---
title: "Apparent dip in cross-section drawing"
subject: "Engineering Geology I"
chapter: 5
key: "semester_3/engineering_geology_i:apparent-dip-in-crosssection-drawing"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/apparent-dip-in-crosssection-drawing
site: BCT Engineering
---

# Apparent dip in cross-section drawing

Engineering Geology I · Chapter 5: Structural Geology

- Asked in: 2062 Baisakh (old course), 1.5+1.5+5 marks
- Video: [Structural Geology Lesson 2: Apparent Dip](https://www.youtube.com/watch?v=-TPje1Ef-x4) — Blake Splitter
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/apparent-dip-in-crosssection-drawing
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-5

## Question

Define true dip and apparent dip. Describe with illustration why it is important to consider apparent dip to draw a cross-section when the line of cross-section is not perpendicular to the strike line.

## Short answer

True dip is the maximum inclination measured perpendicular to strike; apparent dip is the smaller inclination along any other direction. In a cross-section not perpendicular to strike, drawing the bed with true dip overestimates steepness and depth. Apparent dip must be calculated, e.g. true dip 40° and angle α=30° gives apparent dip 22.8°.

## Answer

#### True dip
The **true dip** is the maximum inclination of a bed to the horizontal, measured in a direction perpendicular to the strike.

#### Apparent dip
The **apparent dip** is the inclination of the bed measured along any direction other than the true dip direction. It is smaller than the true dip and is related by $\tan \delta_a = \tan \delta \sin\alpha$, where $\alpha$ is the angle between the strike and the section line.

#### Why apparent dip is needed in a cross-section
A geological cross-section is a vertical slice along a chosen line. The bed in this slice shows only the dip that exists in the direction of the section line.
- If the section line is **perpendicular to the strike**, the section shows the true dip.
- If the line is **oblique to the strike**, the bed in the section has the apparent dip. Drawing the true dip there would make the bed too steep, so the thickness, depth and position of the bed at depth would be wrong.

```text
 Plan:        strike
        ----------------
          \ alpha |
           \      | true dip direction
 section    \     v
 line  -------> (apparent dip direction)

 Section along oblique line:
   true dip 40 deg (wrong)  /|
   apparent dip 22.8 (correct)/__|
```

#### Illustration by numbers
True dip = 40°, section line at $\alpha = 30°$ to the strike.

$$
\tan\delta_a = \tan 40° \times \sin 30° = 0.8391 \times 0.5 = 0.4195 \Rightarrow \delta_a = 22.8°
$$

If the true dip of 40° were drawn, the bed would reach a depth much greater than the real depth, so a wrong depth of the rock bed is predicted for a tunnel or foundation. For $\alpha = 60°$ the apparent dip is 36.0°, and for $\alpha = 90°$ it equals the true dip (40°).

Hence, for oblique sections the apparent dip must be calculated (or taken from a chart or the stereographic net) before drawing the beds.

## Check yourself

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

1. True dip is measured
   A. along the strike
   B. perpendicular to the strike, as the maximum inclination
   C. in any direction
   D. parallel to the section line always
   Answer: B. True dip is the maximum inclination, at right angles to strike. (Key point: True dip definition)

2. When the section line is perpendicular to the strike, the section shows
   A. the true dip
   B. zero dip
   C. the apparent dip smaller than true
   D. a dip equal to alpha
   Answer: A. At alpha = 90° apparent dip equals true dip. (Key point: Section perpendicular to strike)

3. What happens if true dip is drawn in an oblique section?
   A. The bed is drawn too flat
   B. Nothing changes
   C. The bed is drawn too steep, giving a wrong depth and position
   D. Only the strike changes
   Answer: C. The oblique section shows a smaller apparent dip. (Key point: Danger of using true dip)

4. True dip 40°, alpha 30°: apparent dip
   A. 40°
   B. 20°
   C. 36.0°
   D. 22.8°
   Answer: D. tan = 0.8391 x 0.5 = 0.4195, giving 22.8°. (Key point: Numerical illustration)

## 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
- [Strike-dip relationship and apparent dip](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Astrikedip-relationship-and-apparent-dip) — Engineering Geology I, chapter 5, asked 2×
- [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×
- [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×
- [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×
- [Strike of quartzite bed, dip 70° towards S20°E](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Astrike-of-quartzite-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
- [Perpendicular from point to line](https://www.bctengineer.com/question.md?key=semester_2%2Fmathematics%3Aperpendicular-from-point-line) — Mathematics, chapter 3, asked 5×