---
title: "Field procedure of aerial photogrammetry, relief displacement"
subject: "Surveying II"
chapter: 7
key: "semester_4/surveying_ii:aerial-photogrammetry-field-procedure-relief-displacement"
source: https://www.bctengineer.com/q/semester_4/surveying_ii/aerial-photogrammetry-field-procedure-relief-displacement
site: BCT Engineering
---

# Field procedure of aerial photogrammetry, relief displacement

Surveying II · Chapter 7: Photogrammetry and Remote Sensing

- Asked in: 2079 Chaitra, 2+2 marks; 2075 Bhadra, 6 marks (asked 2 times)
- Video: [photogrammetry surveying - photogrametry || advanced surveying - advanced surveying](https://www.youtube.com/watch?v=6saPK5Etyw4) — 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/aerial-photogrammetry-field-procedure-relief-displacement
- Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-7

## Question

Describe the field procedure of aerial photogrammetry and relief displacement of photogrammetry.

## Short answer

Field procedure includes planning photo scale and overlaps, establishing signalised ground control, flying strips at constant height, processing images, plotting from stereo pairs, and field checking. Relief displacement d = r h / H is the radial image shift due to height, causing scale variation; it allows height measurement and stereoscopic parallax.

## Answer

#### Field procedure of aerial photogrammetry
1. **Planning:** decide the purpose, scale and accuracy; choose the camera, flying height and overlaps (60 % end lap, 25–30 % side lap); prepare the flight plan.
2. **Ground control:** establish horizontal and vertical control points (by traversing, levelling or GPS) that can be identified on the photos, and signalise them with targets before the flight.
3. **Photography:** fly the strips at the planned height and speed in suitable weather; the camera exposes at fixed intervals.
4. **Processing:** develop the film (or download digital images), prepare contact prints and a photo index.
5. **Photo-control identification and plotting:** pass the control to the photos, orient the stereo pairs in a plotter, and draw the map or compute coordinates and contours.
6. **Field completion and checking:** annotate names and features, check the map in the field.

#### Relief displacement
It is the radial shift of the image of a point on a vertical photo, due to its height $h$ above the datum. From similar triangles,

$$
d = \frac{r\,h}{H}
$$

where $r$ is the radial distance of the image from the principal point and $H$ is the flying height above the datum. High points move outward from the principal point, low points move inward, and the displacement is zero at the principal point. Because of this, a photograph of undulating ground is not to scale and cannot be used as a map unless corrected (by rectification or orthophoto production). It also lets heights of objects be found: $h = dH/r$, and it gives the parallax needed for stereoscopic viewing.

## Check yourself

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

1. Which is the correct order of the procedure?
   A. Photography, planning, ground control
   B. Planning, ground control, photography, processing, plotting, field completion
   C. Processing, planning, photography
   D. Ground control, plotting, planning
   Answer: B. Planning comes first, then ground control, flying, processing, plotting and finally field completion. (Key point: Order of field procedure)

2. Why are control points signalised before the flight?
   A. To increase tilt
   B. To reduce overlap
   C. To change scale
   D. So they can be identified on photos
   Answer: D. Targets make control points visible on photos, allowing control to be passed to the photos. (Key point: Ground control)

3. What is d in relief displacement?
   A. d = rH/h
   B. d = rh/H
   C. d = hH/r
   D. d = r/(hH)
   Answer: B. d = rh/H with r the radial distance and H the flying height above datum. (Key point: Relief displacement formula)

4. Why can a photo of undulating ground not be used directly as a map?
   A. The photo is too large
   B. Overlap is too high
   C. It has no principal point
   D. Relief displacement makes scale vary, so it needs rectification/orthophoto production
   Answer: D. Displacement and varying scale mean photos must be corrected before being used as maps. (Key point: Consequence)

## Related in Surveying II

- [Short note: Relief displacement expression](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Arelief-displacement-expression) — Surveying II, chapter 7, asked 5×
- [Relief displacement on a vertical photograph](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Arelief-displacement-sketch-vertical-photo) — Surveying II, chapter 7
- [Traverse field procedure between two known stations](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atraverse-field-procedure-between-known-stations) — Surveying II, chapter 1
- [Three-wire stadia field procedure for topo map](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Athree-wire-stadia-field-procedure) — Surveying II, chapter 2
- [Stadia principle and field procedure](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Astadia-principle-field-procedure-topo) — Surveying II, chapter 2
- [ODM principle, tacheometric field procedure](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Aodm-principle-field-procedure-theodolite) — Surveying II, chapter 2

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