Definition and illustration in chain surveying

DEFINITIONS AND ILLUSTRATIONS

A. Survey Stations

Survey stations are the points at the beginning and the end of a chain line. They may also occur at any convenient points on the chain line. Such stations may be:

1. Main stations

2. Subsidiary stations and

3. Tie stations

   

1. Main stations Stations taken along the boundary of an area as controlling points are known as  ‘main survey lines’. The main survey lines should cover the whole area to be surveyed. The main stations are denoted by ‘ ’ with letters A, B, C, D, etc. The chain lines are denoted by “__ … __ ... __...__...__...__”.
2. Subsidiary stations Stations that are on the main survey lines or any other survey lines are known as “Subsidiary stations”. These stations are taken to run subsidiary lines for dividing the area into triangles, for checking the accuracy of triangles and for locating interior details. These stations are denoted by ‘’ with letters S₁, S₂, S₃, etc.
3. Tie stations  These are also subsidiary stations taken on the main survey lines. Lines joining the tie stations are known as tie lines. Tie lines are mainly taken to fix the directions of adjacent sides of the chain survey map. These are also taken to form ‘chain angles’ in chain traversing when triangulation is not possible. Sometimes tie lines are taken to locate interior details. Tie stations are denoted by ‘’ with letters T₁, T₂, T₃. Etc.

B.     Base Line

The line on which the framework of the survey is built is known as the ‘baseline. It is the most important line of the survey. Generally, the longest of the main survey lines is considered the base-line. This line should be taken through the fairly level ground and should be measured very carefully and accurately. The magnetic bearings of the base-line are taken to fix the north line of the map.

C.    Check Line

The line joining the apex point of a triangle to some fixed point on its base is known as the ‘check line’. It is taken to check the accuracy of the triangle. Sometimes this line helps to locate interior details.

D.    Offset

The lateral measurement taken from an object to the chain line is known as ‘offset’. Offsets are taken to locate objects with reference to the chain line. They may be of two kinds:

1. Perpendicular offsets
2. Oblique Offsets

1. Perpendicular offsets:-

When the lateral measurements are taken perpendicular to the chain line, they are known as perpendicular offsets.

Perpendicular offsets may be taken in the following ways:

(a) By setting a perpendicular by swinging a tape from the object to the chain line. The point of minimum reading on the tape will be the base of the perpendicular

(b) By setting a right angle in the ratio 3: 4: 5

(c) By setting a right angle with the help of the builder’s square or tri-square

(d) By setting a right angle by cross-staff or optical square.

2. Oblique offsets Any offset not perpendicular to the chain line is said to be oblique. Oblique offsets are taken when the objects are at a long distance from the chain line or when it is not possible to set up a right angle due to some difficulties. Such offsets are taken in the following manner.

Suppose AB is a chain line and p is the corner of a building. Two points ‘a’ and ‘b’ are taken on the chain line. The chainages of ‘a’ and ‘b’ are noted. The distances ‘ap’ and ‘bp’ are measured and noted in the field book. Then ‘ap’ and ‘bp’ are the oblique offsets. When the triangle abp is plotted, the apex point p will represent the position of the corner of the building.

Perpendicular offsets are preferred for the following reasons:

(a) They can be taken very quickly

(b) The progress of the survey is not  hampered

(c) The entry in the field book becomes easy

(d) The plotting of the offsets also becomes easy

3. The number of offsets The offsets should be taken according to the nature of the object. So, there is no hard and fast rule regarding the number of offsets. It should be remembered that the objects are to be correctly represented and hence the number of offsets should be decided on the field. Some guidelines are given below:

(a) When the boundary of the object is approximately parallel to the chain line, perpendicular offsets are taken at regular intervals

(b) When the boundary is straight, perpendicular offsets are taken at both ends of it

(c) When the boundary line is zigzag, perpendicular offsets are taken at every point of bend to represent the shape of the boundary accurately. In such a case, the interval of the offsets may be irregular

(d) When a road crosses the chain line perpendicularly, the chainage of the intersection point is to be noted

(e) When a road crosses a chain line obliquely, the chainages of intersection points ‘a’ and ‘b’ are noted. Then at least one offset is taken on both sides of the intersection points. More offsets may be taken depending on the nature of the road. Here, perpendicular offsets are taken at ‘c’ and ‘d’

(f) When the building is small, its corners are fixed by perpendicular or oblique offsets and the other dimensions are taken directly on the field and noted in the field book.

