular ‘CC1’ and ‘DD1’ are erected, the line ‘C1D1’ is extended until the building is crossed. On the extended line, two points ‘E1’ and ‘F1’ are selected. Then perpendiculars ‘EE1’ and ‘FF1’ are so erected that.

                                        EE1 = FF1 = =DD1 = CC1

Thus the points ‘C’, ‘D’, ‘E’ AND ‘F’ will lie on the same straight line ‘AB’

    DE =   D1E1

             

Conventional symbol

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.

SELECTION OF SURVEY STATIONS

SELECTION OF SURVEY STATIONS

The following points should be remembered during the selection of survey stations:

1. The stations should be so selected that the general principle of surveying may be strictly followed.

2. The stations should be intervisible.

3. The stations should be selected in such a way that well-conditioned triangles may be formed.

4. The base line should be the longest of the main survey lines.

5. The survey lines should be taken through fairly level ground, as far as practicable.

6. The main survey lines should pass close to the boundary line of the area to be surveyed.

7. The survey lines should be taken close to the objects so that they can be located by short offsets.

8. The tie stations should be suitably selected to fix the directions of adjacent sides.

9. The subsidiary stations should be suitably selected for taking check lines.

10. Stations should be so selected that obstacles to chaining are avoided as far as possible.

11. The survey lines should not be very close to main roads, as survey work may then be interrupted by traffic.

TYPES OF EQUIPMENTS FOR CHAIN SURVEY

The following equipment is required for conducting chain survey:

1. Metric chain (20 m)                             =          1 no.
2. Arrows                                                 =          10 nos. 
3. Metallic tape (15 m)                            =          1 no. 
4. Ranging rods                                       =          3 nos. 
5. Offset rod                                            =          1 no. 
6. Clinometer                                          =          1 no.
7. Plumb bob with thread                        =          1 no.
8. Cross staff or optical square               =          1 no.
9. Prismatic compass with stand            =          1 no. 
10. Wooden pegs                                      =          10 nos. 
11.  Mallet                                                  =          1 no.
12. Field book                                           =          1 no.
13.  Good pencil                                        =          1 no.
14. Pen knife                                             =          1 no.
15. Eraser (rubber)                                    =          1 no.

CROSS-STAFF AND OPTICAL SQUARE

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.

1. Open
2. French
3. Adjustable

The open cross-staff is commonly used.

1) 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 cross-staff 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 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 backwards along the chain line

2) French cross-staff:

It consists of an octagonal brass tube with slits on all the eight sides. It has an alternate vertical sighting slit and an opposite vertical window with a vertical fine wire or horsehair on each of the four sides. These are used for setting out right angles. On the other sides are vertical slits, which are at 45° to those mentioned above for setting out angles of 45°. The sights are about 8 cm apart.

It carries a socket at the base so that it may be mounted on the pointed staff when in use. The sights being too close, it is not as accurate as of the open cross-staff.

French Cross staff

 

For setting out a right angle at a given point on a chain line, it is held vertically over the given point and turned until one of them saw cuts or (the pair of sights) is ranged along the chain line. Then the line of sights through the other saw cut or the pair of sights) is at right angles to the chain line and a ranging rod can be fixed in this direction at a convenient position.

To find the foot of the perpendicular from an object on a chain line, the cross-staff is held vertically at an approximate position of the required object and turned until one of the saw cuts or (the pair of sights) is ranged along the chain line.

If on looking through the another saw cut or (the pair of sights) is seen the object from which the foot of the perpendicular is to be drawn, then the point where the cross-staff is held is the correct position of the foot of the perpendicular from the given object. If not, the cross-staff is moved forward or backwards along the chain line until the line of sight through the saw cut or (the pair of sights) at right angles to the chain line bisect the given object.

3) Adjustable cross-staff:

It consists of a brass metal cylindrical tube about 8 cm in diameter and 10 cm in deep (divided in the centre). The upper cylinder can be rotated relatively t the lower one by a circular rack and pinion arrangement actuated by mill headed screw. Both are provided sighting slits.

The lower part is graduated to degree and subdivision, which the upper one carries a vernier.it has a magnetic compass at the top which may serve to take the bearing of the line.

