Chain Surveying

Chain Surveying is the simplest and oldest form of land surveying of an area using linear measurements only. It can be defined as the process of taking direct measurement, although not necessarily with a chain. In chain surveying the linear measurements are only made and no angular measurements are taken. Chain survey is suitable for the survey of areas that are fairly flat and small areas with simple details. This method is generally known as chain surveying because the principle instrument or equipment used in this method is the chain. The term taping can also be used for measurements with a tape. Tapes are more accurate and easier to carry and use than chains. However, chain surveying is ideally suited for rough use in difficult terrains.

General Procedure in Chain Surveying

a) Reconnaissance: Walk over the area to be surveyed and note the general layout, the position of features and the shape of the area.

b) Choice of Stations: Decide upon the framework to be used and drive in the station pegs to mark the stations selected.

c) Station Marking: Station marks, where possible should be tied into a permanent object so that they may be easily replaced if moved or easily found during the survey. In soft ground wooden pegs may be used while rails may be used on roads or hard surfaces.

d) Witnessing: This consists of making a sketch of the immediate area around the station showing existing permanent features, the position of the stations and its description and designation. Measurements are then made from at least three surrounding features to the station point and recorded on the sketch. The aim of witnessing is to re-locate a station again at much later date even by others after a long interval.

e) Offsetting: Offsets are usually taken perpendicular to chain lines in order to dodge obstacles on the chain line.

f) Sketching: Sketching the layout on the last page of the chain book, together with the date and the name of the surveyor, the longest line of the survey is usually taken as the base line and is measured first.

Equipment Used in Chain Surveying

The equipment used in chain surveying can be divided into three, namely

1. Those used for linear measurement. (Chain, steel band, linear tape)
2. Those used for slope angle measurement and for measuring right angle (e.g. Abney level, clinometer, cross staff, optical squares)
3. Other items (Ranging rods or poles, arrows, pegs etc.).

1) Chain

It is used to measure the distance between two points on the ground. The chain is composed of 100 or 150 pieces of galvanized mild steel wire 4mm in diameter called links. The ends of each link are bent into a loop and connected together by means of three rings. The ends of the chain are provided with brass handles for dragging the chain on the ground, each with a swivel joint so that the chain can be turned round without twisting. The length of the chain is measured from the outside of one handle to the outside of the other. Metallic tallies are fixed at various distinctive points of the chain to facilitate quick reading of a chain in surveying measurements.

Chain is designed for hard usage and is sufficiently accurate for measuring the chain lines and offsets of small surveys. Chains are made up of links which measure 200mm from centre to centre of each middle connecting ring and surveying brass handless are fitted at each end. To avoid confusion in reading, chains are marked similarly from both end (E.g. Tally for 2m and 18m is the same) so that measurements may be commenced with either end of the chain.

Types of chains

Generally, chains are of two types

1. Metric Chain
2. Non-metric Chain

1) Metric Chain

Metric chains are either 20m or 30m in length. A metric chain is prepared with 100 or 150 pieces/ links of galvanized mild steel wire of diameter 4mm. One metre is divided into 5 links each of 0.2m. The least count of metric chain is 0.2m. The ends of the pieces are bent to form loops and connected together by means of three oval shaped rings which gives flexibility to the chain. Two brass handles are provided at the two ends of the chain with swivel joints so that chain can be turned round without twisting. The outside of the handle is the zero point or the end point of the chain. The length of the chain is measured from the outside of one handle to the outside of the other. The length of a link is the distance between the centres of the two consecutive middle rings as shown in the Fig.1. The end links include the length of handle. Tallies are provided for marking 5m, 10m, etc for chains of 20m and 30m lengths and marked with letter “m” to distinguish the metric chain from non-metric chain. Small brass rings are provided at every meter length, except where tallies are attached. In metric chains readings are started from ends, increasing towards the centre. The length of chain whether 20m 0r 30m is indicated on the handle for easy identification.

Fig. 1 Metric Chain

2) Non – Metric Chain

In this type of chains other than metrics unit are used. Nowadays metric chains are used everywhere, therefore this type of chains become obsolete. Generally, these chains are 3 types.

a) Engineer’s Chain

The Engineer’s chain is 100ft. long and consists of 100links, each link being 1ft long. It is used in all engineering surveys.

b) Gunter’s Chain/Surveyor’s Chain

The Gunter’s chain is 66ft. long and is divided into 100 links each 0.66ft long. It is very convenient for measuring distances in miles and furlongs.

