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Surveying is the science of determining the relative positions of points on or near the Earth's surface. It forms the backbone of civil engineering projects, enabling engineers to design and construct infrastructure with precision. Among various surveying methods, chain surveying is the simplest and most fundamental technique used primarily for small, flat areas.
Chain surveying involves measuring linear distances using a chain or tape and recording the relative positions of points by measuring offsets. It is widely used in India and worldwide for tasks such as plotting land boundaries, preparing site plans, and preliminary layouts for roads and buildings.
In this chapter, we will explore chain surveying from first principles, focusing on metric measurements (meters and centimeters), which are standard in India and internationally. Understanding chain surveying lays the foundation for more advanced methods like compass or theodolite surveying.
Chain surveying is based on a few fundamental principles that ensure accurate and reliable measurements:
By combining these measurements, the positions of all points can be plotted accurately on a map.
Why use offsets? Measuring every point directly along the chain line would be inefficient and sometimes impossible due to obstacles. Offsets allow surveyors to measure perpendicular distances from the main line to locate points accurately without complex angle measurements.
Chain surveying requires simple but precise equipment to ensure accuracy. The main items include:
Proper care of equipment is crucial for accuracy:
Chain surveying follows a systematic process to ensure accurate data collection:
graph TD A[Reconnaissance and Planning] --> B[Marking Stations and Ranging] B --> C[Measuring Distances (Chaining)] C --> D[Measuring Offsets] D --> E[Recording Field Notes] E --> F[Plotting the Survey]
Before starting, surveyors inspect the site to identify boundaries, obstacles, and suitable locations for stations. Planning helps decide the main survey lines and the sequence of measurements.
Stations are marked using pegs at key points like corners or changes in direction. Ranging rods are placed at these stations, and the chain is aligned straight between them using visual sighting along the rods. This process is called ranging.
The chain is stretched tightly between stations, and the distance is measured by counting the number of full chains and any partial lengths. Arrows are placed at chain-length intervals to avoid counting errors.
Points away from the main survey line are located by measuring perpendicular offsets using a tape or chain. Offsets are always taken at right angles to the main line to maintain accuracy.
All measurements are recorded immediately in a field book with clear labels, including distances, offsets, and station names. Accurate record-keeping prevents data loss and errors during plotting.
After fieldwork, the survey data is plotted on graph paper or digitally to create a scaled map of the surveyed area.
Detecting Errors: One common check is the closing error, which occurs when the survey does not close perfectly at the starting point. It is calculated by comparing the measured coordinates of the final point with the initial point.
Applying Corrections: The closing error is distributed proportionally along the survey lines to adjust the measurements before final plotting. This ensures the map is as accurate as possible.
Step 1: Mark the four corners of the rectangle with pegs (stations A, B, C, D).
Step 2: Measure the length AB (40 m) using the 20 m chain. Count 2 full chains (2 x 20 m = 40 m) and place arrows at 20 m intervals to avoid counting errors.
Step 3: Similarly, measure BC (30 m). Since 30 m is 1 full chain plus 10 m, measure one full chain and then a partial length of 10 m.
Step 4: Repeat for CD and DA sides.
Step 5: Use ranging rods to ensure the chain is straight and aligned along each side.
Answer: The plot is measured by chaining along the sides with proper ranging and marking stations at corners.
Step 1: Calculate total perimeter length \( P \):
\( P = 50 + 40 + 60 + 55 = 205 \, m \)
Step 2: Use the correction formula:
\( C = \frac{E}{P} \times L_i = \frac{3}{205} \times 50 = 0.7317 \, m \)
Step 3: Since the figure is too large by 3 m, subtract correction from the 50 m chain length:
Corrected length = \( 50 - 0.7317 = 49.2683 \, m \)
Answer: Apply a correction of approximately 0.73 m to the 50 m chain length.
Step 1: Identify points around the obstacle where chaining is possible.
Step 2: Measure the distance from the starting station to a point before the obstacle.
Step 3: Measure a perpendicular offset from the chain line to the obstacle's edge.
Step 4: Chain around the obstacle by measuring the sides of the detour triangle formed.
Step 5: Use offsets to calculate the actual distance along the original chain line using Pythagoras theorem.
Answer: Offsets allow accurate measurement around obstacles without losing alignment.
Step 1: Convert distances to scale:
Step 2: Draw the main survey lines AB, BC, CD, and DA sequentially using a ruler and protractor if needed.
Step 3: At points B, C, and D, draw perpendicular lines representing offsets at the scaled lengths.
Step 4: Mark the offset points on these perpendiculars.
Answer: The plotted figure represents the surveyed area accurately to scale.
Step 1: Calculate number of 100 m units:
\( \frac{450}{100} = 4.5 \) units
Step 2: Multiply by rate:
\( 4.5 \times 500 = 2250 \, \text{INR} \)
Answer: The estimated cost of surveying is INR 2250.
When to use: During initial setup of survey lines to avoid cumulative errors.
When to use: While measuring long distances to maintain accuracy.
When to use: During data collection in the field.
When to use: To maintain accuracy and reduce measurement errors.
When to use: Before final plotting to ensure accuracy of the survey map.
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