---
title: "Strike-dip relationship and apparent dip"
subject: "Engineering Geology I"
chapter: 5
key: "semester_3/engineering_geology_i:strikedip-relationship-and-apparent-dip"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/strikedip-relationship-and-apparent-dip
site: BCT Engineering
---

# Strike-dip relationship and apparent dip

Engineering Geology I · Chapter 5: Structural Geology

- Asked in: 2074 Asoj, 3 marks
- Video: [Mastering Structural Geology lecture 5 (Relationship between True and apparent dip ||Numericals||](https://www.youtube.com/watch?v=1pSyVQJItuA) — Alli Mohammed
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/strikedip-relationship-and-apparent-dip
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-5

## Question

What is the relationship between strike and dip? How do you calculate apparent dip amount from the measured true dip amount?

## Short answer

Strike and dip are perpendicular: the dip direction is always 90° to the strike. Apparent dip in a section making angle α with the strike is tan δ_a = tan δ × sin α, where δ is true dip. Example: true dip 30°, α=45° gives apparent dip 22.2°.

## Answer

#### Relationship between strike and dip
- The strike is the bearing of the horizontal line on an inclined plane, and dip is the maximum inclination of the plane.
- The dip direction is always **at right angles (90°)** to the strike. So when the strike is known, the dip direction is strike ± 90° (the side must be seen from the field), and when the dip direction is measured, strike = dip direction ± 90°.
- Example: strike N30°E, dip 40° towards SE (azimuth 120°).

```text
      N      strike
      |    /
      |  /
  ----+/------ E
     /|  \
        dip direction (90 deg to strike)
```

#### Apparent dip from the true dip
Apparent dip is the inclination of the bed measured along a vertical section that makes an angle $\alpha$ with the strike. It is calculated by:

$$
\tan \delta_a = \tan \delta \times \sin \alpha
$$

where $\delta$ = true dip, $\delta_a$ = apparent dip, $\alpha$ = angle between the strike and the section line.

Example: $\delta = 30°$, $\alpha = 45°$ gives $\tan\delta_a = 0.5774 \times 0.7071 = 0.4082$, so $\delta_a = 22.2°$. The apparent dip is always less than the true dip, and is zero along the strike.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the relation between strike and dip direction?
   A. They are at right angles (90°)
   B. They are parallel
   C. They are 45° apart
   D. They are opposite (180°)
   Answer: A. Dip direction = strike +/- 90°. (Key point: Strike-dip relation)

2. Compared with true dip, apparent dip is
   A. always larger
   B. always equal
   C. always smaller (or equal at alpha = 90°)
   D. always 90°
   Answer: C. Because sin(alpha) is at most 1. (Key point: Apparent dip comparison)

3. True dip 30°, alpha 45°: apparent dip
   A. 30°
   B. 22.2°
   C. 42.4°
   D. 17.5°
   Answer: B. tan(da) = tan30 x sin45 = 0.4082, da = 22.2°. (Key point: Example result)

## Close variants set in other exams

- [Calculating apparent dip from true dip](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Acalculating-apparent-dip-from-true) — Engineering Geology I, chapter 5, asked 2×

## Related in Engineering Geology I

- [Fold classification, strike, true and apparent dip](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Afold-classification-strike-true-and) — Engineering Geology I, chapter 5
- [Dip amount, strike; fold identification criteria](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Adip-amount-strike-fold-identification) — Engineering Geology I, chapter 5
- [True dip versus apparent dip](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Atrue-dip-versus-apparent-dip) — Engineering Geology I, chapter 5, asked 2×
- [Primary and secondary structures; strike and dip](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aprimary-and-secondary-structures-strike) — Engineering Geology I, chapter 5, asked 2×
- [Apparent dip in cross-section drawing](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aapparent-dip-in-crosssection-drawing) — Engineering Geology I, chapter 5
- [Strike of limestone bed, dip N40°W, 64°](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Astrike-of-limestone-bed-dip) — Engineering Geology I, chapter 5

## Related in other subjects

- [True dip from two apparent dips](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Atrue-dip-from-apparent-dips) — Engineering Geology II, chapter 6
- [Reserve: limestone dip 30, apparent 20 m](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Areserve-limestone-dip-30) — Engineering Geology II, chapter 7
- [True thickness from apparent thickness 210 m](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Abh-quartzite-thickness-210-attitude) — Engineering Geology II, chapter 6