FIGURE 2.3 Accurate photogrammetric reconstruction of the imaged terrain requires overlapping images and metadata. (Top) Overlapping frame images produced by an aircraft; metadata include aircraft location determined from a constellation of GPS satellites, orientation determined from an inertial navigation system, and/or GPS-determined ground control (red triangle). (Bottom) Overlapping images produced by linear array scans from a satellite.
determine all three ground coordinates of any target point. Unless one of the three coordinates is known, such as the elevation from a digital elevation model, two or more images are required to accurately recover all three dimensions (Figure 2.4). Imagery, sensor and platform parameters, and metadata such as that from GPS and INS (inertial navigation system) are used in the photogrammetric exploitation.
Most photogrammetric activities deal with cameras and sensors that are carefully built and calibrated to allow direct micrometer-level measurements. However, an important branch of photogrammetry deals with less sophisticated instruments, such as those found on mobile phones, which require careful modeling and often self-calibration. This branch is gaining importance as the availability of imagery from nonmetric cameras grows.
Many digital photogrammetric workstations enable the overlap area of two images to be viewed stereoscopically. Automated algorithms are commonly used to extract 3D features with high accuracy. Frequently, however, human judgment is required to edit, or sometimes to override, the results from such algorithms.
Photogrammetry began as a branch of surveying and was used for constructing topographic maps and for military mapping. It is still sometimes taught in surveying departments. Technological advances in surveying, the growth of photogrammetry, and the inclusion of related fields, such as geodesy, remote sensing (Box 2.2), cartography, and GIS, made the title “surveying” or “surveying engineering” inadequate for a department. The name geomatics or geomatics engineering was introduced to better capture this range of activities (see Box 2.1). At present, photogrammetry is taught in geomatics departments, as well as in other departments, such as geography and forestry.
Photogrammetry has gone through three stages of development: analog, analytical, and digital (Blachut and Burkhardt, 1989). Analog instruments were built to optomechanically simulate the geometry of passive imaging and to allow the extraction, mostly graphically, of information in the form of maps and other media. As computers became available, mathematical models of sensing were developed and algorithms were imple-mented