The error due to bad ranging is:
Cumulative positive
Understanding Errors in Ranging
In the context of surveying, ranging is a fundamental process used to align intermediate points along a straight line between two main stations. This is especially important when measuring distances that are longer than the measuring tape or chain being used, requiring multiple segments.
Bad ranging occurs when these intermediate points are not placed precisely on the true straight line connecting the two terminal points. This misalignment introduces errors into the measurement of the total distance.
Types of Surveying Errors
Surveying measurements are subject to various types of errors. Understanding these types helps in identifying their causes and effects on the final result. The two main categories of errors are:
- Compensating Errors: These are errors that are random in nature. They are equally likely to be positive or negative and tend to cancel each other out over a large number of observations. Think of small, accidental mistakes like slightly misreading a scale.
- Cumulative Errors: These errors consistently occur in the same direction (either always positive or always negative) and accumulate over the series of measurements. Their effect increases with the number of measurements taken or the total distance measured.
Why Bad Ranging Causes Cumulative Positive Error
When ranging is done improperly, and intermediate points are not perfectly aligned on the straight line, the actual path measured between the start and end points becomes a series of short lines connecting the misaligned intermediate points. Geometrically, the sum of the lengths of two sides of a triangle is always greater than the length of the third side.
If intermediate point B is supposed to be on the straight line AC, but is placed slightly off the line (say, at B'), the measurement goes from A to B' and then from B' to C. The sum of distances AB' + B'C will be greater than the true straight-line distance AC. This happens regardless of which side of the line B' is placed on (as long as it's not on the line itself). Similarly, if the tape sags between points, the curved path is longer than the straight horizontal distance.
Since each segment measured with bad ranging technique will likely be slightly longer than the corresponding segment on the true straight line, this small positive error occurs repeatedly for every tape length measured. These small positive errors add up over the entire length of the line being measured. This consistent addition of positive error is what makes it a cumulative positive error.
In essence, bad ranging makes the measured distance greater than the true distance, and this difference accumulates along the length of the line.
Analyzing the Options
- Compensation: This is incorrect because the error from bad ranging does not randomly cancel out; it systematically increases the measured distance.
- Cumulative positive: As explained, bad ranging leads to measuring a longer path than the true straight line, resulting in positive errors that accumulate. This option is consistent with the effect of bad ranging.
- Cumulative negative: This would imply the measured distance is consistently shorter than the true distance. Bad ranging typically causes the measured distance to be longer, not shorter.
- b) and c): Since bad ranging results in a cumulative positive error, it cannot simultaneously result in a cumulative negative error.
Revision Table: Common Surveying Errors
| Error Type | Direction of Error | Accumulation | Impact on Measurement | Example |
|---|---|---|---|---|
| Compensating | Random (Positive or Negative) | Tends to cancel out over many trials | Reduced total error with more measurements | Minor reading errors, accidental tape misplacement |
| Cumulative Positive | Always Positive | Adds up over distance | Measured distance is too long | Bad ranging, tape too long, temperature expansion of tape |
| Cumulative Negative | Always Negative | Adds up over distance | Measured distance is too short | Tape too short, neglecting slope correction (when horizontal needed) |
Additional Information on Bad Ranging
Bad ranging is often considered a type of systematic error because it results from a consistent mistake in procedure (not placing points correctly on the line). Systematic errors are one cause of cumulative errors. The error introduced by bad ranging is proportional to the length being measured; over a longer line, the effect of small misalignments in each segment adds up significantly.
Proper ranging techniques, involving careful alignment using ranging poles and sighting, are essential to minimize this type of cumulative positive error and achieve accurate distance measurements in surveying.
Important Questions from Fundamental Concepts
Mean Sea Level for India is :
Which of the following quantities are each equal to one acre?
A. 43560 sq ft
B. 40 gunthas
C. 10 sq gunter's chain
D. 4850 sq yds
Choose the correct answer below.
The capacity of a telescope of producing a sharp image is called it
1 Nautical mile equals to how many metres?
In surveying optical square is used to setting out right angles. The horizon glass is placed at an angle of ______ with the horizon sight and index glass is placed at an angle of ______ with the index sight.
