Photogrammetry, << FOH toh `gram` uh tree, >> involves the use of photographs or digital images (pictures in the form of numeric code) in obtaining accurate information about physical objects. Such information includes the size of the objects as well as their positions with respect to one another and on a map projection. Photogrammetry is an especially important tool in mapmaking.
Photogrammetrists begin their work by obtaining images of the area or objects of interest. They may use any of several types of photogrammetry. Interpretative photogrammetry involves recognizing in images such objects as roads, rivers, and vegetation. Metric photogrammetry involves taking measurements of objects in images and using known geometric relationships to determine positions, distances, elevations, areas, and volumes. Aerial photogrammetry uses images of Earth taken from an airplane or satellite, and terrestrial photogrammetry uses images taken from ground level.
Devices called remote sensors can capture information along portions of the electromagnetic spectrum not visible to the naked eye. Earth’s curvature, camera lens distortions, refraction (bending) of light rays by the atmosphere, and other geometric factors affect the objects’ positions in the image. Before producing maps, photogrammetrists must make adjustments for these effects to find the correct positions of objects.
In metric photogrammetry, a camera takes overlapping photographs of the area to be mapped. A device called a stereoplotter then combines information from these photographs to produce a distortion-free, three-dimensional computer model. Using this model, a photogrammetrist can produce maps or related products.
Photogrammetric techniques were first used in 1840, when the French physicist and astronomer François Arago demonstrated the use of photographs in surveying elevations and other surface features of land. In 1859, Aime Laussedat, a colonel in the French Army, announced the first successful use of photographs in surveying and mapping.
