Geometric fundamentals of Photogrammetry; Elements of Vertical Photograph; Relief displacement, Image Parallax, Stereo model- Photo mosaics; Image interpretation Keys; Ground Control for Aerial Photography

1. Geometric Fundamentals of Photogrammetry

The geometric fundamentals of photogrammetry explain how 3D ground features are represented as 2D images using the principles of perspective geometry.

Perspective projection – Formation of an image from a single camera position.
Camera station (Exposure station) – Position of the camera during exposure.
Principal point – Centre of the photograph.
Principal axis – Line joining the camera lens and principal point.
Focal length (f) – Distance between the camera lens and image plane.
Scale – Ratio of image distance to ground distance.

Concept:
Photogrammetry uses camera geometry to measure distances, heights, and coordinates accurately.

2. Elements of Vertical Photograph

A vertical photograph is taken with the camera axis nearly perpendicular (90°) to the Earth's surface.

Key Elements

Principal point (PP)
Nadir point (NP)
Isocentre (IC)
Fiducial marks
Photo centre
Flight line
Scale
Overlap (Forward and Side overlap)

Concept:
These elements help determine image orientation and accurate mapping.

3. Relief Displacement

Relief displacement is the apparent outward movement of objects from the centre of a photograph due to differences in elevation.

Height (H)
Flying height (h)
Radial displacement
Photo centre

Concept:
Higher objects (buildings, trees, hills) appear displaced farther from the photo centre than lower objects.

4. Image Parallax

Parallax is the apparent shift in the position of an object between two overlapping photographs.

Absolute parallax
Differential parallax
Air base
Stereo pair

Concept:
Parallax is used to calculate the height and elevation of ground features.

5. Stereo Model

A stereo model is a three-dimensional (3D) view created by observing two overlapping aerial photographs.

Stereo pair
Binocular vision
Stereoscope
Base-height ratio

Concept:
The stereo model enables accurate measurement of terrain height, slope, and landforms.

6. Photo Mosaics

A photo mosaic is a composite image formed by joining several overlapping aerial photographs into one continuous image.

Types:

Uncontrolled mosaic
Semi-controlled mosaic
Controlled mosaic

Concept:
Photo mosaics provide a larger and continuous view for mapping and land-use studies.

7. Image Interpretation Keys

Image interpretation keys are the visual characteristics used to identify objects in aerial photographs or satellite images.

Tone/Colour
Texture
Shape
Size
Pattern
Shadow
Site
Association

Concept:
These keys help distinguish natural and human-made features accurately.

8. Ground Control for Aerial Photography

Ground control refers to accurately surveyed reference points on the Earth's surface used to correct and georeference aerial photographs.

Ground Control Points (GCPs)
GPS/GNSS
Coordinates
Georeferencing
Image rectification
Check points

Concept:
Ground control improves the positional accuracy of aerial photographs and ensures reliable photogrammetric mapping.

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