Rephotography is the act of repeat photography of the same site, with a time lag between the two images; a "then and now" view of a particular area. Some are casual, usually taken from the same view point but without regard to season, lens coverage or framing. Some are very precise and involve a careful study of the original image. Long a technique for scientific study, especially of changing ecological systems, it became formalized as a form of photographic documentary in the middle 1970s. The founding work in this style was the Rephotographic Survey project, conceived in 1977 by the project's chief photographer, Mark Klett. This project engaged 120 sites of government survey photographs from the American West first recorded in the 1870s. The resulting book, Second View, The Rephotographic Survey Project, included precise rephotographs of the same locations 100 years later along with an essay by Klett on the methodology and problems encountered with rephotography. Klett revisited these sites a third time for his 2005 book Third View with a new team of photographers including Byron Wolfe, Michael Marshall and Toshi Ueshina. The acknowledged master of urban rephotography is generally agreed to be Camilo José Vergara, who has worked since the 1970s in most of the major urban centers of the United States. Unlike the Rephotographic Survey, which recorded dramatic changes over approximately a century, Vergara's pictures record often small, incremental changes: the abandonment and adaptive reuse of housing project sites, changes in use of buildings and sites as sociological and economic conditions change.