Chain and Tape
Part of Surveying
How to make and use linear measurement tools — from knotted ropes to metal chains — for accurate distance measurement.
Why This Matters
Measuring distance accurately is the foundation of all practical surveying. You cannot divide land fairly, lay out a straight road, or build a properly proportioned structure without reliable distance measurement. For millennia, the primary tool for this was not a sophisticated instrument but a simple measured length of rope, cord, or metal — something anyone could make and use.
The chain and tape tradition encompasses everything from a knotted string used by an Egyptian field surveyor to divide floodplain farmland, to the Gunter’s chain used by English surveyors to measure estates, to the steel tape that modern construction workers still carry. The principles have not changed: stretch something of known length between two points, count how many times it fits, record the remainder. The skill lies in doing this consistently, maintaining tension, following straight lines, and correcting for slope.
A community that can make reliable measuring lines and use them correctly can accomplish all the distance measurement needed for land division, construction layout, and map-making. Advanced instruments improve convenience and accuracy, but the chain and tape remain usable long after everything else has failed.
Types of Measuring Lines
Knotted cord: The simplest option. Take a length of cord and tie knots at regular intervals — every meter, every foot, or every local unit. This is quick to make and easy to read: count the knots. The major disadvantage is inconsistency — knots compress under tension, and different tensions give different effective lengths. Use knotted cord only for rough measurements where precision beyond a few percent is not needed.
Marked rope: Better than knotted cord. Stretch a rope under standard tension (about 5 kg is typical) and mark it at exact intervals using paint, burned marks, or tied threads. Always use the same tension when measuring — make a simple tension gauge from a spring or weighted lever. Natural-fiber rope stretches when wet, so wet it before marking and before measuring, or accept that wet and dry readings will differ by up to 2%.
Twisted wire measuring line: Twist multiple strands of wire together to make a flexible but dimensionally stable measuring line. Wire does not stretch like rope. Mark it at intervals with wrapped thread or bent tags. Protect wire from kinking — a kinked wire is permanently deformed and will read shorter than its true length.
Surveyor’s chain (Gunter’s chain): The traditional English surveyor’s chain consists of 100 links of metal wire, each link 20.1 cm long, totaling one chain (66 feet or about 20.1 m). Tally rings at every tenth link allow quick counting. To make a chain, form 100 identical oval links from wire, connect them end to end, and attach a handle at each end. The consistency of link length is critical — measure and adjust each link before connecting.
Linen or cotton tape: A flat woven tape marked in units. More flexible than a chain, packs into a small roll, but stretches more under tension. Acceptable for construction layout but less accurate than chain for land measurement.
Making a Reliable Measuring Chain
To construct a chain accurate enough for land surveying, you need wire of consistent diameter, a form to shape links uniformly, and a way to verify the final length.
Materials: Wire of about 2-3 mm diameter, strong enough not to stretch under field tension but workable with simple tools. Iron wire, copper wire, or bronze wire all work. Steel wire is best but hardest to work.
Link form: Cut or carve a wooden mold with an oval cavity of the correct link size. Each link should be 20 cm from inside edge to inside edge (adjust to whatever unit length you choose — 20 cm links give a 20-meter chain with 100 links, which is a convenient length). Wrap wire around the mold, cut the loops, and close each link into an oval.
Assembly: Connect links by threading the next link through the previous before closing. Check that each connection moves freely. Stiff connections introduce length errors when the chain is pulled around curves.
Calibration: Measure out a known distance — a baseline of known length — and check your chain against it. A chain that reads 100 links but only spans 19.5 meters needs adjustment: either add links, replace short links with slightly longer ones, or record the correction factor (1.025 in this case — multiply all chain readings by 1.025).
Handle rings: Attach large rings to each end as handles. The rings also serve as the zero and 100-link marks — the ring face, not the wire, is the measuring end.
Proper Measuring Technique
Accurate chaining requires consistent technique more than it requires precision equipment. These procedures apply whether you are using a rope, a tape, or a metal chain.
Two-person crew: One person (the “follower” or “rear chainman”) holds the rear end at the starting point. The other (“leader” or “head chainman”) carries the far end forward with a set of taping pins.
Taping pins: Short metal pins or pointed stakes, carried in a set of 10. The follower starts with all 10. At each chain length, the leader drives a pin at the end of the chain, and the follower picks up the pin from the previous setup. When the follower has recovered all 10 pins, one “full set” (10 chain lengths) has been measured. Keep count of sets.
Straight-line measurement:
- Before measuring, align the line using stakes driven at intervals along the survey line. Sight from one end to the other and drive intermediate stakes to maintain the line.
- The follower holds the zero end exactly on the starting mark.
- The leader moves forward, dragging the chain, and the follower calls “good” when the chain is straight and on line.
- The leader pulls the chain taut to standard tension and drives a pin at the end mark.
- Both walk forward to the new pin. Repeat.
Tension: Inconsistent tension is a major source of error. For a 20-meter chain or tape, use a consistent pull of about 5 kg (roughly the tension needed to hold a 5 kg weight). Make a tension handle — a wooden grip with a spring or weighted lever that indicates when the correct tension is applied.
Slope correction: If the ground slopes, you have two options:
- Step chaining: Keep the chain horizontal by lifting the downhill end off the ground. Use a plumb bob to project from the suspended end down to the ground. This is the simplest approach for moderate slopes.
- Slope measurement and correction: Measure the slope distance directly, then measure the angle of slope. Horizontal distance = slope distance × cosine(slope angle). For small angles, horizontal distance ≈ slope distance − (slope distance × slope angle² / 2) in radians.
For construction layout, step chaining is usually adequate. For land area calculation, slope correction is important because area is computed from horizontal distances.
Sources of Error and How to Avoid Them
| Error Source | Effect | Prevention |
|---|---|---|
| Wrong tension | Short or long readings | Use tension indicator; standard pull every time |
| Sagging chain | Reads too long | Support chain at intervals; keep off rough ground |
| Misalignment | Reads too long | Align intermediate stakes; use range poles |
| Kinks in chain | Reads too short | Straighten chain before each measurement |
| Incorrect temperature | Small errors in metal | Note temperature; apply correction if needed |
| Miscounting pins | Error by one chain length | Use tally system; double-check pin count |
Temperature correction for metal chains: Metal expands when warm and contracts when cold. For a 20-meter steel chain, a 10°C temperature change causes about 2 mm of change in length. This is small for most applications but matters for precise work. Correction = L × α × ΔT, where L is chain length, α is the coefficient of thermal expansion (about 0.000012 per degree C for steel), and ΔT is the temperature difference from calibration temperature.
Measuring Irregular Ground
When the ground is too rough for direct chaining, indirect methods extend your reach:
Offset method: If an obstacle blocks the direct measurement line, measure sideways (perpendicular) from your line to clear the obstacle, proceed parallel to the original line, then measure back. The total distance along the original line equals the parallel distance. Only works if you can maintain a true right angle for the offsets — use a cross-staff or your baseline-establishment right-angle techniques.
Triangle method: When a body of water or impassable ground prevents you from chaining a distance, set up a baseline perpendicular to the impassable distance and measure angles to a point on the far side. Calculate the distance using the tangent ratio or by constructing a scale drawing.
Mark Your Tape
Whatever measuring line you use, mark it clearly at major intervals and keep a written record of its length at calibration, the calibration temperature, and any corrections needed. A tape that has been stretched, kinked, or spliced should be re-calibrated before further use.