Traditional methods for measuring the water depth rely on sonar soundings. However airborne techniques offer the advantages of increased survey speed and operation over dangerous waters that may be affected by very strong tides and the presence of shoals and reefs that limit the operation of surface vessels. The airborne lidar method has been used very successfully for mapping coastal waters and relies on the difference in the time between a laser pulse (infra red wavelength) reflected from the sea surface and a separate laser pulse (blue-green wavelength) reflected from the sea floor. The depth of investigation, typically 50 to 70 m in ideal conditions, depends strongly on water clarity (turbidity), as well as other factors (e.g. sea state, surf zone, sea bottom reflection) and weather conditions. These lidar systems provide dense depth soundings, typically a grid with a laser spot spacing of 4 to 5 m, and they meet the accuracy standards of the International Hydrographic Organisation (IHO).