Short Answer:

An offset in surveying is the distance measured from a main survey line (like a baseline) to any point or object nearby. Offsets help in locating details such as buildings, trees, boundaries, or other features that are not directly on the main survey line.

There are two main types of offsets: perpendicular offsets and oblique offsets. Perpendicular offsets are measured at 90 degrees from the survey line, while oblique offsets are measured at any angle other than 90 degrees. These help in drawing accurate maps or plans of the area being surveyed.

Detailed Explanation:

Offset and its types

In chain surveying, an offset is a short lateral measurement taken from the main survey line (usually the baseline) to reach important features or details that need to be plotted. These features may include boundaries, walls, trees, poles, buildings, or any other objects that are part of the area being surveyed but do not lie directly on the survey line.

Offsets are necessary because not all features in the field fall directly on the survey line. By measuring the exact distance from the line to the object, surveyors can correctly represent the position of every detail on a map or drawing. The use of offsets makes the survey more complete, accurate, and easy to understand.

There are two main types of offsets:

1. Perpendicular Offsets: These are measured at a right angle (90°) from the main survey line. They are preferred because they give accurate and easy-to-plot positions. Instruments like an optical square, cross staff, or set square are used to ensure the offset is truly perpendicular. These are the most commonly used type in chain surveying because they provide direct and simple measurements.
2. Oblique Offsets: These are measured at an angle that is not 90 degrees to the main survey line. They are used when it is difficult to set a right angle due to obstacles or uneven ground. In such cases, surveyors measure the distance along the main line and the angle or slant distance to the object. The object’s position is then calculated using trigonometry. This method is slightly more complex and less accurate than perpendicular offsets but useful in difficult conditions.

Sometimes, long offsets (more than 15 meters) and short offsets (less than 15 meters) are also mentioned based on distance, but the key classification is based on the angle.

Offsets are very important in chain surveying because they help capture all necessary details of the field. Without offsets, many objects would be missed in the final map. These measurements allow a complete picture of the land area to be drawn, especially when many features are away from the main survey line.

Conclusion:

An offset is a side measurement from the main survey line to objects or features around it. It plays a key role in accurately showing the location of these features on maps. Perpendicular offsets are easier and more accurate, while oblique offsets are used when right angles are not possible.