A tape is 30 metres long. The line you need to measure is 300 metres. So you lay the tape down ten times — and unless every one of those ten positions lies exactly on the straight line between the two ends, you have measured a zigzag instead of a distance.
Ranging is the process of preventing that. This post covers why it is needed and the three methods of doing it.
Why Ranging Is Needed
For the measurement of the length of a survey line, the chain or tape is stretched along the line joining the end stations.
| Situation | What Happens |
|---|---|
| Distance less than the chain length | The chain can be stretched directly, without establishing intermediate points |
| Distance greater than the chain length | It becomes necessary to establish intermediate points in line with the end stations, before any measurement begins |
This ensures that the straight distance is measured between the end stations.
The Definition
The process of establishing intermediate points on a straight line is known as Ranging or Ranging out.
An equivalent way of putting it: the process of locating a number of points on a long survey line.
Why the Error Is Always One-Sided
This is worth understanding rather than accepting.
If the intermediate points wander off the true line, the tape follows a slightly bent path. And a bent path between two fixed points is always longer than the straight one — never shorter. It is geometrically impossible for a zigzag to come out short.
So poor ranging produces an error that is always positive and always in the same direction. It is a cumulative or systematic error, which means it does not cancel out however many times you measure. That is exactly why ranging must be done before measurement, not corrected afterwards.
The Three Methods of Ranging
- Direct ranging
- Indirect ranging
- Random line method
1. Direct Ranging
When ranging rods are placed on intermediate points along the chain line by direct observation from either of the end stations, the process is known as direct ranging.
Let A and B be the two end stations, and c, d, e, f and so on be the intermediate points to be established.
How It Works
The key phrase is direct observation. A surveyor stands at station A and looks towards the ranging rod at station B. An assistant then moves a rod left or right at each intermediate position until, from A’s viewpoint, that rod exactly hides behind the one at B.
When three rods appear as one, they are in a straight line — because the eye is following a genuine straight ray of light.
The Condition
Direct ranging requires one thing above all: the two end stations must be intervisible. You have to be able to stand at A and see B. If you cannot, this method is unavailable, and the next one is needed.
2. Indirect Ranging (Repeated Alignment)
When the end stations are not intervisible — due to rising ground between them, or due to the long distance between the ends — then indirect ranging is done, to lay out a straight line between inaccessible points or points across a mountain.
Note the alternative name, repeated alignment, which describes the technique better than “indirect” does.
The Two Causes of Non-Intervisibility
| Cause | Why It Blocks the View |
|---|---|
| Rising ground between the stations | A hill physically obstructs the line of sight |
| Long distance between the ends | Even with no obstruction, a distant rod becomes too small to see, and the curvature of the earth eventually intervenes |
The Principle Behind Repeated Alignment
The situation looks impossible at first. Neither end can see the other, so how can anything be aligned between them?
The answer is to work with two intermediate points that can each see one end and each other. Two assistants stand between A and B, each visible to the other. The first aligns themselves with station A and the second assistant; then the second aligns with station B and the first assistant. Each correction improves the other’s position.
Repeating this back and forth — hence repeated alignment — brings both points progressively onto the true line AB, until neither needs further adjustment. It is a successive approximation, and it converges.
3. Random Line Method
The third method listed. It is used where obstacles make it impossible to run the required line directly, so a convenient arbitrary line is run instead and the true line is then established by calculation from it.
The Three Methods Compared
| Method | When Used | Requirement |
|---|---|---|
| Direct ranging | Normal conditions | End stations must be intervisible |
| Indirect ranging | Stations not intervisible due to rising ground or long distance | Two intermediate points visible to each other and to one end each |
| Random line method | Where the direct line cannot be run at all | A convenient auxiliary line |
How This Connects to Errors and Corrections
Ranging appears twice more in this chapter, and it is worth linking the three mentions together.
- In errors in chaining, “bad ranging of intermediate points” is listed as a source of error.
- In tape corrections, the correction for misalignment exists specifically because “if the survey line is not accurately ranged out, the error due to misalignment occurs”.
- That correction is stated to be always negative, because “the measured distance will always be greater than the correct distance”.
All three statements are the same fact viewed from different angles. Ranging done well prevents the error; ranging done badly requires a correction that is always in the same direction.
Quick Revision Notes
- Ranging is needed only when the line is longer than one chain length.
- Ranging or ranging out = the process of establishing intermediate points on a straight line.
- It ensures the straight distance is measured between end stations.
- Three methods: direct ranging, indirect ranging, random line method.
- Direct ranging: ranging rods placed on intermediate points by direct observation from either end station; requires the ends to be intervisible.
- Indirect ranging, also called repeated alignment, is used when ends are not intervisible due to rising ground or long distance.
- Indirect ranging is used to lay out a straight line between inaccessible points or points across a mountain.
- Bad ranging produces a systematic positive error, because a bent path is always longer than a straight one.
Mistakes Students Commonly Make
- Saying ranging is needed for every line. It is needed only when the line exceeds one chain length.
- Listing only two methods. There are three, including the random line method.
- Forgetting the alternative name for indirect ranging — repeated alignment.
- Giving only rising ground as the reason for non-intervisibility. Long distance is also stated.
- Thinking poor ranging can make a measurement come out short. It always makes it longer.
- Doing ranging after measurement. It must be completed before measurement begins.
Conclusion
Ranging exists to answer a simple problem — a tape shorter than the line means several tape lengths, and several tape lengths mean several chances to drift off the straight. Direct ranging solves this by eye whenever the two ends can see one another. When they cannot, because of a hill or sheer distance, repeated alignment works two intermediate points back and forth until both settle onto the true line. And because a bent path is always longer than a straight one, poor ranging produces an error that never cancels out — which is why the work is done before any measurement is taken.
Frequently Asked Questions
What is ranging in surveying?
The process of establishing intermediate points on a straight line between the end stations, so that the straight distance is measured rather than a zigzag.
When is ranging necessary?
When the distance between the end stations is greater than the chain or tape length. If the distance is less than one chain length, the chain can be stretched directly without intermediate points.
What are the three methods of ranging?
Direct ranging, indirect ranging, and the random line method.
What is direct ranging?
Placing ranging rods on intermediate points along the chain line by direct observation from either of the end stations.
What condition must be satisfied for direct ranging?
The two end stations must be intervisible, so that a surveyor at one end can see the rod at the other.
What is indirect ranging?
Ranging carried out when the end stations are not intervisible, due to rising ground between them or the long distance between the ends. It is also called repeated alignment.
Why is indirect ranging called repeated alignment?
Because two intermediate points are aligned alternately, each with reference to one end station and the other intermediate point, and the process is repeated until both settle onto the true line.
Why does bad ranging always increase the measured length?
Because a bent or zigzag path between two fixed points is always longer than the straight line joining them, so the error is systematic and always positive.
How does ranging relate to the correction for misalignment?
The correction for misalignment exists precisely because a line that is not accurately ranged out is measured as longer than it truly is. The correction is therefore always negative.