General Information
This section presents basic information about the techniques and goals of the science of photogrammetry. Much of the material in this section is summarized from the text "Remote Sensing and Image Interpretation", by T. Lillesand and W. Kiefer, and the reader is referred to this primary source for more detailed information. In addition, entire texts are devoted to the subject, such as "Elements of Photogrammetry" by Wolf.
Photogrammetry can be defined as the science of obtaining reliable measurements and producing maps by means of photography. Photogrammetric techniques are required to accurately determine relationships of features on aerial photographs, including ground distances and angles, the heights of objects, and terrain elevations.
When a vertical aerial photo is taken, the only point on the scene that is directly below the camera axis is called the ground principal point. Lines drawn to connect marks located along opposite sides of the photo (fiducial marks) intersect precisely at the principal point. This relationship, together with the flying height and the camera focal length, form the basic coordinate system used to quantify various elements of the image.
Identifiable points on the ground (ground control points) are used to determine distances and geometric properties of the image, including camera height above the ground, degree of tilt at the time the photo was taken, and distance between photo centers, and to correct for distortions on the image. There are two types of distortion that commonly occur on aerial photographs: relief displacement and stereoscopic parallax. Relief displacement causes tall objects to appear to lean away from the center of the image, and parallax is the apparent shift of an object with respect to a point of reference (this can be seen in successive photographs, where there is a variation in ground elevation).
In order to correct (or rectify) these distortions, overlapping photos and ground control points are integrated to produce a stereo model of the terrain, which forms the basis of photogrammetric mapping and orthophotography. Ground control, or accurate geodetic data, is essential for all photogrammetric operations.
Several types of stereo plotters are used to produce maps from which image distortions have been corrected, but all consist of a projection system, a viewing system, and a measuring system. Stereoplotters essentially duplicate the position and orientation of the aerial camera at the time an individual stereo pair was acquired. The stereo model is then projected orthographically (perpendicular) to a map sheet, and adjusted by scaling and leveling to correct image distortions. By the mid 1990's it became practical to perform this work with scanned photoimages in a computer.
The most widespread use of photogrammetry is in the construction and revision of topographic maps. The US Geological Survey (USGS) relies on photogrammetric techniques to produce and update its topographic map series. Orthophotos are being produced for many parts of the country, and combine the constant scale of a map with the accuracy and detail of an aerial photograph. Modern mapping and orthophotography are just two of the products of the science of photogrammetry.