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Image preparation and viewing A typical hard copy aerial photograph is 23 cm square and has a number of characteristics. Four tick marks, known as fiducials, are located either at the four corners or at the centre of the four sides of the photograph. Opposite fiducials have a line joining them and where these intersect at the centre of the photograph is termed the principal point. Aerial photographs in digital form may not have these tick marks. The camera lens is therefore immediately above this principal point when the photograph is taken. Further information is often located along the edge of the photograph such as the time, date and altitude at which the photograph was taken. A serial number is also often included. In undertaking an image interpretation there are several factors that must also be considered by the interpreter. These include collecting any relevant sources such as maps, field reports etc. and identifying the special viewing equipment that may be available. It is also good practice to ensure images are systematically labeled
and indexed in order to cross-reference with other data sources such
as maps. Boundary delineations can be made directly on the images or
in a digital form if the necessary equipment is available. Delineations
are most often made on clear acetate and points are marked in order
to ensure the correct lining up of the image and acetate should they
be separated at any time.
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