Definition Of Chain Surveying
Chain surveying is the process of surveying a given piece of land using linear measurements only. Chain surveying method occurs in other aspects of surveying, hence a good knowledge of chain surveying is very vital in surveying as a whole.
Equipment Needed In Chain Surveying
1. Line measuring equipment e.g. The chain, the steel band and tapes.
2. The Abney level for measuring slope angles.
3. The cross staff and the optical square for setting out right angles.
4. The ranging rod for ranging out straight lines.
5. The chain arrows for holding the chain or tape in position and for counting chain lengths.
Line Measuring Equipment
1. The Chain:
This is the oldest instrument used in distance measurement. Chains are made of tempered steel wire and are made of links which measure 200mm from centre to centre of each middle connecting ring. Swiverling brass handles are fitted at each end and the total length is measured over the handles. The Gunter’s chain is a typical example of the chain.
2. The Surveying Band:
This is made of steel strip six mm in width carried on a four arm open frame winder. A handle is fitted for returning the band into its frame after use. They are obtainable in lengths of 30m, 50mm and 100mm. The steel band is a much more accurate measuring instrument than the chain.
Tapes may be made of synthetic, glass fibre being typically coated or plain steel. They are obtainable in lengths of 20m or 30m.
For the synthetic types, the major graduations are shown at whole metre positions and lengths, with minor graduations at hundredths and 50mm intervals indicated.
Those made of glass fibre have a PVC coating and are graduated every 10mm and figured every 100mm. Whole metre figures are shown in red at every metre. Tapes are used for measuring relatively short distances especially off-sets.
4. The Cross Staff
It consists essentially of an octagonal brass book with slits cut in each face such that opposite pairs form sight lines. The instrument may be mounted on a short ranging rod. To set out a right angle, sights are taken through any two pairs of slits whose axes are perpendicular.
5. The Optical Square:
The optical square is a more accurate instrument for setting out right angles than the cross staff. There are two types of the optical square, one using two mirrors and the other a prism. The minor type consists essentially of two plane mirrors placed at an angle of 45degree to each other.
They may simply be held in a frame or a small round shallow box with three openings on the sides A.B and D. One of the mirrors, I called the index glass is wholly silvered.
The other mirror, H called the horizontal glass is half silvered and half plane. The horizon glass is set at 60degree to the line of the EP1 while the index glass is set at 75degree to line IP2 which is at right angle to EP1.
A ray of light reflected by two plane mirrors in succession is sent through an angle equal to twice the angle between the mirrors.
Consequently, a ray light whose path is along the line P2I is reflected to H and hence to E. All these mirrors are set at 45degree to each other a light ray is thus sent through 90degree.
6. The Abney Level
It consists of a rectangular, telescopic tube without lenses about 125mm long with a graduated arc attached. A small bubble tube is fixed to the vernier arm and once the image of the bubble is seen reflected in the eye piece, the angle of the line of sight can be read off with the aid of the reading glass. The vernier allows readings to be taken to 10 minutes.
7. Ranging Rods
Ranging rods are made of wood or hollow metal. They are available in lengths of 2 or 3m and are painted in alternate white and red bands for easy identification. They have pointed metal studs which enable them to be driven into the ground.
Uses Of Ranging Rods
1. Ranging rods are used as targets.
2. They are used for ranging out lines to facilitate chaining especially in catenary taping.
8. The Chain Arrows
A chain arrow is made of a long stud wire pointed at one end and formed into a ring at the other end for easy carrying.
They are used for counting chain lengths when measuring a long line. They are also used in holding the tape in position.
Field Procedure In Chain Surveying
In a recce diagram drawn after a recce exercise for the survey of a five-sided piece of land by chain survey method. The measurement (Linear only) commences from the line AB (Method of linear measurement has been described in chapter 1) then BC, CD, DE and EA. But since only linear measurements are taken, the lines AD and BD also have to be measured to make plotting the survey possible since no angles are measured. With all the lines measured the survey is completed.
Uses Of Chain Survey
1. It is used in the quick survey of small areas to determine shapes and sizes.
2. It is used in the survey of details in traverse survey.
Principles Of Chain Surveying
The basic principles in chain surveying are illustrated below:
1. Take the case of an area on ground with three straight boundaries AB, BC and CA. If these three sides are measured, this simple survey can be plotted in the following ways:
a. First the line AB, is drawn to scale on paper to form a base line.
b. The length AC is set to the same scale on a pair of compasses and an arc of radius AC is drawn.
c. By same method and arc of radius BC is drawn to intersect the first.
d. Point C is represented by the intersection of the arcs. These points A, B and C on the ground.
e. Where the area on ground is bounded by more triangle, that area can be plotted by triangles. This follows that the four sided figure one of the diagonals must be measured.