Chain surveying in Forestry
General--Chain surveying necessitates the setting out on the ground a system of straight lines, called chain lines, forming well defined figures and referring to these lines the features of the ground to be surveyed by means of offsets. It deals with only linear measurements in the field; it is a convenient method suitable for small areas and open ground with simple details. Relative position of the chain lines is determined by recording the angles made by them with each other with an angle measuring instrument, as in Prismatic Compass Survey, or by laying out lines to form a series of triangles and measuring the sides as in Chain Survey, involving only linear measurements.
Types of Chains
The surveying chains are of following types: Revenue chain—It is mainly used in cadastral survey. It is 33 ft long having 16 links. Gunters chain: This is named after its inventor Edmund Gunter and is also called surveyor's chain. A Surveyor's chain is 66 ft long and is divided into 100 links. It is easy to measure the distance by this chain. 10 gunter's chains = 1 furlong 80 Gunter's chains = 1 mile 10 Square Gunter's chains = 1 acre Engineer's chain—This is 100 ft long and is also divided into 100 links so that each link is equal to 1 ft. The constructional details of A Surveyor’s chain are same as that of a Gunter's chain.
Metric chain-The most commonly used metric chains in surveying are 20 and 30 m chains. The 20 m chain is divided into 100 links while the 30 m chain is divided into 150 links such that each link is 20 cm long. The chain is shown in Fig. 2.2.
Steel band—It is a thin and long steel band of about 20 or 30 m length. The band width and thickness is about 16 mm and 0.25 to 0.70 mm respectively. It is lighter than chain and can be used with better accuracy.
Testing of chain The standard chain measured with a tension of 8 kg and checked against certified steel band or tape which has been standardized at 20° C, every metre length should be accurate within 2 mm and overall length of the chain should be within the limits given below. 20 metre chain ± 5 m.m. 30 metre chain ± 8 m.m. While using the chain, the links gets bent thereby reducing the length of the chain. Due to the following reasons, the length of chain increases : (a) Wear and tear (b) Due to the continuous use of the chain links joints gets widened. (c) Pulling through hedges and fences or rough use. Due to these reasons there is need for constant check of its length before commencing the work every day, otherwise mistakes in measurement will be occur.
Testing of chain is carried out by comparing with 1. Chain laid down along with a stand and chain by the side of some important public building. 2. Surveyor's office may keep a steel tape for testing.
Adjustment of chain In case, the chain is longer, it may be corrected by 1. Shortening of the joints 2. By setting right the rings by hammering 3. Replacement of worn parts 4. By removing rings and 5. By adjusting handle links.
If the chain is shorter, it is adjusted by 1. Straightening links 2. By replacing smaller rings by larger ones 3. Adjusting the handle links 4. By adding new rings
Tapes
When greater accuracy is required in measurement and the ground to be surveyed is not very rough, then tapes can he used. Tapes are available both in feet and metres. For surveying mostly 30 metre tape is used Three types of tapes. namely. the linen or cloth tape. metallic tape and steel tapes are commonly used.
- Linen or cloth tape—It is made of varnished strip of woven linen 12 to 16 mm wide with a brass handle at Tcro end whose length is included in the length of the tape.
- Metallic tape ---A metallic tape is made up from good quality cotton or linen and is reinforced with fine brass or copper wires. They are also not suited for very precise measurements.
- Steel tape --It is made of steel ribbon or stainless steel and very accurate. Steel tapes are used for very precise measurements and for checking the accuracy of chain lengths. It is very delicate and should not be used for rough works.
Accessories in Chain Surveying
- Arrows--An arrow is used to mark the end of the chain or tape during linear measurement. It is made of metallic wire or a thin rod. It is pointed at one end for fixing on the ground and a loop is provided at the top for holding it.
- Peg;-- It is made of wood. It is used to mark the stations. The peg is about 15 cm long and 2.5x 2.5 cm in cross section. It is driven in the ground by a hammer.
- Ranging rods—Ranging rods are used for ranging lines and to mark stations which must be seen from a long distance. They are made of light, strong and thin bamboo, Fig. 2.7 Peg seasoned wood or mild steel rods. Pointed metal shoe is provided at the bottom to facilitate fixing in the ground. The rod is alternately painted white and black or white and red. The length of a ranging rod is usually 2 to 3 m.
4.Ranging poles—They are similar to ranging rods but heavier in cross section. The length may be upto 6 m and is used for ranging very long lines.
5, Plumb bob--A plumb bob consists of a metal weight made of brass point with a pointed end. It is suspended by a string and is used to locate points directly below or above another point
- Offset rod—It is similar to ranging rod or ranging pole and is 3 to 4 metres long. It is divided into meters and decimeters.
- Parambulator—This is used to measure a distance quickly and approximately. It looks like a bicycle wheel and is provided with a fork and handle. With the help of the handle, the wheel is pushed through the distance to be measured.
