FUNDAMENTALS OF SURVEYING : Methods of linear measurement

1)                  Third order: used in chain surveying and other minor surveys.

2)                  Second order: in the measurement oftraverse line in which theodolite is used for measuring directions.

3)                  First order: triangulation survey for determination of length ofbase line. Chain surveying

Ø   It is that type of survey in which only linear measurements are taken on the ground and no angular measurements are taken.

Ø   Basically it is a land survey.

Ø   Suitable for surveys of small extent on open ground or to take simple details.

Ø    The principle of chain survey (or chain triangulation) is to provide skeleton or frame work consisting of a number of connected triangles.(as triangle is the only figure which can be plotted from the lengths of its sides measured in the field)

Definitions

Main station

Prominent point on the chain line and it can be either at the beginning or at the end of chain line. Subsidiary or Tie Station

It is the station selected anywhere on the chain line and subsidiary or tie lines may run through it. Check lines or proof lines

These are the lines which are run in the field to check the accuracy of main framework oftriangles.

Offsets

These are the lateral distances measured from survey lines. Perpendicular (simply offset) and

oblique offsets are the two types.

Tie lines

A line which joins subsidiary or tie stations to the main survey line is called tie line. The main

objective behind running a tie line is to take the details of nearby objects but it also serve the

purpose of a check line.

Base line

It is the long survey line which runs through the middle ofthe area to be surveyed. The framework

of triangles is built upon the base line.

Well conditioned triangles

A triangle is said to be well conditioned if it can be plotted accurately by the intersection of arcs

from the ends of base line. The best one is the equilateral triangle.

If site conditions do not permit an equilateral triangle, then the triangle in which no angle is less than

30^o or more than 120^o can be treated as a well conditioned triangle.