(g) When the building is large, zigzag in shape and oblique to the chain line, then the corners are fixed by perpendicular or oblique offsets. Then the full plan of the building is drawn on a separate page along with all the dimensions. This page should be attached to the field book at the proper place.

(h) When the object is circular, perpendicular offsets are taken at short and regular intervals

4. Limiting length of offset The maximum length of the offset should not be more than the length of the tape used in the survey. Generally, the maximum length of offset is limited to 15m. However, this length also depends upon the following factors:

(a) The desired accuracy of the map

(b) The scale of the map

(c) The maximum allowable deflection of the offset from its true direction and

(d) The nature of the ground

Problems on limiting length of offset

Problem 1  An offset was laid out 6⁰ from its true direction and the scale of the map was 20 m to 1 cm. Find the maximum length of offset for the displacement of a point on the paper not to exceed 0.03 cm.

Solution Let AB be the actual length of offset which was laid out 6⁰ from its true direction. So, BC is the displacement of the point.

            Let the maximum length of offset,     AB = L m

            From triangle ABC, (BC/AB) = sin 6⁰

                        

or         BC = AB sin 6⁰ = L sin 6⁰ m  (displacement of the ground)

Since the scale is 1 cm to 20 m, 20 m on the ground represents 1 cm on the paper.

Therefore, Lsin 6⁰ on the ground represent ((Lsin 6⁰ ) / 20) cm on the paper. 

  According to the given condition ((Lsin 6⁰ ) / 20) = 0.03       

                  = 5.740 m

            Therefore, the maximum length of offset should be 5.740 m.

Problem 2  The length of the offset is 20 m and the scale of the plan 10 m to 1 cm. If the offset is laid out 3⁰ from its true direction, find the displacement of the plotted point on the paper

(i) perpendicular to the chain line, and

(ii) parallel to the chain line.

Solution Let AB be the actual length of offset, which is 15 m long and deflected by 3⁰ from its true direction.

Here,

            BC = Displacement parallel to the chain line

                        CD = displacement perpendicular to chain line

(i)  CD = AD – AC = AB - AC

             = 20 – 20 cos 3⁰

             = 20 (1 – cos 3⁰) m (displacement on the ground)

            Since the scale is 1 cm to 10 m,

            10 m on the ground = 1 cm on the map

            20(1 – cos 3⁰  ) m = {(20(1-cos3°))/10}

                                                         =  0.00274 cm on the map

            Required displacement perpendicular to the chain line

          = 0.00274 cm (on paper)

(ii) BC = AB sin 3⁰ = 20 sin 3⁰ = 1.0467 m (displacement on ground)

Displacment parallel to chain = (1.0467/10) = 0.10467 cm (on paper)

     

E. Degree of Accuracy

Degree of accuracy is determined before the starting of any survey work. It is worked out according the following factors:

(a) Scale of plotting

(b) Permissible error in plotting

During the reconnaissance survey, the length of the main survey lines is approximately determined by the pacing method. One pace or walking step of a man is considered to equal 80 cm. When the length of the survey lines or the extent of the area to be surveyed is approximately known, the scale of the map may be assumed. Again, the permissible error in plotting may be obtained from the concerned department. Then the degree of accuracy in measurement is ascertained.

Let us now consider an example.

Suppose the scale of plotting is 10 m to 1 cm and the allowable error is 0.02 cm.

Then,  1 cm on the map = 1000 cm on the ground

        0.02 cm on the map = 1000 x 0.02 = 20 cm on the ground

So, the measurement should be taken nearest to 20 cm.

Selection of chain station, large scale and small scale and Well & Ill triangle

 It should be noted that plotting triangles requires no angular measurements to be made, if the three sides are known.

Chain surveying is recommended when:

1. The ground surface is more or less level

2. A small area is to be surveyed

3. A small-scale map is to be prepared and

4. The formation of well-conditioned triangles is easy

Chain surveying is unsuitable when:

1. The area is crowded with many details

2. The area consists of too many undulations

3. The area is very large and

4. The formation of well-conditioned triangles becomes difficult due to obstacles

A. Large-Scale and Small-Scale Maps

When 1 cm of a map represents a small distance, it is said to be a large-scale map.