It is used for setting out angles of any magnitude.

B. Optical Square

An optical square is also used for setting out right angles. It consists of a small circular metal box of diameter 5 cm and depth 1.25 cm. It has a metal cover which slides around 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. An 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 unsilver 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. Then he should move forward or backwards 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.

Plotting procedure in surveying

 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

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.

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.

The Field Book

  

THE FIELD BOOK

The notebook in which field measurements are noted is known as the ‘field book’. The size of the field book is 20 cm x 12 cm and it opens lengthwise. Field books may be of two types:

1.      Single –line, and

2.      Double-line.

1.      Single-line field book :

In this type of field book, a single red line is drawn through the middle of each page. This line represents the chain line, and the chainages are written on it. The offsets are recorded, with sketches, to the left or right of the chain line. The recording of the field book is started from the last page and continued towards the first page. The main stations are marked by ‘’ and subsidiary stations or tie stations are by ‘’

2.      . Double-line field book:

In this type of field book, two red lines, 1.5 cm apart, are drawn through the middle of each page. This column represents the chain line, and the chainages are written in it. The offsets are recorded, with sketches, to the left or right of this column. The recording is begun from the last page and continued towards the first. The main stations are marked by ‘’ and subsidiary or tie stations by ‘’ this type of field book is commonly used.

A. Problems on Entering Records in Field Book

Problem 1 While measuring a chain line AB, the following offsets are taken. How would you enter the field book?

(a) A telegraph post is 10 m perpendicularly from chainage 2.5 m to the right of the chain line.

(b) A road crosses obliquely from left to right at chainage 10 m and 14 m. Perpendicular offsets are 2m and 3m to the side of the road from chainage 5m and 20 m respectively.

(c) A tube-well is 5m perpendicularly from chainage 30 m to the left of the chain line.

(d) Total chainage of AB is 45 m.

Problem 2 The base-line AC of a chain survey is measured and the following records are noted. Make the necessary entries in a field book.

(a) The corners of a building are 9 and 9,5m from chainage 7.5 and 18 m to the left of the chain line. The building is 7m wide.

(b) A 4 m wide road runs about parallel to the right of the chain line. Offsets are 2, 2.1, 2.2, and 2.15m at chainages 0, 20, 40, and 55.5m respectively.

(c) A check line is taken from the sub-station at chainage 25 m to the left.

(d) The total chainage of the base-line is 55.5m.

(e) The fore bearing and back bearing of the base-line are 30⁰30’ and 210⁰3C’ respectively.

Problem 3 Enter the field book according to the following field notes:

(a) Chainage of line AB is 95.5m

(b) The offsets to the pond at the left of the chain line are as follows:

Chainage – 10, 15,20,25,30 m

Offset – 16, 12,10,14,20 m

(c)    The offsets to the river at the right of the chain line are:

Chainage – 5,25,40,80 m

Offset -13, 17, 19, 19.5m

B. Precautions to be Taken While Entering the Field book

1. All measurements should be noted as soon as they are taken.

2. Each chain line should be recorded on a separate page. Normally it should start from the bottom of one page and end on the top of another. No line should be started from any intermediate position.

3. Overwriting should be avoided.

4. Figures and hand-writing should be neat and legible.

5. Index-sketch, object-sketch and notes should be clear.

6. Reference sketches should be given in the field book, so that the station can be located when required.

7. The field book should be entered in pencil and not in ink.

8. If an entry is incorrect or a page damaged, cancel the page and start the entry from a new one.

9. Erasing a sketch, measurement or note should be avoided.

10. The surveyor should face the direction of chaining so that the left-hand and right-hand objects can be recorded without any confusion.

11. The field-book should be carefully preserved.

12. The field-book should contain the following:

 (i) name,

(ii) Location, and

(iii) Date, of the survey,

(iv) Name of party members, and

(v) Page index or chain line.

3.8. 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 the framework of the survey. During this investigation, he should examine the indivisibility 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 a 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 fieldwork 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 the survey lines and noting them in the field book

Ranging and chaining is started from the baseline, which should be measured carefully. The magnetic bearings of the base-line are measured by the 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 fieldwork is completed.