Devices & Theory of Photogrammetry
The basic principle of photographic devices rely on the fact that rays of light reaching the suface of the film during exposure, pass through the camera lens. The lens can mathematically be considered as a single point. The internal geometry of the camera needs to be known to take measurments of objects from photographs. Therefore the important components are the focal length, lens distortion and the principal point. Relating to these factors, the photographic devices fit into three main categories:
- Metric Cameras: Stable and precisely known internal geometries and low lens distortions, hence they tend to be expensive. This type of camera is used for aerial surveying where the camera is mounted to the aircraft, pointing downwards.
- Stereometric Cameras: Consist of two metric cameras mounted at each end of a bar of known length. When the cameras have parallel viewing angles, they have similar properties to the two images produced by human eyes. The overlapping area of the two images produces a stereopair that can be seen in 3D.
- Amateur Cameras: The internal geometry is not known and often unstable. These cameras tend to be used for low accuracy applications.
Reliant on the device used and the amount of available results, photogrammetry techniques fall into three main categories:
Mapping from a single photograph
Only useful for planar objects. However, to get good measurement results, they often have to rectified using one of the following methods:
- Paper strip
- Optical
- Numerical
- Monoplotting
- Digital
Stereo-photogrammetry
The overlapping part of stereo-pair images can be viewed and mapped in 3D. Aerial photographs are often made using this technique. An aircraft, mounted with special metric cameras, flies along a path to ensure overlap in the images. Mapping of the stereo-pair images can be achieved using one of the following techniques:
- Analogue
- Analytical
- Digital
Mapping from several photographs
Using control and triangulation points from several photographs it is possible to digitally reconstruct a 3D object. With the continual improvement of computer hardware and software, this method is used extensively for many applications including film visual effects.
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