Photogrammetric Methods

The main purpose of photogrammetry is to define the quantitative parameters of objects using images taken by photographic or digital cameras. The typical output of photogrammetry is a map or three-dimensional (3D) model of some real object or scene. Photogrammetry is widely used to compile topographic maps of Earth's surface.

Photogrammetry is based on principles of ste-reometry—a branch of geometry that studies the projections of 3D objects into 2D space; in particular, photogrammetry deals with the appearance of real 3D objects in two overlapping photographs (image stereopair). The fundamental geometric principle used by photogrammetry is triangulation. By taking photographs from two (or more) different locations, so-called lines of sight can be developed from each camera to points on the object. These lines of sight are usually called projection rays owing to their optical and geometrical nature.

A standard approach to classifying photogram-metric methods is based on the relative location of a camera and objects during a photographic process: In terrestrial, or close-range, photogram-metry, the photographic camera and the objects are both placed on Earth's surface, and the camera is pointed horizontally (parallel to the ground surface), while in aerial photogrammetry, the [Page 2174]camera is above the object and is pointed vertically, toward the ground.

In close-range photogrammetry, the photographs are taken by terrestrial photographic cameras (phototheodolites), placed on nonmovable tripods to take pictures from the ground. As vertical aerial pictures are taken from moving platforms (aircraft and satellites), the cameras are specifically designed to take measurable photographs while the platform flies along the path. An aerial photographic camera is an assembly of high-precision optical lenses and mechanical, electrical, and electronic components. Aerial photographic cameras are designed to take pictures using panchromatic or color photographic films that can be as long as 120 meters, with picture frames as wide as 40 cm (centimeters; 23 cm × 23 cm is a typical size of a frame image). Apart from classical photographic films, modern image acquisition systems use different digital technologies and acquire imagery directly in digital form.

When obtaining vertical aerial photographs, the aircraft normally flies in a series of lines, called flight runs. Photos are taken in rapid succession looking straight down at the ground, often with a 60% overlap between adjacent photos. The overlap ensures total coverage along a flight line and also facilitates stereoscopic viewing. The distance the camera moves between exposures is called the air base. Sequences of photographs in one single flight run form a strip of overlapping images; several runs, in turn, compose a block (mosaic) of aerial photographs covering large areas.

Aerial photographs often contain imprinted information such as the date and time of image acquisition, camera focal length, flight altitude, and other data, allowing calculation of image scale and approximate tilt of the particular frame. Fiducial marks, usually imprinted alongside photographs, allow precise measurement using pho-togrammetric instruments.

The accuracy of photogrammetric measurements in photographs is described in terms of micrometers (µm). A standard stereocomparator (photogrammetric equipment used for precise measurement) records the coordinates of points in an image as accurately as 5 µm; top-precision comparators target a 1- to 2-µm accuracy of image measurements.

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