For example,

        

When 1 cm of the map represents a large distance, it is called a small-scale map.

For example,

A map having an RF of less than 1/500 is considered to be large-scale. A map of RF more than 1/500 is said to be small-scale.

WELL-CONDITIONED AND ILL-CONDITIONED TRIANGLES

A triangle is said to be well-conditioned when no angle in it is less than 30⁰ or greater than 120⁰ . An equilateral triangle is considered to be the best-condition or ideal triangle

Well-conditioned triangles are preferred because their apex points are very sharp and can be located by a single ‘dot’. In such a case, there is no possibility of relative displacement of the plotted point.

A triangle in which an angle is less than 30⁰ or more than 120⁰ is said to be ill-conditioned

Well-conditioned triangles are not used in chain surveying. This is because their apex points are not sharp and well defined, which is why a slight displacement of these points may cause a considerable error in plotting.

 RECONNAISSANCE SURVEY AND INDEX SKETCH

Before the commencement of any survey work, the area to be surveyed is thoroughly examined by the surveyor, who then thinks about the possible arrangement of the framework of the survey. This primary investigation of the area is termed as reconnaissance survey or reconnoitre.

During the reconnaissance survey, the surveyor should walk over the area and note the various obstacles and whether or not the selected stations are intervisible. The main stations should be so selected that they enclose the whole area. The surveyor should also take care that

The neat hand sketch of the area which is prepared during the reconnaissance survey is known as the ‘index sketch’ or ‘key plan’. The index sketch shows the skeleton of the survey work. It indicates the main survey stations, sub-stations, tie stations, base-line, arrangement for the framework of triangles and the approximate positions of different objects. This sketch is an important document for the surveyor and for the person who will plot the map. It should be attached to the starting page of the field book

Ranging Out Survey Lines at level ground

Ranging Out Survey Lines

While measuring the survey lines, the chain or the tape has to be stretched along the survey line along that joins two terminal stations. When the line to be measured has a smaller length compared to the chain, then the measurement goes smooth. If the length of the line is greater, the survey lines have to be divided by certain intermediate points, before conducting the chaining process. This process is called ranging.

The process of ranging can be done by two methods:

1. Direct Ranging
2. Indirect Ranging

1. Direct Ranging

Direct ranging is ranging conducted when the intermediate points are intervisible. Direct ranging can be performed by eye or with the help of an eye instrument.

Ranging by Eye

As shown in figure-1 below, let A and B are the two intervisible points at the ends of the survey line. The surveyor stands with a ranging rod at the point A by keeping the ranging rod at the point B. The ranging rod is held at about half metre length.

The assistant then takes the ranging rod and establishes at a point in between AB, almost in line with AB. This is fixed at a distance not greater than one chain length from point A.The surveyor can give signals to the assistant to move traverse till the rod is in line with A and B. In this way, other intermediate points are determined.

Ranging by Line Ranger

The figure-2 below shows a line ranger that has either two plane mirror arrangement or two isosceles prisms that are placed one over the other. The diagonals of the prism are arranged and silvered such that they reflect incident rays.

In order to handle the instrument in hand a handle with hook is provided. The hook is to enable a plumb- bob to help transfer the point to the ground.

In order to range the point ‘P’, initially, two rods are fixed at points A and B. By eye judgment, the surveyor holds the ranging rod at P almost in line with AB.

The lower prism abc receives the rays coming from A which is then reflected by the diagonal ac towards the observer. The upper prism dbc receives the rays from B which is then reflected by the diagonal bd towards the observer. Hence the observer can see the images of the ranging rods A and B, which might not be in the same vertical line as shown in figure-2(c).

The surveyor moves the instrument till the two images come in the same vertical line as shown in figure-2(d). With the help of a plumb bob, the point P is then transferred to the ground. This instrument can be used to locate the intermediate points without going to the other end of the survey line. This method only requires one person to hold the line ranger.

2. Indirect Ranging

Indirect ranging is employed when the two points are not intervisible or the two points are at a long distance. This may be due to some kind of intervention between the two points. In this case, the following procedure is followed.

As shown in figure-3, two intermediate points are located M1 and N1 very near to chain line by judgment such that from M1, both N1 and B are visible & from N1 both M1 and A are visible. At M1 and N1 two surveyors stay with ranging rods. The person standing at M1 directs the person at N1 to move to a new position N2 as shown in the figure. N2 must be inline with M1B.

