OFFSET IN CHAIN SURVEY

Offset

The lateral measurement taken from an object to the chain line is known as ‘offset’. Offsets are taken to locate objects with reference to the chain line. They may be of two kinds. 

1. Perpendicular offset and

2. Oblique offset.

Perpendicular offset: When the lateral measurements are taken perpendicular to the chain line, they are known as perpendicular offsets (Fig. 10) 

Perpendicular offsets may taken in the following ways.

    1. By setting a perpendicular by swinging a tape from the object to the chain       line. The point of minimum reading on the tape will be the base of the             perpendicular (Fig. 11.a)

.       2. By setting a right angle in the ratio 3:4:5 or 5:12:13 (Fig. 11.b)

    3. By setting a right angle with the help of builder’s square or tri-square (Fig. 11.c)

      4. By setting a right angle by cross-staff or optical square.

Perpendicular offset are preferred for the following reasons:

(i)                  The can be taken very quickly.

(ii)                 The progress of survey is not hampered.

(iii)                The entry in the field book becomes easy.

(iv)               The plotting of the offsets also becomes easy.

Oblique offset: Any offset not perpendicular to the chain line is said to be oblique offset. Oblique offset taken when the objects are at the long distance from the chain line or when it is not possible to set up a right angle.

Oblique offsets are taken in the following manner.

Suppose AB is a chain line and p is the corner of a building. Two points ‘a’ and ‘b’ are taken on the chain line. The chain ages of ‘a’ and ‘b’ are noted. The distance ‘ap’ and ‘bp’ are measured and noted in the field book. Then ‘ap’ and ‘bp’ are the oblique offsets (Fig. 12). When the triangle ‘abp’ is plotted, the apex point ‘p’ will represent the position of the corner of the building.


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