PHOTOGRAMMETRY AND AERIAL SURVEY

Introduction

Photogrammetry is the science and technology of obtaining reliable information about physical objects and the environment through the process of recording, measuring, and interpreting photographic images and patterns of electromagnetic radiant imagery. Aerial surveying is a key component, where photographs taken from airborne platforms (like planes or drones) are used to map the ground below.

Brief Overview

graph TD; Photo[Photogrammetry] --> Terrestrial[Terrestrial Photogrammetry] Photo --> Aerial[Aerial Photogrammetry] Aerial --> Forward[Forward Overlap] Aerial --> Lateral[Lateral Overlap] Terrestrial --> Surface[Earth's Surface Cameras]

1. Principles of Photogrammetry

Definition: "Photo" (picture), "gram" (drawing/record), and "metry" (measurement). It is essentially 3D measurement from 2D photographs.
Stereoscopy: The core principle relies on stereovision. By taking two photographs of the same object from slightly different positions (a stereo pair), the brain (or a computer) can calculate depth and elevation, creating a 3D model.
Types:
- Terrestrial Photogrammetry: Camera is located on the Earth's surface (tripods, vehicles).
- Aerial Photogrammetry: Camera is mounted on an aircraft pointing vertically downward or obliquely.

MCQs - Principles of Photogrammetry

Photogrammetry fundamentally deals with extracting what kind of information from photographs?
a) Chemical composition
b) Biological data
c) Geometric and 3D measurements
d) Temperature profiles
Answer: c
The technique of overlapping two images from different perspectives to create a 3D effect is called:
a) Orthophotography
b) Stereoscopy
c) Remote viewing
d) Parallax
Answer: b
Terrestrial photogrammetry refers to photographs taken from:
a) Satellites
b) Drones
c) The Earth's surface
d) High-altitude aircraft
Answer: c
In photogrammetry, a "stereo pair" consists of:
a) Two cameras mounted side-by-side
b) Two overlapping photographs of the same area from different angles
c) A photograph and a map
d) Two identical photographs printed in different colors
Answer: b
The Greek root "gram" in photogrammetry means:
a) Light
b) Measurement
c) Record or drawing
d) Distance
Answer: c
The fundamental geometric principle used to determine distances and elevations from a stereo pair is known as:
a) Triangulation
b) Parallax
c) Resection
d) Intersection
Answer: b
The term "photo" in photogrammetry is derived from Greek, meaning:
a) Picture
b) Light
c) Lens
d) Color
Answer: b
In terrestrial photogrammetry, the camera axis is typically oriented:
a) Horizontally
b) Vertically downwards
c) Towards the zenith
d) Randomly
Answer: a
Which of the following is NOT a common platform for aerial photogrammetry?
a) Drones
b) Helicopters
c) Tripods
d) Airplanes
Answer: c
The primary difference between terrestrial and aerial photogrammetry is the:
a) Type of camera used
b) Location of the camera platform
c) Number of photographs taken
d) Processing software
Answer: b

2. Aerial Surveying & Flight Planning

Flight Lines: An aircraft flies in straight parallel lines over the terrain to be mapped.
Overlap: Crucial for stereoscopy and continuous mapping.
- Forward Overlap (End lap): Overlap between consecutive photos in the same flight line (usually 60% to ensure every point is seen in at least two photos).
- Lateral Overlap (Side lap): Overlap between adjacent flight lines (usually 20-30% to prevent gaps).
Scale: The scale of an aerial photograph is the ratio of focal length ($f$) to the flying height above ground ($H$). Scale = $f/H$.

MCQs - Aerial Surveying

Why is forward overlap (end lap) necessary in aerial surveying?
a) To make the photos look brighter
b) To allow for stereoscopic viewing and 3D modeling
c) To use less film
d) To fly the plane faster
Answer: b
What is the standard recommended percentage for forward overlap (end lap)?
a) 10%
b) 25%
c) 60%
d) 90%
Answer: c
The overlap between adjacent parallel flight lines is called:
a) End lap
b) Side lap
c) Vertical overlap
d) Oblique overlap
Answer: b
If a camera's focal length is 150 mm and the flying height is 3000 m, the scale of the photograph is:
a) 1:2000
b) 1:20000
c) 1:5000
d) 1:15000
Answer: b ($Scale = 150mm / 3000000mm = 1:20000$)
Aerial surveying relies on flight lines that are generally:
a) Circular
b) Zig-zag
c) Straight and parallel
d) Random
Answer: c
A higher flying height, assuming the focal length remains constant, will result in a photograph with a:
a) Larger scale
b) Smaller scale
c) Unchanged scale
d) Distorted scale
Answer: b
Which overlap ensures that adjacent flight lines do not leave any unmapped gaps?
a) Forward overlap
b) Side lap
c) Vertical overlap
d) End lap
Answer: b
The distance between the optical center of the lens and the focal plane is called the:
a) Flying height
b) Camera baseline
c) Focal length
d) Air base
Answer: c
End lap in aerial surveying is typically maintained around what percentage?
a) 20-30%
b) 40-50%
c) 60%
d) 80%
Answer: c
If the scale of an aerial photograph is 1:10000 and the flying height is 2000 m, what is the camera's focal length?
a) 150 mm
b) 200 mm
c) 250 mm
d) 300 mm
Answer: b

3. Types of Aerial Photographs

Vertical Photographs: The camera axis is pointed as vertically as possible (within $3^\circ$ of the plumb line). Used heavily for topographic mapping and orthophoto creation.
Low Oblique Photographs: The camera axis is intentionally tilted. The horizon is not visible. Good for recognizing ground features.
High Oblique Photographs: The camera axis is tilted enough that the horizon is visible in the photograph. Excellent for sweeping views and military reconnaissance.

