The purpose of this chapter is to review the principles of photogrammetry. The chapter contains background information and references that support the standards and guidelines found in the previous chapters. Section I reviews the basic elements of photogrammetry with an emphasis on obtaining quantitative information from aerial photographs. Section II discusses basic operational principles of stereoplotters. Section III summarizes the datums and reference coordinate systems commonly encountered in photogrammetric mapping. Section IV discusses the principles of aerotriangulation. Section V provides background information for mosaics and orthophotographs. A more generalized nontechnical overview of photogrammetry may be found in Appendix C. Section I Elements of Photogrammetry 10-2. General The purpose of this section is to review the basic geometry of aerial photography and the elements of photogrammetry that form the foundation of photogrammetric solutions. 10-3. Definition Photogrammetry can be defined as the science and art of determining qualitative and quantitative characteristics of objects from the images recorded on photographic emulsions. Objects are identified and qualitatively described by observing photographic image characteristics such as shape, pattern, tone, and texture. Identification of deciduous versus coniferous trees, delineation of geologic landforms, and inventories of existing land use are examples of qualitative observations obtained from photography. The quantitative characteristics of objects such as size, orientation, and position are determined from measured image positions in the image plane of the camera taking the photography. Tree heights, stockpile volumes, topographic maps, and horizontal and vertical coordinates of unknown points are examples of quantitative measurements obtained from photograph.