||Experimental study is an important methodology for ocean engineering research. However, measuring any physical parameters in the |
field involves the instrumentation to overcome a wide range of problems, such as robustness to the severe environment, limitation
of power supply and data storage, and also simplicity of operation. Taking seabed profile or bedform measurement as an example,
conductivity point gauge or ultra-sound non-contact profiler is usually adopted. Limited by the time needed per sampling, these
approaches are costly in operation if dense grid points are required to describe the variation of a long transect. In addition to
this drawback, the surface will be disturbed after conductivity-based contact probing. We propose using an off-the-shelf,
non-metric CCD camera along with a simple calibration methodology as an alternative to carry out the measurement. As the semiconductor
technology advances drastically, nowadays high quality CCD cameras are available with inexpensive prices. Recently, CCD camera emerges
as a convenient input sensor for many applications. However, generally it requires a delicate optical and geometrical calibration of
the camera before it can be used to carry out 2D or 3D measurement of the target. The optical parameters are focal length, distortion
of lens, optical axis offset, CCD array linearity and etc; and geometrical parameters are position and orientation of the CCD camera.
Some of these parameters are sensitive to the setup of the system, and a re-calibration is needed whenever the system is disassembled
or moved. We propose using a template on which grid points of known locations are used to construct several sub-mappings between measurement
coordinate system and image pixel coordinate system. This simple procedure is effective to meet the accuracy requirement for several applications.
In this work, this idea is adopted and verified in three different experiments: An underwater laser line scanner, a cross-section wave tank bedform
profiling and Particle Imaging Velocimetry.