In aerial photogrammetry, perspective projection describes how a three-dimensional terrain is captured as a two-dimensional image by an aerial camera.
Understanding Perspective Projection
Perspective projection is a fundamental concept in aerial photogrammetry, influencing how measurements and maps are derived from photographs taken from an aircraft. Unlike other types of projections where rays might be parallel, perspective projection involves rays converging to a single point, much like how the human eye or a camera lens works.
Based on the reference, aerial photographs are typically acquired using an optical lens system. These photographs record the terrain images in the central of perspective projection. This means that features on the ground are projected onto the camera's image plane as if viewed from a single point in space – the camera's lens center at the moment of exposure.
Key Characteristics
- Central Projection: All light rays from the terrain pass through a single point (the lens center) before hitting the image sensor or film.
- Optical Lens System: This type of projection is characteristic of images captured using standard cameras with lenses.
- Scale Variation: Objects closer to the camera appear larger than objects further away, leading to variations in scale across the image. This is a defining characteristic of perspective projection.
Perspective vs. Other Projections
The reference highlights a key difference:
- Aerial Photographs: Utilize central perspective projection.
- Satellite Images: Often record data using parallel and/or orthogonal projection.
This distinction is crucial for processing. Parallel/orthogonal projections maintain consistent scale (apart from sensor limitations), whereas perspective projection inherently introduces scale distortion that needs to be corrected for mapping and measurement.
| Feature | Aerial Photographs | Satellite Images |
|---|---|---|
| Projection Type | Central Perspective Projection | Parallel and/or Orthogonal |
| Recording System | Optical Lens System | Various (e.g., Scanners, Arrays) |
| Scale | Varies across image | More consistent (ideally uniform) |
Implications for Photogrammetry
The use of perspective projection in aerial photographs has several important implications:
- Relief Displacement: Tall objects lean away from the center of the photograph due to the perspective view. This displacement is directly related to the object's height and its distance from the image center.
- Scale Distortion: The scale of the photograph is not uniform across the image. It is largest at the point directly beneath the camera (nadir) and decreases towards the edges.
- Geometric Correction: To create accurate maps or 3D models, the distortions caused by perspective projection, along with other factors like tilt and terrain relief, must be corrected. This process often involves using ground control points (GCPs) or reference marks, as mentioned in the reference, which are precisely located points on the ground identifiable in the photograph.
In essence, understanding perspective projection is fundamental to analyzing aerial photographs, calculating distances, areas, and heights, and ultimately creating accurate spatial data products.
