Terms Used in Chain Survey, Offsets, Field Book and Method of Chaining on Sloping Ground

Chain surveying is one of the simplest and most commonly used methods in surveying. It involves linear measurements and is widely used for small areas. To perform chain surveying effectively, it is important to understand various terms, types of offsets, use of field books, and methods of chaining, especially on sloping ground.

Important Terms Used in Chain Survey

• Survey Station: A fixed point on the ground where measurements are taken.
• Main Survey Line: The line joining main stations and forming the framework of the survey.
• Base Line: The longest and most important line in the survey area.
• Check Line (Proof Line): A line used to check the accuracy of the survey.
• Tie Line: A line joining two points on main survey lines to locate details.
• Offset: The lateral distance from a survey line to locate details.
• Ranging: The process of establishing intermediate points on a straight line.

Offset

Offset is the perpendicular or oblique distance measured from the survey line to locate objects such as buildings, trees, fences, etc. Offsets help in detailing the features of the surveyed area.

Types of Offsets

1. Perpendicular Offset

This type of offset is taken at right angles (90°) to the survey line. It is the most accurate and commonly used method.

2. Oblique Offset

This offset is taken at an angle other than 90°. It is used when it is difficult to take a perpendicular offset.

Limit of Offset

The length of an offset should be kept as short as possible to maintain accuracy. Long offsets may lead to errors in plotting. Generally:

• Perpendicular offsets should not exceed 15 meters
• Oblique offsets should be avoided or kept minimal

Field Book

A field book is a notebook used by surveyors to record measurements, observations, and details taken during fieldwork. It is an essential document for plotting and reference.

All measurements should be recorded clearly and systematically in the field book to avoid confusion and errors.

Types of Field Book

1. Single Line Field Book

In this type, a single central line is drawn on each page to represent the chain line. Offsets are recorded on either side.

2. Double Line Field Book

This type has two parallel lines in the center representing the chain line. It provides more clarity and space for recording data.

Entry of Field Book

Proper entry in the field book is very important. The following points should be followed:

• Start with a neat heading including date, location, and survey details
• Draw chain lines clearly
• Record chainage (distance) along the central line
• Note offsets on left or right side of the chain line
• Use standard symbols for features
• Avoid overwriting and keep entries neat

Accurate field book entries ensure correct plotting and reduce chances of errors.

Method of Chaining on Sloping Ground

In sloping ground, direct measurement along the slope gives a longer distance than the actual horizontal distance. Therefore, special methods are used to measure correct horizontal distances.

1. Stepping Method

In this method, the chain is held horizontally in steps. One end of the chain is placed on the ground while the other is held at a higher level. This process is repeated step by step along the slope.

• Provides accurate horizontal distance
• Suitable for steep slopes

2. Slope Correction Method

In this method, the slope distance is measured, and a correction is applied to obtain the horizontal distance using calculations.

3. Hypotenuse Allowance Method

This method involves measuring along the slope and applying trigonometric calculations to determine the horizontal distance.

Precautions During Chaining

• Ensure proper alignment of chain
• Keep chain straight and horizontal
• Avoid sagging of chain
• Use proper ranging techniques
• Record measurements accurately

Importance in Surveyor Trade

Understanding these terms and methods is essential for accurate surveying. Proper use of offsets, correct field book entries, and appropriate chaining methods ensure reliable results.

These concepts form the foundation of surveying and are widely used in practical fieldwork.

Conclusion

Chain surveying involves various terms, techniques, and methods that must be clearly understood by trainees. Offsets help in locating details, while field books ensure proper recording of data.

Chaining on sloping ground requires special methods to obtain accurate horizontal distances. By mastering these concepts, surveyor trainees can perform fieldwork efficiently and produce accurate survey results.