CHAIN SURVEYING

General:- Chain Surveying-Terms commonly used in Chain Surveying-

Operations in Chain Surveying- offsetting--Exampies on Limiting Length of

offsets-Instruments used for setting out right angles, Cross-staff, optical

square, prism squareConventional signs or symbols yield-work in Chain

Surveying-Recording field notes--Plotting a Chain Surveying Obstacles in

chaining-Examples on obstacles in chainingSome typical probłems in

ranging and chaining-Construction of scales--Exercise No. 3.

General : According to the place of work, surveying is classified into

(i) Land Surveying (ii) Marine Surveying and (ii) Aerial Surveying. Land

surveying can be done either by traversing or by triangulation. In travering.

a series of connected lines whose lengths are measured with chain or tane

and directions are determined with the angular instruments are run around

the area to be surveyed; whereas in triangulation, the area is divided into

network of triangles, the leng ths of whose sides are measured directly or

indirectly. A triangle can very easily be plotted if the length of its sides are

known.

Chain Surveying :

It is the method of surveying in which the area is divided into network

of triangles and the sides of the various triangles are measured directly in

the field with chain or tape and no angular measurements are taken. It is

the simplest kind of surveying and is most suitable when the area to be

surveyed is small in extent and is fairiy level and open with simple details.

It is unsuitable for large areas and for areas crowded with many details and

over difficult country as in such cases the formation of triangles becomes

difficuit.

surveyed into a network of connected triangles as a triangle is the oniy

simple figure that can be plotted from the lengths of its sides measured in

the field. Since triangulation forms the principle of chain surveying, the

chain survey is also sometimes called as chain triangulation.

If the area to be surveyed is triangular in shape and if the lengths and

sequence of its three sides are recorded, the plan of the area can be easily

drawn.

But if the area has more than three straight boundaries, for example

as in fig. 3.1 (a) it is no longer sufficient to measure lengths of the sides

only. The field measurements must be so arranged that the area can be

plotted by laying down triangles. Several arrangements should be.made

satisfying this condition but only twO are given here as shown in fig. 3.1.

(b) and (c).

3.2. Terms commonly used in Chain Surveying :

1. Frame work : The system of lines or triangles covering the area to

be surveyed is called Framne work or skeleton or survey such as ABCDE in

fig. 3.2. The arrangement of triangles depends upon the nature ánd shape

of the area to be surveyed.

Since an equilateral triangle can be more accurately plotted than an

obtuse-angled triangle, therefore, as far as possible, the triangles formed in

a chain survey should be nearly equilateral. The triangles in which the

angles are neither very acute nor very obtuse i.e. all angles are greater than

30° and less than 120° are called well conditioned or well shaped triangles

and are always preferred in a chain survey. A triangle which is almost an

equilateral one is the best suited for plotting work and is known as the best

conditioned triangle. The triangles having angles less than 30° or greater

than 120 are known as bad conditioned or ill conditioned triangles and

Should always be avoided. If however they cannot be avoided, great care

must be taken during their chaining and plotting.

2. Survey Stations. The ends of a chain line denote the Survey

Stations. These are (i) main survey stations, and (ii) subsidiary or tie

stations.