Discontinuity Spacing Calculator
Calculates the mean discontinuity (joint) spacing along a scanline from the measured length and the number of discontinuities counted, plus the true (normal) spacing, the discontinuity frequency, and the ISRM spacing description. It supports rock-mass characterisation for engineering geology, slope and tunnel work.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the scanline length L in metres and the number of discontinuities N counted along it.
- Optionally enter the angle θ (decimal degrees) between the scanline and the discontinuity-set normal for a true-spacing correction.
- Read the mean spacing, true spacing, frequency λ and the ISRM spacing class.
How it works
The mean apparent spacing along the scanline is simply L ÷ N, and the discontinuity frequency is its reciprocal λ = N ÷ L per metre. This apparent spacing is only the true spacing when the scanline runs perpendicular to the set (θ = 0°).
When the scanline is oblique to the set, the true spacing measured normal to the joint planes is (L/N) × cos θ, where θ is the angle between the scanline and the set normal. The mean spacing is then classified using the ISRM (1978) scale from very close to extremely wide.
Worked example
10 m scanline, 40 joints, θ = 60°. Mean spacing = 10 ÷ 40 = 0.25 m (ISRM 'Wide', 0.2–0.6 m). Frequency λ = 40 ÷ 10 = 4 per m. With the scanline 60° from the set normal, true spacing = 0.25 × cos 60° = 0.125 m.
Common mistakes
- Measuring the angle from the joint plane instead of the joint normal — θ is the angle to the set normal, so a scanline perpendicular to the set is θ = 0°.
- Reporting apparent scanline spacing as true spacing when the scanline crosses the set obliquely.
- Mixing units — keep the scanline length in metres so the spacing classes and frequency read correctly.
Frequently asked questions
How do I calculate mean joint spacing?
Divide the scanline length by the number of discontinuities intersected: mean spacing = L ÷ N. Its reciprocal is the discontinuity frequency λ = N ÷ L.
What is the difference between apparent and true spacing?
Apparent spacing is measured along the scanline. True spacing is measured perpendicular to the joint set and equals apparent spacing × cos θ, where θ is the angle between the scanline and the set normal.
What are the ISRM spacing classes?
Very close <0.02 m, Close 0.02–0.06 m, Moderate 0.06–0.2 m, Wide 0.2–0.6 m, Very wide 0.6–2 m, Extremely wide >2 m.
What is discontinuity frequency λ?
It is the number of discontinuities per metre along the scanline (λ = N ÷ L), the inverse of mean spacing, and it feeds parameters like RQD estimates.
Should I correct spacing for scanline orientation?
Yes, when the scanline is not perpendicular to the set. Apply the cos θ correction so the reported spacing reflects the true spacing normal to the joint planes.
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Tip: Enter any known values to calculate the remaining results.
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