Next, a person at N2 directs the person at M1 to move to a position M2 such that it is inline with N2A. Hence, the two persons are in points are M2 and N2.

The process is repeated until the points M and N are in the survey line AB. Finally, it reaches a situation where the person standing at M finds the person standing at N in line with NA and vice versa. Once M and N are fixed, other points are fixed by direct ranging.

 

 

Various instruments are used in Chain surveying.

 

A) Chain

Chains are the measuring instrument used in surveying formed by the 100 links of 4mm galvanized mild steel wire. These links are joined by 3 circular or oval wire rings. These rings provide flexibility to the chains.

Every aspect of life requires some measuring units. Measurements are used to do the work precisely and accurately. Let it be from kitchen to office, everywhere measurements are used. So as in engineering calculation or measurements holds a very greater role in construction or surveying or any other aspect.

There are various units of measurements such as meters, centimetres, feet, inches, acre, yards and the list goes on. Same as units there is the various instrument used in the measurements of any entity. One of the instruments used in measurement is chains.

                                                                   Chain

Parts of Chains used in Surveying

The chain consists of many small parts used for handling or reading the measurements.

• At the ends, the chain is provided with brass handle with swivel joint so that it can be easy to roll or unroll the chain without twisting and knots.
• At every 10^th link is provided with a tally of one teeth, 20^th link with a tally of two teeth and so on till 40^th link. This is provided for the easy reading of measurements.
• At the centre of the chain is provided with a circular talley used for easy reading.

Types of Chains used in Surveying

Depending upon the length of the chain, these are divide into following types,

1. Metric chains
2. Steel band or Band chain
3. Gunter’s chain or surveyor’s chain
4. Engineer’s chain
5. Revenue chain

1. Metric chains

Metric chains are the most commonly used chain in India. These types of chains comes in many lengths such as 5, 10, 20 and 30 meters. Most commonly used is 20m chain. Tallies are provided at every 2m of the chain for quick reading. Every link of this type of chain is 0.2m. The total length of the chain is marked on the brass handle at the ends.

2. Steel band or Band chain

These types of chain consisting of a long narrow strip of steel of uniform width of 12 to 16 mm and thickness of 0.3 to 0.6 mm. this chain is divided by brass studs at every 20cm or instead of brass studs, band chain may have graduated engraving as a centimetre.

For easy use and workability, band chains are wound on steel crosses or metal reels from which they can be easily unrolled. These steel bands are available in 20m and 30m length and the width of about 12-16mm.

3. Gunter’s chain or surveyor’s chain

Gunter chain comes in standard 66ft. This chain consists of 100links, each link being 0.66ft or 7.92inches. The length 66ft is selected because it is convenient in land measurements.

10 square Gunter’s chains = 1 Acre

10 Gunter chains = 1 Furlong

80 Gunter chains = 1 mile

4. Engineer’s chain

This chain comes in 100ft length. Its consist of 100 links each link being 1ft long. At every 10 links, a brass ring or tags are provided for indication of 10 links. Readings are taken in feet and decimal.

5. Revenue Chain

The standard size of this type of chain is 33ft. The number of links is 16, each link being 2   ft. This chain is commonly used in the cadastral survey.

*Testing and Adjustment of Chain

As the chain is a metal made, it may undergo many changes due to temperature effect or human error and etc. So for all lengths of the chain a tolerance are given,

5m chain = + or – 3mm

10m chain = + or – 3mm

20m chain = + or – 5mm

30m chain = + or – 8mm

*Chain length shorten due to

1. Bending of links.
2. Sticking of mud in the rings

*Chain length increases due to

• Opening of small rings.
• Wearing of surfaces.