   10 Gunter’s chain = 1 furlong
   50 Gunter’s chain = 1 mile

   10 square Gunter’s chain = 1 acre

c) Revenue Chain

The standard size of this type of chain is 33ft. The number of links are 16. This chain is commonly used in cadastral survey.

Suitability of Chains

The chains are suitable for the following cases.

• It is suitable for ordinary or preliminary works as its length alters due to continuous use.
• Its length gets shortened due to bending of links and gets lengthened by flattening of the rings.
• Being heavier, a chain gets sagged considerably when suspended at the ends.
• It can be easily repaired in the field.
• Measurement readings can be taken very easily.
• It is only suitable for rough works.

Merits of Chains

• They can be read easily and quickly.
• They can withstand wear and tear.
• They can be easily repaired or rectified in the field.

Demerits of Chains

• They are heavy and take too much time to open or fold.
• They become longer or shorter due to continuous use.
• When the measurement is taken in suspension the chain sags excessively giving incorrect measurements.

2) Steel Bands

This may be 30m, 50m or 100m long and 13mm wide. It has handles similar to those on the chain and is wound on a steel cross. It is more accurate but less robust than the chain. The operating tension and temperature for which it was graduated should be indicated on the band.

Fig. 2 Steel Band

3) Tapes

Tapes are used where greater accuracy of measurements are required, such as the setting out of buildings and roads. They are 15m or 30m long marked in metres, centimetre and millimetres. Depending upon the material tapes are classified as the following.

a) Cloth or Linen Tape

Linen tapes are closely woven linen and varnished to resist moisture. They are generally 10 metres to 30 metres in length and 12mm to 15 mm in width. These tapes are liable to stretch in use and should be frequently tested for length. They should never be used on work for which great accuracy is required. Cloth tapes are generally used for measuring offset measurements only due to following reasons.

• It is easily affected by moisture and shrunk.
• Its length gets altered by stretching.
• It is likely to twist and tangle.
• It is not strong as a chain or steel tape.
• It is light and flexible and it does not remain straight in strong wind.
• Due to continuous use, its figures get indistinct.

b) Metallic Tape

A linen tape reinforced with brass or copper wires to prevent stretching or twisting of fibres is called a metallic tape. As the wires are interwoven and the tape is varnished, these wires are not visible to naked eyes. These tapes are available in different lengths but tapes of 20m and 30m lengths are very common. These are supplied in leather case with winding machine. Each metre is divided into decimeters and each decimeter is sub-divided into centimeters.

c) Steel Tape

These are much more accurate and are usually used for setting out buildings and structural steel works. Steel tapes are available in various lengths up to 100m (20m and 30m being the most common) encased in steel or plastic boxes with a recessed winding lever or mounted on open frames with a folding winding lever. Steel tapes are available with different accuracy of graduation. At the end of the tape a brass ring is provided. The length of metal ring is included in the length of tape. A steel tape of lowest degree of accuracy is generally superior to a metallic or cloth tape for linear measurements.

d) Invar Tape

Invar tapes are made of an alloy of nickel (36%) and steel (64%) having very low co-efficient of thermal expansion (0.000000122 per 1ºC). These are 6mm wide and are available in length of 30m, 50m and 100m. These tapes are used for high degree of precision required for base measurements.

e) Fibre Glass Tapes

These are much stronger than lines and will not stretch in use.

4) Arrows

Arrow are made of tempered steel wire of diameter 4mm. One end of the arrow is bent into a ring of diameter 50 mm and the other end is pointed as shown in Fig.2. Its overall length is 400mm. Arrows are used for counting the number of chains while measuring a chain line. Generally, 10 arrows accompany a chain. A piece of coloured cloth, white or red ribbon is usually attached or tied to the end of the arrow to be clearly seen on the field.

Fig. 3 Arrow

5) Pegs

Pegs are used to mark definite points on the ground. These are made of hard timber and are tapered at one end. They are usually 15cm length with 3 to 5cm diameter circular in shape (or) 3 to 5cm square in shape. The pointed end of peg is covered by iron shoe for easy driving into the ground. Wooden pegs usually 2.5cm square and 15cm deep are used to mark the position of survey stations. Pegs are driven with a mallet and nails are set in the tops.

