What is Chain SURVEY?
Chain survey is the easiest type of survey in which area to be surveyed is divided into a number of triangles.
Because all the geometrical figures only whose shape and size are determined when the length of the sides are known.
The perpendicular distance, called offsets, of various objects in the field from the line, are measured and recorded in a book called field book.
From this records in the field book, the whole area can be plotted on a drawing sheet to a reduced scale.
Linear measurement- Basics
One of the basic function in surveying
Always horizontal not sloping distance
No way to check the error other than remeasuring
Linear measurement methods:
Direct measurements Instruments such as
Pacing
Passometer
Pedometer
Odometer or Speedometer
Rough methods for preliminary survey
Check of large mistakes in measurement made by more precise method
Unsuitable in irregular and sloping ground
Precision – 1/50 to 1/200
Different types of Chain in Chain survey
It is a steel wire with links connected by steel rings. It has brass handles at both ends. There are many types of chains of which the Engineer's Chain and Gunter's chain are commonly used in most of the country.
Engineer's Chain:
Gunter's chain:
The Engineers chain is 100 ft long and Gunter's chain 66 ft. The Engineer's chain consists of 100 links each one a foot long and at every 10 links a tally is attached to facilitate reading. The Gunter's chain is also divided into 100 links each link is 0.66 ft or 7.92 inches.
Metric chain:
Revenue chain:
Revenue chain is 33 ft long chain. It consists of 16 links. This chain is used for length measurements in feet & inches for small areas.
Steel Band or Band Chain:
Steel bands are preferred than chains as they are more accurate and easy to use. The disadvantages are they get broken down easily, and it's difficult to service in the field. The length of the tape is 20 and 30 m, 12 to 16 mm wide and 0.3 to 0.6 mm thick. They are numbered at every meter. Brass studs divide them at every 20 cm.
Taping / Chaining
Chaining or taping carries same meaning
Use of either chain or tape.
One of the accurate method of direct measurements
This method is basis for most surveying.
Precision
1/1000 to 1/5000 (ordinary land survey)
Use of chain or tape
Chain (Early 1600s)
Steel tape in use by early 2000s
Tape is very common nowadays.
Instruments for chaining/taping
Arrows -10 arrows
Pegs (Station position)
(Terminal points of survey line)
Ranging rods
(Used to range intermediate points on survey line
Length 2 m ( very common) or 3 m
Bands of 20 cm painted alternate colour( red & white, black & white))
Ranging poles
Similar to ranging rod but big in size
Used in case of long lines
Used to range intermediate points on survey line
Plumb bob
Verticality of ranging pole
Transferring the points to ground
Also used for centring purpose in other surveying methods.
Chain (Absolute equipment at present)
Metric chain (available in 5,10,20 meter)
Gunter’s or surveyor’s chain (66 ft of 100 links)
Engineer’s chain (100 ft of 100 links)
Revenue chain (33 ft of 16 links)
Steel band or band chain ( 20 or 30 m)
Tape
Cloth or Lenin tape
Metallic tape
Steel tape
Invar tape (Alloy of nickel 36% & steel)
Definitions:
Pegs: These are wooden blocks of conical shape %sect in fixing stations.
Arrow: They are of steel wire 15 inches long pointed at one end, and the other end is looped for a convenience of handling. They are used for making chain length on the ground.
Ranging Rods: They are about 10 ft. Long, 1.5 inches diameter round or hexagonal wooden poles painted with black and white alternative bands Each band is of one-foot length.
Offset Staff: They are wooden rods 10 feet long. Each foot is painted black and white alternately. They are used for measuring short lengths.
Optical square:
It is used to find the foot of the perpendicular from a given object in the field to a given chain line to take the offset. It consists of a wedge-shaped hollow brass box of about 2 inches sides and 1.25 inches depth with a brass handle about 3 inches long fixed at the bottom. Two plane mirrors set at 45 degrees are fixed to the inclined sides of the box. There are two slits above these mirrors. In using it, a ranging rod is held at an object for. Which an offset is to be taken. A man is holding the optical square in his right-hand stands on the chain line. He looks towards the front ranging rod on the chain line with the open face of the optical square towards the ranging rod at the object. Now the man looks through one mirror while the other mirror is turned towards the object. Then he walks along the chain line forwards and backward till he sights the image of the ranging rod at the object in the mirror and the front ranging led in the slit in the same line. The position of the man on the chain line gives the exact point at which the perpendicular from the object meets the chain line.
Procedure of chain surveying:
Field work in Chain surveying:
In includes reconnaissance. Selection of the station, measurement of lines and taking offsets of different objects in the field
Reconnaissance:
This is the preliminary survey in which to the survey party will examine the plot to be surveyed in order to know as to how the works can be executed in the best possible ways. The party will note all details like roads, buildings, canals, ditches, culverts and the difficulties and obstacles that may arise during the carrying out of the work. The party should locate the suitable points for stations by driving pegs. Sometimes a small triangle or a circle is made around the stations and the pegs are inserted into the centers. The party should then make a rough sketch of the plot showing the possible stations and from there the arrangement of different lines. It is important to give a north line on the rough sketch and though the sketch is not prepared according to the scale, it should represent the approximate positions of the different things in the plot and hence to be a good guide for further work.
Stations:
These are points on the ground fixed by driving pegs. Every station should be located with respect to three permanent objects i.e. the distances from these objects to the stations should be measured very accurately and recorded in the field book. The advantage of taking this measurement is that if in future the peg at the station is lost, then it can be located again by knowing descriptions and distances of these objects.
The selection of a particular station depends upon the fallowing important considerations:
The triangle should be a well-defined one, i.e. nearly equilateral triangle. ·
Every main station should be visible from the other two
There should be a minimum number of obstacles in Ranging and chaining
The chain line should run near the boundary of the plot
The chain line should be as few as possible
The chain line should be over an approximately leveled ground · In the case of chaining along the road, it is always better to run chains on one side of the road to avoid interruptions by vehicles.
It is better not to cross the road frequently
Offsets should not exceed one chain
Check and tie lines should be provided in sufficient number so that all the main lines, of sets and other details, can be checked thoroughly
Tie lines and check lines:
A tie line is one which connects two points on the two main lines of the triangle. It helps in taking offsets of the objects falling within the triangle and which are too far away from the main line. A checking line is also a tie line which helps in checking the accuracy of the work after plotting in a drawing sheet. A checking line or tie line is never extended beyond the main lines.
Measurement of lines and taking offsets:
In Fig the main station A is located with respect to three permanent objects and a ranging rod is fixed to the station. One ranging rod is fixed at main station B and another at an intermediate point in between A & B. The three rods will be in a straight line when only the intermediate rod is visible if a man looks from A to B. Now measurement of line AB is taken by the chain. The chain should be properly stretched so that there is no sag in it. As the measurement proceeds, offsets are taken on both side's of the line AB and recorded in the field book. In this way, all the lines including tie and check lines are measured and offset taken and recorded in the field book.
Advantages and disadvantages of chain Survey
Advantages
This type of survey work is suited for a small plain ground
It requires simple instruments
Plotting of maps is very simple and easy
disadvantages
But this type of surveying is not suitable for undulation land where chaining operation is tedious and subject to errors. This method is not generally recommended for a crowded city with a large number of buildings and obstacles because it cannot be divided into well-conditioned triangles. In the case of route surveying.
i.e the survey work of a road, irrigation canal, railways, water and sewer lines, tunneling etc, this method is not recommended at all.
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