Tapes are available in a variety of materials, lengths and weights, and the choice matters more than it first appears. A tape that stretches under pull, shrinks in damp weather or expands in the sun will quietly corrupt every measurement taken with it.
This post covers the four types, in ascending order of accuracy and cost.
1. Cloth or Linen Tape
| Property | Detail |
|---|---|
| Material | Closely woven linen or synthetic material |
| Finish | Varnished to resist moisture |
| Lengths available | 10 to 30 m |
| Widths | 12 to 15 mm |
Three Disadvantages
- It is affected by moisture and gets shrunk.
- Its length gets changed by stretching.
- It is likely to twist and does not remain straight in strong winds.
All three come from the same root cause — cloth is soft and flexible. That makes the tape light and convenient, but it also means the material itself changes shape in response to water, tension and wind.
Note that varnishing only resists moisture; it does not prevent shrinkage entirely, which is why moisture still appears in the disadvantage list.
2. Metallic Tape
A linen tape with brass or copper wires woven into it longitudinally to reduce stretching. The wires are not visible because it is varnished.
| Property | Detail |
|---|---|
| Lengths available | 20 to 30 m |
| Accuracy | An accurate measurement device |
| Common use | Measuring offsets |
As it is reinforced with brass or copper wires, all the defects of linen tapes are overcome.
The Clever Idea Here
A metallic tape is a linen tape with a skeleton. The wires run longitudinally — along the length, which is precisely the direction in which stretching would cause error.
Metal barely stretches under the pull a surveyor applies, so the wires hold the length constant while the cloth around them keeps the tape light and flexible enough to roll up. You get the convenience of cloth with the dimensional stability of metal.
The note that the wires are not visible because it is varnished matters in practice — you cannot tell a metallic tape from a plain linen one by looking at it, which is why they must be labelled.
3. Steel Tape
| Property | Detail |
|---|---|
| Material | Steel or stainless steel strips |
| Accuracy | More accurate than metallic tapes |
| Lengths available | 1 to 50 m |
| Widths | 6 to 10 mm |
| End fitting | Brass ring, whose outer end is the zero point of the tape |
Steel tapes cannot be used in ground with vegetation and weeds.
Two Practical Points
The zero point is the outer end of the brass ring, not the start of the graduations and not the ring’s centre. Measuring from the wrong point puts a constant error into every reading — a systematic error, which never averages out.
The vegetation restriction is worth understanding. A steel tape is a thin, flexible strip that must be pulled taut in a straight line. Grass, weeds and undergrowth catch it, hold it off line and put unknown bends into it. The tape then measures a longer, wavy path rather than the straight distance — and there is no way to know by how much.
4. Invar Tape
Made of an alloy of nickel (36 %) and steel (64 %), with a very low coefficient of thermal expansion of 0.122 × 10−6 per °C.
| Property | Detail |
|---|---|
| Composition | 36 % nickel, 64 % steel |
| Coefficient of thermal expansion | 0.122 × 10−6 / °C |
| Lengths available | 30, 50 and 100 m |
| Width | 6 mm |
Advantages
- Highly precise.
- Less affected by temperature changes in comparison to other tapes.
Disadvantages
- It is soft, hence deforms easily.
- It requires much attention in handling, hence not used for ordinary works.
Why Invar Exists
The name comes from “invariable”, and the whole point of the alloy is that its length barely changes with temperature.
Consider what that is worth. A steel tape left in the sun heats up and grows longer, so every distance measured with it reads short. The temperature correction can allow for this, but only if you know the tape’s temperature accurately — and on a hot day a tape lying on bare ground may be far hotter than the air around it.
An invar tape sidesteps the problem. Its expansion coefficient is roughly one thirtieth that of steel, so a temperature error that would matter with steel becomes negligible with invar.
The Price Paid
The disadvantages are unusually blunt, and they are honest. The same nickel-steel alloy that resists temperature is soft. Bend it, kink it or tread on it and it deforms — and a deformed precision tape is worse than no precision tape, because it looks correct while reading wrong.
Hence the conclusion that it is not used for ordinary works. Invar is reserved for baseline measurement and other high-precision tasks where the care it demands is justified.
