geology · structural
True Dip from Apparent Dip
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Inputs
°
°
Description
Recovers the true (maximum) dip of a plane from an apparent dip observed in a vertical section or cross-section that is not perpendicular to strike, given the structural bearing (angle from strike) of that section. This is the inverse of apparent_dip_true_dip and is typically the more common field workflow: an apparent dip is what's directly observable in an outcrop, road cut, or seismic section, while true dip is the value usually needed for mapping or volumetrics.
Variables
| Symbol | Unit | Description |
|---|---|---|
| δ | ° | True Dip |
| α | ° | The dip observed in a vertical section or cross-section that is not perpendicular to strike (e.g., measured in an outcrop, road cut, or seismic section). |
| β | ° | The horizontal angle, measured from the strike direction, to the vertical section or observation line along which the apparent dip was observed. |
Assumptions
- The surface is genuinely planar over the section of interest.
- β is the acute angle (0-90°) between the strike line and the section/observation direction along which the apparent dip was measured.
- The observed apparent dip is a true apparent dip of the same planar surface, not a value distorted by outcrop weathering, topographic slope, or measurement along a non-planar exposure.
Limitations
- Not valid across a fold hinge or fault, where the planar-attitude assumption breaks down.
- Highly sensitive to error in β (and in the observed apparent dip) when the section is nearly parallel to strike (small β) — recovered true dip can swing sharply for small input errors in that regime; see the calculator's own warning flag.
- Does not determine the true dip direction (azimuth) independently — only its magnitude; the true dip direction must be established separately (e.g., from a three-point solution or two non-parallel apparent-dip observations).
- Uses the same tan(α)=tan(δ)×sin(β) relationship as apparent_dip_true_dip, corroborated across multiple independent secondary structural-geology sources; see that calculator's own citation disclosure for the same Ragan (2009) sourcing caveat.
Use Cases
- → Recovering true dip from an outcrop or road-cut observation: An apparent dip measured directly in an exposure that isn't perpendicular to strike, combined with a known or mapped strike, gives the plane's true dip without needing a separate three-point solution.
- → Recovering true dip from a seismic section: A dip picked on a 2-D seismic line oriented obliquely to structural strike can be corrected to true dip once the line's bearing relative to strike is known.
Related Calculations
Region Notes
Global
Recovered true dip is always ≥ the input apparent dip for the same plane — if this calculator ever returns a true dip less than the entered apparent dip, check whether β was measured as the angle from strike (correct) rather than from the dip direction.
References
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