What is true width versus core length in a drill result?

Drill intervals are measured along the drill core ("downhole" or "core length"). True width is the thickness of the mineralised zone measured perpendicular to its plane. Unless a hole intersects the zone at exactly 90°, core length overstates true width.

Example: a 20 m core-length intercept through a zone the drill cut at a shallow angle might be only 12 m true width. Responsible reporting states true width or gives the relationship ("intervals are reported as drilled core length; true widths are estimated at 70–80% of core length"). NI 43-101 and Australia's JORC Code both require disclosure of this relationship when known.

Why it matters: tonnage in a resource model depends on true width, not core length. A company quoting only core lengths — or drilling deliberately oblique to inflate widths — can make a thin zone look thick. Always check the footnotes for a true-width statement before taking gram-metres at face value.