*Chains maybe tested with respect to

• Steel tape
• Permanent test gauge
• Pegs are driven in the field at required distances
• Permanent test gauge made with dressed stones

*If the chain is found long, then

• Close the joins of the rings
• Reshape the elongated rings
• Remove one or two rings
• Replace worn out rings

*If the chain is found short, then

• Straighten the links
• Replace the small rings with a big one
• Insert additional rings

Flattening the circular rings

*Advantages and Disadvantages of Chains in Surveying

a) Advantages of Chains in Surveying

• Chain survey is the simplest and commonest method used in surveying exercises
• The equipment used to conduct chain survey are simple to use,
• The equipment used in chain survey can easily be replaced. For example, measuring rods can be replaced with measuring tape.
• This method does not involve a complicated mathematical calculation. I know this is the relief to those who are afraid of mathematics
• In chain survey few people are needed to conduct the survey. Normally chain survey team has three people Booker, leader and follower.

b) Disadvantages of Chains in Surveying

• Simple chain survey cannot be conducted in built-up areas and large areas.
• Simple chain survey is subject to several chances of errors of accumulation which may cause by the problem of the chain. The chain linkage may fail to stretch up properly and result in inaccurate data. Also clogging of a chain may read to an error in reading.
• It is time-consuming
• It may not be conducted in areas with steep slopes or waterlogged areas. Chain survey is usually conducted in dry areas with gentle slopes. It becomes more complicated when a survey is conducted in areas that are too wet.

Chain survey becomes more complicated method when there are raised points (obstacles) in between areas to be surveyed.

2) Tapes are grouped into four categories depending upon the material used for its construction

o    Cloth or Linnen tape

o    Woven metallic tape

o    Metric steel tape

o    Inver tape

o    Synthetic tape

o    Cloth or linen tape

It is made of a varnished strip of woven linen 12 to 15 mm wide. It is available in lengths of 10m, and 30m and 30m it is little used in surveying, but can be used for making subsidiary measurements such as offsets of a building.

a) Woven metallic tape

It is made of cloth strip woven with fine brass wires. It is also 16mm wide and available in length of 10, 15, 30 and 60m. The brass wires durable prevent the tape from twisting and stretching. It is more durable than cloth tape. Hence, it is used for general purposes.

b) Metric steel tap

It is made of steel or strain – less steel, may be provided with a vinyl coating. It is also 6mm to 10mm wide and available in length of 10, 15, 30 and 60m. it is used to measure the distance accurately.

c) Inver tape

It is made of an alloy of steel and nickel. It is also 6mm wide and available in length 15, 30, and 100m. It has great accuracy. Hence, it is used for work of the highest precision such as measurement of the baseline in the triangulation survey and in city work.

d) Synthetic tape

It is made of glass – fibre having P.V.C coating. It maintains its length. It is strong and durable. So it is used for measuring length with a good degree of precision. It is available in 5m, 10m,20m and 30m length.

 

C) Arrows

Arrows or chain pin is a rod of iron or steel, of diameter 4mm. it is 300 to 400mm in length. The arrow has a loop of diameter 50mm at one end whereas the other end is pointed for a length of 15mm. they are used for recording the chain length measured.

 

D) Pages

It is made of hardwood and is 2.5cm square in cross-section and 150mm long. It is tapered
At another end to facilitate easy driving. It is used to mark the position of the survey station or the endpoints of survey line. The pegs are driven into the ground using a mallet or wooden hammer such that its length of about 40 mm project above the surface of the ground.

E) Ranging rods

These are made of timber and steel. They are circular, octagonal in shape and of diameter 2 to 3cm. they are provided with a tapering edge and shade with cross shoe 150mm long at the bottom end facilitate easy driving. There is an order to make them visible from a distance, they are pointed alternately black and white or red and white. Ranging is necessary before starting the measurement of a line whose length is more than the chain length so that the measurement is done in a straight line.

F) Offset rods

An offset rod is a similar ranging rod and has a length of 3. They are round wooden rod shoed with a pointed iron shoe at the one end and provided with a notch or a hook at the other. The hook facilities pulling and chain through hedges and other obstructions. 

H) lath

The laths used by plasters for plastering the walls may be used for ranging in a level or open ground with an obstruction such as hedges, walls, or when crossing a depression. They are very light in weight and can be carried from place to place easily. The lower end can be easily sharpened to a point and cut to any desired length when required.

I) Whites

Whites are the pieces of sharpened thick sticks cut from the nearest place in the field. One end of the stick is sharpened and the other end is split. White papers are inserted in the split to improve visibility. Whites are also used for the same purpose as laths.

J) Plumb Bob

The pump is a ball made of brass or bronze of the shape of a pear. It has a fine steel point. There is a hook at the top for attaching a string of nylon. Its length is about 50mm. the plumb bob is used for measuring distance on the sloping ground. used in the centring of various instruments such as a magnetic compass, plane table, dumpy level or theodolite etc.