---
title: "Flying height from photo and map scale"
subject: "Surveying II"
chapter: 7
key: "semester_4/surveying_ii:flying-height-pq-20cm-focal-24"
source: https://www.bctengineer.com/q/semester_4/surveying_ii/flying-height-pq-20cm-focal-24
site: BCT Engineering
---

# Flying height from photo and map scale

Surveying II · Chapter 7: Photogrammetry and Remote Sensing

- Asked in: 2080 Chaitra, 4 marks
- Video: [Scale of a vertical photograph, Advanced surveying - photogrammetric surveying part 09, Globemech,](https://www.youtube.com/watch?v=l-Uh1HU5WBA) — Globe Mech
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/surveying_ii/flying-height-pq-20cm-focal-24
- Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-7

## Question

A line PQ measures 20 cm on an aerial photograph taken with an aerial camera having focal length 24 cm. The same line measures 6 cm on a map drawn to a scale of 1:50000. Calculate the flying height of the aircraft if the average altitude of ground is 350 m.

## Short answer

Find ground distance: 6 cm × 50,000 = 3000 m. Photo scale = 0.20 m / 3000 m = 1:15,000. Flying height above datum H = f/scale + h = 0.24 × 15,000 + 350 = 3950 m.

## Answer

The photo scale is found from a map distance, and then the flying height from $S = f/(H-h)$.

#### Step 1: Ground length of PQ
Map scale 1:50 000, so PQ on the ground:

$$
PQ_{ground} = 6\ \text{cm}\times 50\,000 = 300\,000\ \text{cm} = 3000\ \text{m}
$$

#### Step 2: Photo scale
$$
S = \frac{\text{photo distance}}{\text{ground distance}} = \frac{0.20\ \text{m}}{3000\ \text{m}} = \frac{1}{15\,000}
$$

#### Step 3: Flying height
$$
S = \frac{f}{H-h}\ \Rightarrow\ H - h = f\times 15\,000 = 0.24\times 15\,000 = 3600\ \text{m}
$$

$$
H = 3600 + 350 = 3950\ \text{m}
$$

**Answer:** The photo scale is 1:15 000 and the flying height of the aircraft is **3950 m** above datum (3600 m above the average ground).

## Check yourself

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

1. 6 cm on a 1:50 000 map equals what ground distance?
   A. 300 m
   B. 3000 m
   C. 30 000 m
   D. 12 000 m
   Answer: B. 6 x 50 000 = 300 000 cm = 3000 m. (Key point: Ground length)

2. Photo scale (20 cm photo for 3000 m ground):
   A. 1:150 000
   B. 1:1500
   C. 1:12 000
   D. 1:15 000
   Answer: D. 0.20/3000 = 1/15 000. (Key point: Photo scale)

3. With f = 0.24 m and S = 1/15 000, H - h =
   A. 360 m
   B. 3600 m
   C. 15 000 m
   D. 3950 m
   Answer: B. H - h = f x 15 000 = 3600 m. (Key point: H-h)

4. Adding the ground elevation 350 m, H =
   A. 3250 m
   B. 3600 m
   C. 4300 m
   D. 3950 m
   Answer: D. H = 3600 + 350 = 3950 m above datum. (Key point: Flying height)

## Related in Surveying II

- [Types of aerial photograph, scale relation](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Aaerial-photo-types-scale-relation) — Surveying II, chapter 7
- [Short note: Photo scale, photogrammetry uses](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Aphoto-scale-photogrammetry-uses) — Surveying II, chapter 7
- [Expression for scale of vertical photograph](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Avertical-photograph-scale-expression) — Surveying II, chapter 7
- [Define flying height](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Adefine-flying-height) — Surveying II, chapter 7
- [Clear height of flood light tower (arena)](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Aflood-light-tower-clear-height-arena) — Surveying II, chapter 3

## Related in other subjects

- [Length of PQ across a pond](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aobstacle-pond-chain-line) — Surveying I, chapter 3
- [True length of 1020 m line, 20 m tape](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atape-20m-1020m-line) — Surveying I, chapter 2, asked 2×
- [True length of 680 m line, 20 m tape](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atape-20m-680m-line) — Surveying I, chapter 2, asked 3×
- [Scale for 1.5 km road on map](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Ascale-road-1-5-km) — Surveying I, chapter 1