Procedures
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Two stations are set up on the opposite side of each other of the measured area.
- A survey line which joins the two stations together which was set up by 30m tape and the reading was recorded.
- The survey portion was outlined on a sketch paper.
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Regular interval between the stations is determined, which was depended on the length of the survey line.
- A straight offset line which joins the station and chainage point was measured by 5m tape and the reading was recorded. The ranging rod can be used to fix the 5m tape.
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By repeating Step 5 with different offset lines on the survey portion, data on the irregular shaped area were recorded.
Anticipated Errors & precaution
Anticipated errors
(1) ‘Sagging’ of tape
When the tape is not held taut in measurement e.g. over uneven ground, sagging of the tape occurs.
(2) Variations in length of tape
This occurs particularly in the first 7m lengths as this portion is used frequently. ‘Wetting’ or temperature changes may also result in considerable variations in length.
(3) Error in measurement due to incorrect tape length
Due to using tape over rough ground or under/over stretching of the tape, lengths obtained by taping with a faulty tape may either be too long or too short than the length, which is obtained with a standard tape.
(4) Error in offset length
Errors will arise in taking offsets if there is any appreciable deviation from a right angle. This will increase the true length of offset and also displace the true position of the offset point.
(5) Random errors
Errors will arise when taking the tape figures because it is hard to ensure the reading which approaches 0.1mm.
Precautions
- In planning the main survey lines, check lines are introduced to prove the accuracy of the work; also the subsidiary lines run between known points on the main survey lines.
- By judging with eye the right angle formed between two tapes. This method is inaccurate and error will increase with length
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The zero end of tape is placed at ground level on the point of object and gently swings the taut offset tape side by side along the chain line to obtain the shortest measurement.
- Use optical square to set out right angle in order to get perpendicular direction.
- Generally offset lengths should be kept as short as possible. For a few isolated offsets to unimportant or badly defined features such as to trees or the edge of a pond, longer offsets may be allowed if this will avoid the necessity of an extra survey line.
Difficulties
- It is difficult to obtain the shortest distance, as the offset line should perpendicular to the chain line.
- The ground is not very flat and smooth, hence when placing the tape to measure the distance between two points, it is difficult to hold the tape flatly. Then the measurement will be either too long or too short.
Data collected and calculations
Then, the area of irregular shape at the site=
(3.41+3.605+4.185+5.125+5.975+6.505+6.815+6.935+6.875+6.63+6.19
+5.545+4.995+4.8+4.89) = 82.48㎡
Conclusion
In short, the accuracy of method of using offset line is mostly depending on the data we measured. Therefore, the accuracy may be lower due to different errors including gross, systematic and random error.
In conclusion, the irregular area was calculated to be 82.48㎡ by offset line method.
