EXPERIMENT: 1 SURVEY OF AN AREA BY CHAIN SURVEY (CLOSED TRAVERSE) AIM: To survey an open field by chain survey in order to calculate the area of the open field. INSTRUMENTS: Chain, Tape, Ranging Rods, Arrows, Cross Staff. DESCRIPTION OF INSTRUMENTS: Chains: The chains are composed of 100 pieces of 4 mm diameter galvanized mild steel wires bent into rings at the end and joined to each other by three circular or oval shaped rings. These rings give flexibility to the chain. The ends of chains are provided with swivel joints so that the chain can be turned without twisting. To facilitate easy reading of the chain, brass tallies are provided. End of 10th link from each end is provided with a tally of one tooth, 20th link is provided with a tally of two teeth; 30th link with a tally of three teeth; 40th link with a tally of 4 teeth and the middle of chain is provided with a tally of circular shape. Commonly used metric chains are of 20 m length. They have 100 links with Talley’s at every 2 m. Each link is of 0.2 m length. Simple rings are provided at every one metre length except wherever tallies are provided. The total length of chain is marked on the brass handle. However 30 m chains are also in use. Length of each link is 0.3 m. It is not so convenient as 20 m chain to read, since no rings can be provided at one metre distance and each link needs multiplication with 0.3 to arrive at metre units.
Tapes: Depending upon the materials used, they are classified as: (i) Cloth or linen tape (ii) Metallic tape (iii) Steel tape and (iv) Invar tape. (i) Cloth or Linen Tape: 12 to 15 mm wide cloth or linen is varnished and graduations are Marked. They are provided with brass handle at the ends. They are available in length of 10 m, 20 m, 25 m and 30 m. These tapes are light and flexible. (ii) Metallic Tape: They are made up of varnished strip of waterproof linen inter woven with small wires of brass, copper or bronze. End 100 mm length of tapes are provided with leather or suitable strong plastic materials. Tapes of length 10 m, 20 m, 30 m and 50 m are available in a case of leather or corrosion resistant metal fitted with a winding device. Red and black coloured markings are used for indicating full metres and its fractions in centimetres. iii) Steel Tape: A steel tape consists of 6 to 10 mm wide strip with metal ring at free end and wound in a leather or corrosion resistant metal case. It is provided with a suitable winding device. Tapes are marked indicating 5 mm, centimetres, decimetres and metres. The end 10 cm length is marked with millimetres also. 10 m, 20 m, 30 m, or 50 m tapes are used in surveying. iv) Invar Tape: Invar is an alloy of nickel (36%) and steel. It’s coefficient of thermal expansion is low. Hence errors due to variation in temperature do not affect measurements much. The width of tape is 6 mm. It is available in length 30 m, 50 m and 100 m. It is accurate but expensive. Arrows: When the length of the line to be measured is more than a chain length, there is need to mark the end of the chain length. Arrows are used for this purpose. A typical arrow is shown in Fig. 12.5. Arrows are made up of 4 mm diameter steel wire with one end sharpened and other end bent into a loop. Length of an arrow is approximately 400 mm.
Ranging rods: For ranging intermediate points along the line to be measured, ranging rods and ranging poles are used. Ranging rods are 2 to 4 m long and are made of hard wood. They are provided with iron shoe at one end as shown in finger. They are usually circular in section with 30 mm diameter and are painted with 200 mm colour bands of red and white or with black and white. If distance is more than 200 m, for clear visibility they may be provided with multi coloured flags at their top. The ranging rods are occasionally used to measure short distances since they are painted with alternate colour of band 200 mm. Ranging poles are similar to ranging rods except that they are longer. Their length varies from 4 m to 8 m and diameter from 60 mm to 100 mm. They are made of hard wood or steel. They are fixed in the ground by making 0.5 m holes and then packed to keep them vertical.
Offset Rod: These rods are also similar to ranging rods and they are 3 m long. They are made up of hard wood and are provided with iron shoe at one end. A hook or a notch is provided at other end. At height of eye, two narrow slits at right angles to each other are also provided for using it for setting right angles. A typical offset rod is shown in Fig.12.8. PROCEDURE TRAVERSE): FOR SURVEYING THE GIVEN OPEN FIELD (CLOSED ABCDEF is the required closed traverse open field to be surveyed for calculating the area as shown in Fig 1. From the station A the length of all the opposite corners such as AC, AD and AE are measured with a chain and the longest distance is considered for laying off the main chain line. In this case AD is the longest and a chain line running from A to D is laid. Offsets to corner points B, C, E and F are now laid from the chain line AD either by tape or cross-staff and their foot of offsets are G, I, J, H respectively. All the offset lengths GB, HF, IC and JE are measured either by chain or tape depending on the length of offsets. The distances between all the points AG, GH, HI, IJ and JD are also measured along the chain line.