A tape on its own cannot survey anything. You need something to mark where the line starts, something to mark where each tape length ends, something to keep the line straight, and something to reach into hedges. Those four needs give the four accessories in this topic.
1. Pegs
Pegs are used to mark definite points on the ground, either temporarily or semi-permanently.
The exact point from or to which measurements are to be taken, or over which an instrument is to be set, is often marked on the peg using nails.
Why a Nail Is Needed
This detail is easy to skip past, but it matters.
A peg is a piece of timber several centimetres across. Its top is a surface, not a point — so which part of it is “the station”? Set an instrument over one corner today and the other corner tomorrow, and you have introduced an error of several centimetres before measuring anything.
The nail solves this. It reduces the peg’s broad top to a single unambiguous point that anyone can identify exactly. Precision in surveying always comes back to defining points sharply, and this is the simplest example of it.
2. Arrows
When measuring the length of a long line, the tape has to be laid down a number of times, and the positions of the ends are marked with arrows.
The Counting Check
As arrows are placed in the ground after every chain length, the number of arrows held by the follower indicates the number of chains that have been measured. It provides a check over the length of the line as entered in the field notes.
This is a genuinely elegant piece of field practice, and worth following carefully.
Two people measure a long line. The leader walks ahead pulling the tape, and at the end of each tape length pushes an arrow into the ground. The follower comes up behind, stands at that arrow to start the next length, and collects the arrow as they move on.
So at any moment, the number of arrows in the follower’s hand equals the number of complete tape lengths measured. Nobody has to remember a count, and nobody has to trust the field book.
Recall from the errors topic that miscounting of chain lengths is one of the listed sources of error in chaining. The arrows exist precisely to prevent it — the physical count in your hand cannot be forgotten or misremembered.
3. Ranging Rods
Ranging rods are used to locate intermediate points such that these points lie on the straight line joining the end stations.
Specifications
| Property | Value |
|---|---|
| Diameter | 30 mm |
| Length | 2 or 3 m |
| Painting | Alternate bands of red and white, or black and white |
| Band length | 200 mm |
Why the Bands Are 200 mm
The bands are not merely for visibility. Because each band is exactly 200 mm, the rod itself can be used for the rough measurement of short lengths — count the bands and multiply by 200 mm.
So the painting serves two purposes at once: making the rod conspicuous against any background, and turning it into an approximate measuring stick. A 2 m rod carries ten bands; a 3 m rod carries fifteen.
When Ranging Is Compulsory
Ranging is a must before starting the measurement of a line whose length is more than a chain length, so that the measurement is made along a straight line.
The reason is straightforward. If a line is longer than one tape length, you must lay the tape down several times — and unless the intermediate points are genuinely on the straight line, you end up measuring a zigzag. A zigzag is always longer than the straight distance, so the error is systematic and always in the same direction.
This connects directly to the correction for misalignment in the tape corrections topic, which is negative for exactly this reason.
4. Offset Rods
Offset rods are similar to ranging rods except at the top, where a stout open ring recessed hook is provided.
The Two Extra Features
| Feature | Purpose |
|---|---|
| Hook at the top | The chain can be pulled or pushed through hedges or other obstructions |
| Two short narrow vertical slots, at right angles to each other, passing through the centre of the section at about eye level | Used to align the offset line |
The offset rod is mainly used to align the offset line and measure short offsets.
Why the Slots Are at Right Angles
This is the detail that makes the instrument work, and it repays a moment’s thought.
An offset is a short measurement taken sideways from the survey line to a feature — and it is meant to be perpendicular to that line. If it is not perpendicular, the position of the feature is plotted wrongly.
The offset rod lets you check this by eye. Stand the rod at the point, look through one slot along the survey line, and the second slot then points at right angles to it — straight down the offset direction. Because the two slots are built at exactly 90 degrees, the rod is effectively a simple right-angle sighting device.
Placing them at about eye level is equally deliberate — you can use them standing upright, without stooping or lifting the rod.
