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Upham Beach Video Monitoring: Technical Description
In November, 1996 a video camera was installed on the tenth-floor roof of a building, looking south
along the Upham beach shoreline. It is a black and white digital camera connected directly to a PC.
The system is configured to acquire one still image and one ten-minute time exposure
of the beach once every daylight hour. These images are 640 pixels wide and 480 pixels high with 256
light intensity levels. In the near-field this provides horizontal and vertical resolutions of about 0.1 m
and 0.2 m. In the far-field these degrade to about 1 m and 10 m, respectively.
In order to obtain quantitative information from the video imagery, it is essential to relate the reference
frames of the image and the beach. Holland et al (1997) provide such a technique. There are two types
of requirements for this procedure. One is knowledge of certain technical parameters of the camera
(e.g. lens distortion), and these are measured in the laboratory. The other refers to a basic set of
geometric information about the field deployment. The camera location must be known relative to
a beach reference frame.
In addition, Ground Control Points (GCP's) must be placed within the field
of view of the camera. These GCP's are objects (in our case, discs) visible in the camera imagery. They
are surveyed and given three-dimensional coordinates in the beach reference frame. Two GCP's provide
sufficient information to obtain the geometric transformation between the image and beach coordinate
systems. Additional GCP's allow a solution of least-squares error to be found.
A simple application for the transformation is the conversion of the camera image to an image as seen from directly above the beach. In practice, the geometric transformation is calculated using Matlab software written by Holland et al. (1997).
References
Bowen, A.J.; The Generation of Longshore Currents on a Plane Beach; J. Mar. Res., v27, May 1969.
Holland, K.T.; R.A. Holman; T.C. Lippmann; J. Stanley; and N. Plant; Practical Use of Video Imagery in Nearshore Oceanographic Field Studies; IEEE J. Oce. Eng., v22, Jan 1997.
written by Kurt Hanson |
