---
title: "Short note: Stereoscopy, aerial mapping"
subject: "Surveying II"
chapter: 7
key: "semester_4/surveying_ii:stereoscopy-aerial-mapping"
source: https://www.bctengineer.com/q/semester_4/surveying_ii/stereoscopy-aerial-mapping
site: BCT Engineering
---

# Short note: Stereoscopy, aerial mapping

Surveying II · Chapter 7: Photogrammetry and Remote Sensing

- Asked in: 2073 Bhadra, 4 marks
- Video: [Stereoscopy and Stereoscope](https://www.youtube.com/watch?v=gSegSKMdirw) — Dr. Subhash Karande
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/surveying_ii/stereoscopy-aerial-mapping
- Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-7

## Question

Write a short note on the principle of stereoscopy and importance of aerial mapping.

## Short answer

Stereoscopy fuses overlapping photos taken from different stations to create a 3D model via parallax. Aerial mapping provides fast, uniform, cost-effective maps of large or inaccessible areas, with permanent records for planning and monitoring.

## Answer

#### Principle of stereoscopy
Stereoscopy is the ability to see in three dimensions. A person's two eyes are about 65 mm apart (the eye base). Each eye views an object from a slightly different angle, so the two retinal images differ; the brain fuses them into a 3D impression and judges depth from the **parallax** between the images.

In photogrammetry, two overlapping photographs of the same area taken from different camera stations (air base $B$) serve as the "two eyes". When the left photo is seen by the left eye and the right photo by the right eye (using a lens or mirror stereoscope, or anaglyph or polarised displays), the overlap is seen as a three-dimensional model. Height differences are measured from the differences in parallax: $h = \dfrac{H\,\Delta p}{p + \Delta p}$, where $p$ is the absolute stereoscopic parallax of the lower (base) point, $\Delta p$ the difference in parallax, and $H$ the flying height above the base point.

```text
   O1 <------ B ------> O2     two exposures
    \                 /
     \               /
      \             /
       [ overlap ]  -> 3D model
```

#### Importance of aerial mapping
- Quick preparation of maps of large and inaccessible areas.
- Uniform accuracy and a permanent record of the ground at the time of the flight.
- Cheaper and faster than ground survey for large areas.
- Gives contours, spot heights, planimetric details and orthophotos for the planning and design of roads, dams and cities.
- Allows maps to be updated and changes to be monitored (floods, landslides, urban growth).

## Check yourself

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

1. How does stereoscopic vision give depth?
   A. Using one eye with colour
   B. Each eye sees a slightly different view; the brain fuses them using parallax
   C. Magnification only
   D. Shadow length
   Answer: B. The two eyes (about 65 mm apart) see different images, and the brain judges depth from parallax. (Key point: Principle)

2. In photogrammetry the two 'eyes' are:
   A. Two ground stations
   B. Two contours
   C. Two datum lines
   D. Two overlapping photos from different camera stations
   Answer: D. Overlapping photos taken at air base B give a 3D model when viewed stereoscopically. (Key point: Photogrammetric use)

3. Heights are obtained from:
   A. Photo colour
   B. Differences in parallax
   C. Film speed
   D. Principal point alone
   Answer: B. h = HΔp/(p+Δp), based on parallax differences. (Key point: Height measurement)

4. Which is an importance of aerial mapping?
   A. Cheaper than ground survey for tiny plots
   B. Not weather dependent
   C. No overlap needed
   D. Quick maps of large inaccessible areas with uniform accuracy
   Answer: D. It gives quick, uniform maps of large areas and a permanent record. (Key point: Importance of aerial mapping)

## Related in Surveying II

- [Total station principle, importance and uses](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atotal-station-principle-importance-uses) — Surveying II, chapter 9