All Four Compared
| Cloth / Linen | Metallic | Steel | Invar | |
|---|---|---|---|---|
| Material | Woven linen or synthetic, varnished | Linen + brass or copper wires | Steel or stainless steel | 36 % nickel, 64 % steel |
| Lengths | 10–30 m | 20–30 m | 1–50 m | 30, 50, 100 m |
| Width | 12–15 mm | — | 6–10 mm | 6 mm |
| Accuracy | Lowest | Accurate | More accurate than metallic | Highly precise |
| Typical use | Rough work | Offsets | General measurement | Precise work only |
| Main weakness | Shrinks, stretches, twists | — | Unusable in vegetation | Soft, deforms easily |
The Pattern
Read the accuracy row from left to right and it climbs steadily — linen, metallic, steel, invar. Read the width row and it falls — 12–15 mm down to 6 mm.
That is not a coincidence. Narrower tapes are made of stiffer, more stable material, so they need less width to hold their shape. The trend is a useful memory aid: the more accurate the tape, the narrower it gets.
The Two Constants You Will Need Later
Two material properties from this topic reappear in the tape correction formulas:
| Property | Steel | Invar |
|---|---|---|
| Coefficient of thermal expansion α | Used in temperature correction | 0.122 × 10−6 / °C, also written 1.22 × 10−7 / °C |
| Modulus of elasticity E | 2.1 × 105 N/mm2 | 1.54 × 105 N/mm2 |
Note that 0.122 × 10−6 and 1.22 × 10−7 are the same number written two ways. Both forms appear in textbooks, and seeing them side by side prevents confusion later.
Quick Revision Notes
- Linen tape: woven linen or synthetic, varnished to resist moisture, 10–30 m, width 12–15 mm.
- Linen disadvantages: shrinks with moisture, stretches, twists in strong winds.
- Metallic tape: linen with brass or copper wires woven longitudinally, 20–30 m, used mainly for offsets; overcomes all defects of linen tape.
- Steel tape: steel or stainless steel, 1–50 m, width 6–10 mm, brass ring outer end is the zero point, cannot be used in vegetation and weeds.
- Invar tape: 36 % nickel and 64 % steel, α = 0.122 × 10−6/°C, lengths 30, 50 and 100 m, width 6 mm.
- Invar advantages: highly precise, less affected by temperature.
- Invar disadvantages: soft and deforms easily, needs much attention, not used for ordinary work.
- Accuracy order: linen < metallic < steel < invar.
Mistakes Students Commonly Make
- Getting the invar composition backwards. It is 36 % nickel and 64 % steel.
- Thinking the wires in a metallic tape are visible. They are hidden by varnish.
- Saying the wires run across the tape. They run longitudinally, because that is the direction stretching occurs.
- Taking the centre or inner edge of the brass ring as the tape zero. The outer end of the ring is the zero point.
- Assuming invar is the best tape for all work. It is soft and easily deformed, so it is not used for ordinary works.
- Confusing the two forms of the invar expansion coefficient. 0.122 × 10−6 = 1.22 × 10−7.
- Forgetting that a steel tape is unusable in vegetation and weeds.
Conclusion
Four tapes, arranged in a clear order of accuracy and cost. Linen is cheap, light and unreliable — it shrinks, stretches and twists. Metallic solves all three problems by weaving metal wires into the cloth. Steel is more accurate still but refuses to work in vegetation. And invar, with a thermal expansion coefficient about thirty times smaller than steel’s, gives the highest precision available — at the price of being soft enough to ruin if mishandled, which is why it stays reserved for work that genuinely needs it.
Frequently Asked Questions
What are the four types of tape used in surveying?
Cloth or linen tape, metallic tape, steel tape and invar tape.
What are the disadvantages of a linen tape?
It is affected by moisture and shrinks, its length changes by stretching, and it is likely to twist and does not remain straight in strong winds.
What is a metallic tape?
A linen tape with brass or copper wires woven into it longitudinally to reduce stretching. The wires are not visible because the tape is varnished.
Why does a metallic tape overcome the defects of linen?
Because the metal wires run along the length of the tape, in the direction where stretching would occur, and metal barely stretches under normal pull.
What is the zero point of a steel tape?
The outer end of the brass ring attached at the end of the tape.
Why can steel tapes not be used in vegetation?
Because grass and weeds catch the thin flexible strip, hold it off line and introduce bends, so the tape measures a longer wavy path instead of the straight distance.
What is invar tape made of?
An alloy of 36 percent nickel and 64 percent steel, with a very low coefficient of thermal expansion of 0.122 × 10−6 per degree Celsius.
What are the advantages and disadvantages of invar tape?
It is highly precise and much less affected by temperature changes than other tapes. However, it is soft and deforms easily, and it requires much attention in handling, so it is not used for ordinary work.
What lengths and width is invar tape available in?
Lengths of 30, 50 and 100 m, with a width of 6 mm.