In these process, we take linear measurement between stations or between station and points of any important structure like buildings etc. It involves three steps:
1.Ranging
It involves establishing the intermediate ranging rods in a line with the help of ranging rods in order to make a straight path for chaining between stations. It can be done either by an instrument or simply by eye. The instrument may be a line ranger or a theodolite. Ranging can be classified into two categories
a.Direct Ranging: It is used when we perform whole process on plain ground such that we can see other station clearly. It can be done with by eye or by line ranger.
Line Ranger
It is a small reflecting instrument consist of two rectangular prism placed one above on another. It is used for ranging long lines when there is difficulty to reach one end of line. The Observer take approximate intermediate position between the stations. Then the ray from A enter from one side of ranger and ray from B enter from another end present at opposite side of ranger. The images from both stations are seen directly in upper and lower prism. The observer continuously change the instrument position forward or backward until the images of both station coincide with each other. The centre of instrument give the required position on ground.
b.Indirect Ranging: When there is depression or high ground between the stations then it is difficult for ranging betwen the station. It is also known as Reciprocal Ranging. Supoose if there is hill between two stations then we perform ranging as picture below:
Firstly we take two intermediate stations by approximation. We range A, M1, N1 by moving the rod at M1 which give new point M2 and then range N1 with M2, B give new point N2. These process continuous till all point come in a straight line.
2.Chaining
Two persons are required for chaining- Leader who present at forward end and drag chain forward and Follower who follow the leader and present at rear end of chain. The leader insert arrow in ground while Follower collect the arrow from ground.
If two points are present at lesser distance than the length of chain by estimation then person directly mesured the distance with chain or tape. But if the distance between the station are larger then they took the help of arrows and ranging rod. Firstly the Follower help the Leader to range the rod. After ranging the rod, the leader mark the point and then stretch the chain toward the marked point. The leader insert an arrow at outer edge of chain which represent a full chain length. Then the leader and follower repeat the same process till the end station reached. The follower collected the previous arrows and give it to leader after stock out.
If chaining is at slope ground, then we need only horizontal distance for plotting. The slope of any surface upto 3 taken as level surface. It involves following parts:
a.Direct Method (Stepping Method)
Horizontal distance (AB)= Aa+ab+bc+cd+dB
b.Indirect Method
3.Offset
The method of measuring the distance of any object from chain line is known as Offset. The offset can taken on either side of chain line. It is perform to get idea of how far an object locate fron chain and plotted on sheet. The offsets are classified according to direction and length.
According to direction:
a.Perpendicular Offset: The measurement of distance at right angle from chain line. The offset are generally taken at peripendicular direction in surveying. So an offset mean perpendicular offset in strict case.
b.Oblique Offset: The measuring of distance at angle execpt 90 is called Oblique Offset. It is also know as Tie Line offset. It is taken when distance is larger or when there is need of plotting corner of any building.
In above image, Ab is chain line. P1P is perpendicular offset and QP is Oblique Offset.
According to length:
a.Long Offset: The offsets greater than 15 m.
b.Short Offset: The offset lesser than 15m.
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