MCQs - Types of Photographs

A photograph taken with the camera axis pointing straight down to the ground is a:
a) High oblique photograph
b) Low oblique photograph
c) Vertical photograph
d) Terrestrial photograph
Answer: c
In a high oblique aerial photograph, what key feature must be visible?
a) The aircraft's wing
b) The sun
c) The horizon
d) The nadir point
Answer: c
Which type of photograph is most commonly used for creating accurate topographic maps?
a) High oblique
b) Low oblique
c) Vertical
d) Horizontal
Answer: c
If an aerial photo is tilted but the horizon does not appear in the image, it is a:
a) Vertical photograph
b) Low oblique photograph
c) High oblique photograph
d) Orthophotograph
Answer: b
Oblique photographs are generally easier for untrained people to understand than vertical photographs because:
a) They show true scale everywhere
b) They provide a more natural perspective resembling human vision
c) They are always in color
d) They cover a much smaller area
Answer: b
The point on the ground vertically beneath the perspective center of the camera lens is called the:
a) Principal point
b) Nadir point
c) Isocenter
d) Focal point
Answer: b
For reconnaissance purposes where viewing large areas is more important than precise measurements, which type of photograph is best?
a) Vertical
b) High oblique
c) Low oblique
d) Orthophoto
Answer: b
The intersection of the optical axis with the photographic plane is the:
a) Nadir point
b) Isocenter
c) Principal point
d) Zenith point
Answer: c
Which type of aerial photograph provides a view that most closely resembles a map?
a) Low oblique
b) High oblique
c) Vertical
d) Panoramic
Answer: c
In a truly vertical aerial photograph, the tilt of the camera axis is exactly:
a) 90 degrees
b) 45 degrees
c) 3 degrees
d) 0 degrees
Answer: d

4. Orthophotography and Drones (UAVs)

Orthophoto: An aerial photograph that has been geometrically corrected (orthorectified) such that the scale is uniform. It has the visual characteristics of a photo but the geometric accuracy of a map. Distances can be measured directly on it.
UAVs (Drones): Unmanned Aerial Vehicles have revolutionized photogrammetry. They are cheap, can fly very low (yielding ultra-high resolution), and use advanced software (Structure from Motion - SfM) to automatically stitch thousands of photos into 3D point clouds.

MCQs - Orthophotography and Drones

An orthophotograph is an aerial photo that has been corrected so that it:
a) Shows the horizon perfectly
b) Has a uniform scale and can be used like a map
c) Looks 3D without special glasses
d) Removes all buildings and trees
Answer: b
The geometric correction applied to an aerial photo to make it an orthophoto is called:
a) Stereoscopy
b) Digitization
c) Orthorectification
d) Vectorization
Answer: c
In modern surveying, UAV stands for:
a) Universal Aerial View
b) Unmanned Aerial Vehicle
c) Underground Acoustic Vector
d) Unified Altitude Vision
Answer: b
A major advantage of using drones for photogrammetry over traditional manned aircraft is:
a) They can fly much higher
b) They can carry heavier film cameras
c) They can fly lower, capturing higher resolution data at a lower cost
d) They do not require any overlapping images
Answer: c
Which software technique is commonly used with drone imagery to automatically generate 3D models?
a) AutoCAD
b) SfM (Structure from Motion)
c) Word Processing
d) SQL
Answer: b
The process of removing the effects of image perspective and relief displacement to create a uniform scale is called:
a) Stereomatching
b) Orthorectification
c) Image fusion
d) Georeferencing
Answer: b
Which of the following is a key component produced by SfM software during drone mapping?
a) Vector map
b) Text document
c) 3D point cloud
d) Spreadsheet
Answer: c
Ground Control Points (GCPs) are used in drone surveying primarily to:
a) Control the drone's flight path
b) Improve the absolute accuracy of the final model
c) Increase the camera's resolution
d) Extend battery life
Answer: b
Unlike a regular aerial photograph, an orthophoto allows for accurate measurement of:
a) Distances only
b) Angles only
c) Both distances and angles
d) Temperature
Answer: c
The high resolution of UAV imagery is primarily attributed to their:
a) Expensive sensors
b) Low flying altitude
c) Heavy payload capacity
d) Long flight duration
Answer: b

Fun Facts about Photogrammetry!

Pigeon Cameras: Before planes and drones, in 1907, an eccentric German apothecary invented a miniature camera that was strapped to homing pigeons! The camera had a timer and took pictures as the birds flew back to their coop.
Mapping the Moon: Photogrammetry wasn't just used on Earth. During the Apollo missions, astronauts used specialized photogrammetric cameras to map the lunar surface in 3D for future landing sites.
Hollywood Magic: The same photogrammetry techniques used by surveyors are used by video game developers and VFX artists to scan real-world actors, objects, and landscapes to create hyper-realistic 3D assets for movies and games!

Photogrammetry And Aerial Survey