Photogrammetry

Photogrammetry is the science of making precise measurements from photographs or mapping from photographs, especially through aerial surveying. By definition given by the American Society for Photogrammetry and Remote Sensing (ASPRS), in the Mapping Sciences, Photogrammetry is the art, science, and technology of obtaining reliable information about physical objects and the environment through the processes of recording, measuring, and interpreting photographic images and patterns of electromagnetic radiant energy and other phenomena. It is the first remote sensing technology ever developed. With respect to history, photogrammetry is as old as modern photography itself, and can be dated to mid-nineteenth century.

The three-dimensional coordinates of points on an object are determined by measurements made in two or more photographic images taken from different positions. The technology is termed as stereoscopy. Stereoscopy, stereoscopic imaging or 3-D (three-dimensional) imaging refers to any technique capable of recording three-dimensional visual information or creating the illusion of depth in an image. The illusion of depth in a photograph or two-dimensional image is created by presenting a slightly different image to each eye. In the workflow common points are identified on each image. A line of sight can be constructed from the camera location to the point on the object. It is the intersection of these rays that determines the three-dimensional location of the point and the process is called triangulation. In this digital era more sophisticated algorithms can explore other information about the scene even in some cases reconstructions of 3D coordinates from single camera position.

In the workflow each of the following four main variables can be an input or an output of a photogrammetric method.

1) The 3D co-ordinates define the locations of object points in the 3D space.
2) The image co-ordinates define the locations of the object points' images on the film or an electronic imaging device.
3) The exterior orientation of a camera defines its location in space and its view direction.
4) The interior 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 like, a known distance of two points in space, or known fix points, from which the connection to the basic measuring units is created.

Photogrammetry is used in different fields, such as topographic mapping, architecture, engineering, manufacturing, quality control, police investigation, forensics and geology, as well as by archaeologists to quickly produce plans of large or complex sites and by meteorologists as a way to determine the actual wind speed of a storm where objective weather data cannot be obtained. It is also used to combine live action with computer generated imagery in movie graphics sequences. Photogrammetry uses methods from many disciplines including optics and projective geometry.