All Four Accessories Compared
| Accessory | Purpose | Key Specification |
|---|---|---|
| Pegs | Mark definite points temporarily or semi-permanently | Exact point marked with a nail |
| Arrows | Mark tape-end positions; the count held by the follower checks the number of chains | Placed after every chain length |
| Ranging rods | Locate intermediate points on the straight line between end stations | 30 mm dia, 2 or 3 m long, 200 mm bands |
| Offset rods | Align the offset line and measure short offsets | Hook at top, two slots at right angles at eye level |
How They Divide Up
- Pegs and arrows answer the question where is this point? — they mark positions.
- Ranging rods and offset rods answer the question which direction? — ranging rods keep the survey line straight, offset rods keep the offset perpendicular to it.
Quick Revision Notes
- Pegs mark definite points temporarily or semi-permanently; the exact point is marked with a nail.
- Arrows mark the tape ends; the number held by the follower gives the number of chains measured, providing a check on the field notes.
- Ranging rods: 30 mm diameter, 2 or 3 m long, painted with alternate bands of red and white or black and white, each band 200 mm.
- The 200 mm bands allow the rod to be used for rough measurement of short lengths.
- Ranging is a must for any line longer than one chain length.
- Offset rods are like ranging rods but have a stout open ring recessed hook at the top and two narrow vertical slots at right angles at about eye level.
- The hook lets the chain be pulled or pushed through hedges or obstructions.
- Offset rods are used to align the offset line and measure short offsets.
Mistakes Students Commonly Make
- Saying the leader holds the arrows that give the count. It is the follower, who collects them.
- Forgetting that the ranging rod bands are 200 mm and can be used for rough measurement.
- Giving the ranging rod diameter as 20 or 40 mm. It is 30 mm.
- Saying the offset rod slots are parallel. They are at right angles to each other, which is what makes perpendicular sighting possible.
- Confusing pegs and arrows. Pegs mark stations; arrows mark tape ends.
- Overlooking why a nail is used on a peg — to reduce a broad surface to a single definite point.
- Assuming ranging is optional. It is compulsory whenever the line exceeds one chain length.
Conclusion
Four accessories, each solving one problem. Pegs fix stations, with a nail reducing the peg top to a definite point. Arrows mark tape ends and — through the simple device of the follower collecting them — give a physical count that cannot be misremembered. Ranging rods hold the survey line straight, with 200 mm bands that double as a rough scale. And offset rods, with their hook and their pair of slots set at right angles, keep offsets perpendicular and reach the chain through hedges.
Frequently Asked Questions
What are pegs used for?
To mark definite points on the ground, either temporarily or semi-permanently. The exact point from or to which measurements are taken, or over which an instrument is set, is marked on the peg using nails.
Why is a nail used on a peg?
Because the top of a peg is a broad surface rather than a point. The nail reduces it to a single unambiguous position that anyone can identify exactly.
What is the purpose of arrows?
They mark the positions of the tape ends when a long line is measured in several tape lengths.
How do arrows provide a check on measurement?
Because an arrow is placed after every chain length and collected by the follower, the number of arrows held by the follower equals the number of chains measured. This checks the length recorded in the field notes.
What are the dimensions of a ranging rod?
30 mm in diameter and 2 or 3 m long, painted with alternate bands of red and white or black and white, each band 200 mm in length.
Why are ranging rod bands 200 mm long?
So that the rod can also be used for the rough measurement of short lengths, by counting bands.
When is ranging compulsory?
Before starting the measurement of any line whose length is more than one chain length, so that measurement is made along a straight line.
How does an offset rod differ from a ranging rod?
It has a stout open ring recessed hook at the top, and two short narrow vertical slots at right angles to each other passing through the centre of the section at about eye level.
What are the offset rod slots used for?
To align the offset line. Sighting through one slot along the survey line makes the other slot point at right angles to it, giving the perpendicular direction for the offset.
What is the hook on an offset rod for?
It allows the chain to be pulled or pushed through hedges or other obstructions.