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Changes in the area and condition of samphire marshes with time. In: McComb, A.J., Kobryn, H.T. and Latchford, J.A. (eds) Samphire marshes of the Peel-Harvey estuarine system Western Australia.

Glasson, R.L., Kobryn, H.T. and Segal, R.D. (1995) Changes in the area and condition of samphire marshes with time. In: McComb, A.J., Kobryn, H.T. and Latchford, J.A. (eds) Samphire marshes of the Peel-Harvey estuarine system Western Australia. Peel Preservation Group and Murdoch University, Perth, Western Australia.

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    Abstract

    Aerial photography is especially suited to the study of vegetation, water resources and shoreline mapping (A.S.O.P, 1968). Air photographs provide a perspective of the earth’s geographical features that are generally readily understood. Air photographs also provide a plan view that can be spatially compared to the knowledge an individual may have about a similar area. For those reasons aerial photographs frequently provide working documents for planners and managers. They are however, limited in spatial accuracy because the images suffer geometric distortions, particularly near photograph margins or when terrain varies in height. Also, a single aerial photograph rarely covers an entire study area. There are manual and computer assisted techniques of joining air photographs and also eliminating the geometric distortions within and between individual photographs. An assembly of aerial photographs is called a photo mosaic. Photo mosaics can be "controlled" or "uncontrolled", the former having the geometric distortions removed.

    The amount of geometric distortions in an aerial photograph depends on many factors, including the physical optics of the camera and the orientation of the camera at the instant of exposure. Where the optical axis of the camera is near vertical (<3° from vertical), then the photograph is accepted as vertical. The point where the optical axis of the camera meets the earths surface is the "principal point" of the photograph. Vertical photographs are the most common type of "metric" aerial photograph, or one that is used to derive information about spatial measurements of geographical features. Geometric distortion of aerial photographs increases outwards from the principal point towards the margin. The distortions are increased if photography is acquired at low altitude, or with cameras using wide angle (>70°) and super-wide angle (>100°) lenses. Gross distortions of scale occur on individual photographs when terrain slope changes suddenly, as when a scarp, cliff or portion of a mountain is included in the photograph (Maling, 1989). Some of the distortions in vertical air photography can be avoided by using only central portions of each photograph.

    Publication Type: Report
    Murdoch Affiliation: School of Environmental Science
    Series Name: Samphire marshes of the Peel-Harvey estuarine system Western Australia
    Publisher: Peel Preservation Group and Murdoch University
    URI: http://researchrepository.murdoch.edu.au/id/eprint/6102
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