Wave Gauges

Wave Gauges are made in several ways. Electric wave staffs measure wave height by changes in resistance. More common is using a pressure sensor with a current meters in a technique called PUV. Wave height is measured as changes in pressure. With an array of three or more sensors, wave direction can be determined.
Another method uses Doppler current meters to form a virtual wave array. This is done by bottom mounting an upward-looking, multiplebeamed Doppler so it can measure the range to the surface as well as the orbital wave velocities in a series of bins extending away from the instrument. These mani fold measurements allow one to differentiate among multiple sources of waves—something that a single point sensor cannot do.
Wave heights can also be measured from vessels or piers using microwave altimeters. An accelerometer is used to remove ship motion from the wave amplitude measurement.
Buoys can be used to measure waves if they have an elastic mooring line that allows them to travel with the waves. By using three orthogonal accelerometers, the buoy motion can be used to measure wave height and direction. Another technique uses the Doppler shift of GPS signals to determine buoy movement. By analyzing the signal from several GPS satellites, wave height and direction can be determined.
A good primer on wave theory and measurement may be found at http://cdip.ucsd.edu/?nav=documents&sub=index&xitem=waves#gauging
For info on wave measurement instruments used at the Scripps Institution of Oceanography, see http://cdip.ucsd.edu/?nav=documents&sub=index&xitem=gauge