---
title: "Define principal point"
subject: "Surveying II"
chapter: 7
key: "semester_4/surveying_ii:define-principal-point"
source: https://www.bctengineer.com/q/semester_4/surveying_ii/define-principal-point
site: BCT Engineering
---

# Define principal point

Surveying II · Chapter 7: Photogrammetry and Remote Sensing

- Asked in: 2069 Bhadra, 1 marks
- Video: [Pg Lect3  image Displacement , Plumb point (Nadir), principal point and isocentre](https://www.youtube.com/watch?v=VYeB-vaR5Sg) — Photogrammetry World
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/surveying_ii/define-principal-point
- Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-7

## Question

Define principal point.

## Short answer

Point where the optical axis meets the photograph, at the foot of the perpendicular from the lens. It is the intersection of lines joining opposite fiducial marks.

## Answer

The **principal point** of a photograph is the foot of the perpendicular from the lens (perspective centre) to the photo plane, i.e. the point where the optical axis of the camera meets the photograph. It is located at the intersection of the lines joining opposite fiducial marks.

## Check yourself

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

1. The principal point is:
   A. Intersection of ground diagonals
   B. Highest point
   C. A control point
   D. Foot of perpendicular from lens to the photo plane
   Answer: D. It is where the optical axis meets the photo. (Key point: Definition)

2. It is located by:
   A. The photo corner
   B. Lines joining opposite fiducial marks
   C. The horizon
   D. The sun
   Answer: B. Fiducial mark lines intersect at it. (Key point: Location)

3. The principal point lies on:
   A. The datum
   B. The air base
   C. The shadow
   D. The optical axis of the camera
   Answer: D. It is where the camera's optical axis meets the photograph. (Key point: Axis)

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