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True Thickness Calculator for Drill Intercepts and Strata

Convert a measured apparent interval to idealized plane-normal true thickness using either a known acute line-to-plane angle or complete right-hand-rule plane and measurement-line orientations. True thickness is the shortest distance between two parallel bounding surfaces. A drill core, trench, map width, section, or other oblique measurement records apparent thickness along its own direction. When the acute angle between that line and the plane is known, true thickness equals apparent interval multiplied by the sine of the angle. A 90-degree perpendicular intersection has a correction factor of one; a shallow intersection has a much larger apparent-to-true ratio and greater sensitivity to orientation error. GeoMiner can derive the acute angle in three dimensions from right-hand-rule strike, dip, line azimuth clockwise from north, and signed line inclination from horizontal, where negative is downward and positive is upward. Values remain in the browser. Analytics retain only bounded action, method, and broad angle-band groups—never interval length, unit, strike, dip, azimuth, inclination, angle, thickness, hole, project, location, grade, or notes. The calculation assumes two parallel planar contacts, a straight surveyed measurement line, locally constant orientation, and one coherent tabular unit. Folding, faulting, cleavage, irregular or gradational contacts, vein pinching and swelling, multiple lenses, internal waste, broken or lost core, selective sampling, drilling-induced damage, and downhole deviation can invalidate that model. A nearly parallel line produces an unstable correction: a finite observed intercept may instead reflect irregular, folded, faulted, discontinuous, or non-tabular geometry. If structure orientation or its relationship to the sample is unknown, do not invent a numerical correction. Keep the interval labelled apparent or downhole length and state that true thickness is unknown. Confirm strike convention, dip quadrant, grid versus true north, signed drill inclination, collar orientation, downhole survey source and depth, core orientation quality, contact picks, and whether the same plane orientation applies at both boundaries. Propagate orientation and contact uncertainty instead of reporting unjustified precision. A thickness correction does not establish recovery, representative sampling, grade continuity, geological continuity,

Verification resources

Cross-check terminology, classification, methods, and safety with these authoritative external resources.

  • undefined. Derives true stratum thickness from outcrop width, dip, and elevation relationships; for level ground and a traverse normal to strike, T = H sin(dip).
  • undefined. Documents right-hand-rule plane orientation and the relationship between true dip, apparent dip, strike, and measurement direction.
  • undefined. Distinguishes apparent interval measured along a diamond-drill core axis from corrected true stratigraphic thickness and records the core-axis-to-bedding angle.
  • undefined. Shows the idealized relationship t = drilled thickness × sin(core-to-foliation angle) and discusses error in core-angle and dip observations.
  • undefined. Requires mineral-project drilling disclosure to state the relationship between sample length and true thickness when known, or state that mineralization orientation is unknown.

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Common questions

What is true thickness in geology?
True thickness is the shortest distance between the bounding surfaces of a tabular unit, measured normal to those surfaces. Core, trench, map, and section intervals commonly record a longer apparent interval.
How do I calculate true thickness from a drill intercept?
Multiply apparent interval by the sine of the acute angle between the drill or core axis and the plane. A perpendicular 90-degree intersection needs no geometric correction.
What orientation conventions does this calculator use?
Plane strike uses right-hand-rule azimuth, dip is 0 to 90 degrees, line azimuth is clockwise from north, and signed inclination is negative downward and positive upward from horizontal.
Can I calculate true thickness when mineralization orientation is unknown?
No. Keep the interval labelled apparent or downhole length and state that the relationship to true thickness is unknown.
Does true-thickness correction make a drill result representative?
No. It corrects only idealized planar geometry and does not address recovery, sampling, grade distribution, irregular contacts, continuity, resources, reserves, or economics.

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