J. Raper et al.
404
processing systems. Digital photography has recently been tested as a capture method
(Koh and Edwards, 1996) achieving 1524 by 1012 pixel 8 bit images at a rate of one
frame every 2.75 seconds which required 1.54Mb of storage space. Historically, aerial
photographs are also not georeferenced precisely, photograph positions being plotted
on maps from flightlines and by approximation from photograph content. Some
modern aerial photography (such as the digital photography by Koh and Edwards
(1996)) is now referenced by the Global Positioning Systems (GPS) coordinates for the
plane at the time the photograph was taken. However, in general historical aerial
photographs can only be mosaicked together to make an imagemap if there is
sufficient overlap between the photographs, and if there is ground control visible in the
overlap between the photographs.
Without ground control (or georeferencing of the photographs by GPS)
pattern matching techniques to overlap photographs can be used, although these
techniques are difficult to use when there are no regular geometric shapes such as
buildings visible in the imagery. If there is sparse ground control that is only on some
photographs then aerotriangulation techniques can be used to transfer control from one
photograph to another. However, this is not very accurate when the control is always
on one edge of the photographs, as is often the case in coastal photography. Aerial
photography can be stereocorrelated to generate a surface elevation model accurate to
centimetres if there is heighted ground control visible in the overlap between
photographs. In general the lack of photograph georeferencing or ground control in
air photography at the coast can be a significant barrier to its use in mapping or
stereocorrelation.
Aerial videography is an alternative low cost, lower resolution form of
(generally optical band) machine readable imagery (recorded on magnetic tape) which
is more cost-effective for the end user to obtain than aerial photography (King, 1996;
Um, 1997). Imagery can be shot either vertically or obliquely depending on the aerial
platform available. For the cost of a set of 150 aerial photographs taken with a large
format camera the user can commission multi-hour aerial videography runs along the
coast at the appropriate state of the tide or on geomorphologically appropriate
occasions. Fig. 4 shows a 24 bit videography image of Far Point from April 1997,
Scolt Head Island at low tide with a 1m ground resolution and requiring 1.2Mb of
storage space. This allows ground control to be placed at key sites of interest before
the flight and allows flight planning to ensure that control appears in the actual
imagery obtained. Since aerial surveyors can review the captured imagery whilst in
the air, sites can be reflown to obtain the necessary coverage and ground control
visibility.
404
processing systems. Digital photography has recently been tested as a capture method
(Koh and Edwards, 1996) achieving 1524 by 1012 pixel 8 bit images at a rate of one
frame every 2.75 seconds which required 1.54Mb of storage space. Historically, aerial
photographs are also not georeferenced precisely, photograph positions being plotted
on maps from flightlines and by approximation from photograph content. Some
modern aerial photography (such as the digital photography by Koh and Edwards
(1996)) is now referenced by the Global Positioning Systems (GPS) coordinates for the
plane at the time the photograph was taken. However, in general historical aerial
photographs can only be mosaicked together to make an imagemap if there is
sufficient overlap between the photographs, and if there is ground control visible in the
overlap between the photographs.
Without ground control (or georeferencing of the photographs by GPS)
pattern matching techniques to overlap photographs can be used, although these
techniques are difficult to use when there are no regular geometric shapes such as
buildings visible in the imagery. If there is sparse ground control that is only on some
photographs then aerotriangulation techniques can be used to transfer control from one
photograph to another. However, this is not very accurate when the control is always
on one edge of the photographs, as is often the case in coastal photography. Aerial
photography can be stereocorrelated to generate a surface elevation model accurate to
centimetres if there is heighted ground control visible in the overlap between
photographs. In general the lack of photograph georeferencing or ground control in
air photography at the coast can be a significant barrier to its use in mapping or
stereocorrelation.
Aerial videography is an alternative low cost, lower resolution form of
(generally optical band) machine readable imagery (recorded on magnetic tape) which
is more cost-effective for the end user to obtain than aerial photography (King, 1996;
Um, 1997). Imagery can be shot either vertically or obliquely depending on the aerial
platform available. For the cost of a set of 150 aerial photographs taken with a large
format camera the user can commission multi-hour aerial videography runs along the
coast at the appropriate state of the tide or on geomorphologically appropriate
occasions. Fig. 4 shows a 24 bit videography image of Far Point from April 1997,
Scolt Head Island at low tide with a 1m ground resolution and requiring 1.2Mb of
storage space. This allows ground control to be placed at key sites of interest before
the flight and allows flight planning to ensure that control appears in the actual
imagery obtained. Since aerial surveyors can review the captured imagery whilst in
the air, sites can be reflown to obtain the necessary coverage and ground control
visibility.