Photogrammetry

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Photogrammetry is a remote sensing technology in which geometric properties about objects are determined from photographic images. For example, the three-dimensional coordinates of points on an object are determined by measurements made in two or more photographic images taken from different positions (see stereoscopy).

Common points are identified on each image. A line of sight (or ray) can be constructed from the camera location to the point on the object. It is the intersection of these rays (triangulation) that determines the three-dimensional location of the point. More sophisticated algorithms can exploit other information about the scene that is known a priori, for example symmetries, in some cases allowing reconstructions of 3D coordinates from only one camera position.

Photogrammetry is used in different fields, such as topographic mapping, architecture, engineering, manufacturing, quality control, police investigation, and geology, as well as by archaeologists to quickly produce plans of large or complex sites. It is also used to combine live action with computer generated imagery in movie post-production; Fight Club is an excellent example of the use of photogrammetry in film.

Algorithms for photogrammetry typically express the problem as that of minimizing the sum of the squares of a set of errors. The minimization is itself often performed using the Levenberg-Marquardt algorithm (also known as bundle adjustment).

[edit] Photogrammetric methods

Wiora's data model of photogrammetry [1].

Photogrammetry uses methods from many disciplines including optics and projective geometry. The data model on the right shows what type of information can go into and come out of photogrammetric methods.

The 3D co-ordinates define the locations of object points in the 3D space. The image co-ordinates define the locations of the object points' images on the film or an electronic imaging device. The exterior orientation of a camera defines its location in space and its view direction. The inner orientation defines the geometric parameters of the imaging process. This is primarily the focal length of the lens, but can also include the description of lens distortions. Further additional observations play an important role: With scale bars, basically a known distance of two points in space, or known fix points, the connection to the basic measuring units is created.

Each of the four main variables can be an input or a result of a photogrammetric method.