Fig. 4 Pegs

6) Ranging Rod

Rods, which are used for ranging a line are known as ranging rod. Such rods are made of seasoned timber or seasoned bamboo or steel tubular rods. Sometimes GI pipes of 25mm/30mm diameter are also used as ranging rods. They are generally circular in section of diameter 25mm/30mm and length 2m / 3m. These are used for marking a point in such a way that the position of point can be clearly and exactly seen from some distance away. The rod is divided into equal parts of 20cm each and the divisions are painted black and white or red and white alternatively so that the rod is visible from a long distance. The lower end of the rod is pointed or provided with an iron shoe. Sometimes these are used to mark the permanent points. When they are at a considerable distance, red and white (or) while and yellow flags about 25cm square should be fastened at the top.

Fig. 5 Ranging Rod

7) Ranging Poles

These are similar to ranging rods except that they are heavier in section of length 4m to 6m. They are used for ranging very long lines in undulating ground. Their length varies from 4 m to 8 m and diameter from 60 mm to 100 mm. Ranging poles are commonly used to highlight the ends of survey lines, so that line of sight can be easily seen for extending lines or setting right angles with an optical square.

8) Offset Rod

It is similar to the ranging rod but is usually 3m long. The top is provided with a stout open ring recessed hook for putting or pulling the chain through a hedge or other obstruction like bushes etc. It is used for aligning the offset line and measuring short offsets. They are made up of hard wood and are provided with iron shoe at one end. At height of eye, two narrow slits at right angles to each other are also provided for using it for setting right angles.

9) Plumb Bob

It is used to transfer the end points of the chain onto ground while measuring distances in hilly terrain. It is also used for testing verticality of ranging poles, ranging rods etc. It is used while measuring distances on sloping ground and transfer to the levelled ground. It is made of steel in a conical shape with a thread connected at the centre. Plumb bobs come in a variety of shapes, sizes and weights, as shown in Fig. 6.

Fig. 6  Types of Plumb Bob

10) Cross Staff

This instrument is used for finding the foot of the perpendicular from a given point to a line, and setting out a right angle at a given point on a line. It consists of two pairs of vanes set at right angle to each other with a wide and narrow slit in each vane. The instrument is mounted upon a pole, so that when it is set up it is at normal eye level. It is also used for setting out lines at right angle to the main chain line. To set out a right angles the following three type of cross staffs are used.

   a) Open Cross Staff
   b) French Cross Staff

   c) Adjustable Cross Staff

11) Optical Square

It is used to set out right angles. It is a small compact hand instrument. It consists of a circular metal box about 5cm in diameter and 1.25cm deep. It is protected by a metal cover, which slides round so as to cover the openings and thus protects the mirrors from dust when not in use. H and I are the two mirrors placed at an angle of 45˚ to each other. It is used where greater accuracy is required. There are two types of optical square, one using two mirrors and the other a prism.

Fig. 7 Double Prism Optical Square

11) Clinometer

This instrument is used for measuring angles of ground slopes (slope angle). They are of several form, the common form is the Watking’s Clinometer, which consist of a small disc of about 60mm diameter. A weighted ring inside the disc can be made to hang free and by sighting across this graduated ring angle of slopes can be read off. It is less accurate than Abney level.

12) Abney Level

This instrument is generally used to obtain roughly the slope angle of the ground. It consists of a rectangular, telescopic tube (without lenses) about 125mm long with a graduated arc attached. A small bubble is fixed to the Vernier arm, once the image of the bubble is seen reflected in the eyepiece the angle of the line of sight can be read off with the aid of the reading glass.

Fig. 8 Abney Level

13) Hammer

It is used to give blows to fix the peg on the ground.

14) Laths

Laths are 0.5 to 1.0 m long sticks of soft wood. They are sharpened at one end and are painted with white or light colours. They are used as intermediate points while ranging or while crossing depressions.

15) 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 the visibility. Whites are also used for the same purpose as laths.

Necessary Precautions in Using Chain Surveying Instruments

• After use in wet weather, chains should be cleaned and steel tapes should be dried and wiped with an oily rag.
• A piece of coloured cloth should be tied to arrow (or ribbon – attached) to enable them to be seen clearly on the field.
• Ranging rods should be erected as vertical as possible at the exact station point.
• The operating tension and temperature for which steel bands/tapes are graduated should be indicated.
• Linen tapes should be frequently tested for length (standardized) and always after repairs.
• Always keep tapes reeled up when not in use.