Survey Stations

Survey stations are important points fixed on ground indicating the starting point and the end point of the survey line. These are also the basic control points of the survey. There can be two types of survey stations.

1) Main Stations

Main stations are control points at the ends of the chain lines commanding the boundaries of survey and the lines joining the main stations are called the main survey line or the chain lines (A, B, C, D and E in Fig.1).

2) Subsidiary or Tie Stations

These are stations selected on the main survey lines for running auxiliary lines drawn to locate, measure and plot interior details such as fences, hedges, building, etc. (a and b in Fig.1).

Fig. 1 Layout of Chain Survey

The survey stations are suitably selected with care so that at least main survey stations are mutually visible and survey lines run through as flat ground as possible and are as close to the boundaries as possible. The main survey lines should form well-conditioned triangles. These should be as few as possible and suitably selected so as to avoid obstacles in chaining and ranging.

Survey Lines

The lines joining survey stations are the survey lines. The survey lines between main stations are thus called main survey lines or chain lines. The different survey lines are listed below.

1) Base Line

The longest of the main survey line is normally called base line running primarily through the middle of the area to be surveyed. The framework of triangles shall have one or two base lines since the entire survey is built around base line. It shall be measured with higher care and accuracy. 

2) Chain Line (Main Survey) Line 

The lines that join main stations are termed as chain line or main survey line.

3) Tie or Subsidiary Line

The survey line joining the subsidiary or tie stations on main line is termed tie line. It helps to check the accuracy of surveying and to locate the interior details. The position of each tie line should be close to some features such as paths, buildings, etc.

4) Check Line or Proof Line

A check-line also termed as a proof-line is a line joining the apex of a triangle to some fixed points on any two sides of a triangle. A check-line is measured to check the accuracy of the framework. The length of a checking line, as measured on the ground should agree with its length on the plan. It is preferable to have at least one check line in each triangle of the framework.

Offsets

The details on ground such as fences, buildings and towers, etc. are to be located with reference to main chain lines by means of lateral measurements. These lateral measurements with reference to the chain line are referred to as offsets. Offsets are classified based on its length and inclination to the survey line.

1) Classification Based on Length of Offset

a) Short Offset

Offset whose length is less than 15m is called short offset.

a) Long Offset

Offset whose length is greater than 15m is called long offset.

2) Classification of Offset Based on the Inclination to the Survey Line

a) Perpendicular Offset

Perpendicular offsets are the lateral distances taken at right angles (normal) to the chain line.

b) Oblique Offset

If the inclination of offset line to chain line is anything other than 90^o, the offsets are termed as oblique offsets.

Fig. 2 Offsets 

Factors Affecting Survey Station Selection 

• Stations should be visible from at least two or more stations. 
• As far as possible, main lines should run on level ground. 
• All triangle will be well conditioned triangle. 
• Each triangle should have at least one check line. 
• Survey lines should be as few as possible. 
• Obstacles to ranging and chaining should be avoided. 
• Sides of the larger triangles should pass as close to the boundary lines as possible. 
• Trespassing and frequent crossing of the roads should be avoided.