Using lasers to measure depths and heights

While echosounders are the most common way of measuring water depths today, lasers can be used in shallow, clear water. Lasers also can be used to measure heights on land near the shore, or to locate shoreline structures.

Bathymetric lidar

  • Illustration of the coverage of a LIDAR flight line, compared to the coverage of the multibeam lines required to survey the same area.

    Illustration of the coverage of a LIDAR flight line, compared to the coverage of the multibeam lines required to survey the same area.

  • Composite of LIDAR data acquired during 3.5 hours of flight, and multibeam data aquired during 2 days of hydrographic survey, overlayed over a nautical chart showing the area of Winona and Grimsby, Ontario.

    Composite of LIDAR data acquired during 3.5 hours of flight, and multibeam data aquired during 2 days of survey, overlayed over a nautical chart showing the area of Winona and Grimsby, Ontario.

  • Coverage of LIDAR survey in Cape Ray, Newfoundland

    Coverage of LIDAR survey in Cape Ray, Newfoundland.

  • Result of LIDAR survey and postprocessing in Cape Ray, Newfoundand.

    Result of LIDAR survey and postprocessing in Cape Ray, Newfoundand.

  • Result of LIDAR survey and postprocessing in Cape Ray, Newfoundand.

    Result of LIDAR survey and postprocessing in Cape Ray, Newfoundand.

  • Result of LIDAR survey and postprocessing in Cape Ray, Newfoundand.

    Result of LIDAR survey and postprocessing in Cape Ray, Newfoundand.

Lidar, which stands for light detection and ranging, uses the time of flight of a laser pulse to determine distance. Since light travels at roughly 3 billion metres per second (m/s), this requires very precise timing to measure distance accurately. Bathymetric lidar uses a blue-green laser, which penetrates clear water easily. Airborne bathymetric lidar (ALB) deploys two lasers from an aircraft (fixed-wing or helicopter): a red laser measures the height of the sea surface since it does not penetrate the water; and the blue-green laser measures the distance to the seabed, provided enough backscatter energy returns to the aircraft. The time difference between the two laser returns gives the depth of water, after some sophisticated signal processing.

Bathymetric lidar works very well in depths up to about 50 metres, when the water is clear and the seabed is light-coloured, such as around coral reefs. It is not suitable for measuring depth if the water is turbid or if there are suspended materials in the water column, such as air bubbles, fish or kelp.

 

Topographic lidar

  • Terrestrial test of Riegl VZ400 mobile LiDAR that would be mounted on CHS' multibeam launch 'Harlequin' during summer 2012.

    Terrestrial test of Riegl VZ400 mobile LiDAR that would be mounted on CHS' multibeam launch "Harlequin" during summer 2012.

  • Bay Bulls Harbour, Avalon Peninsula, Newfoundland. Topographic LIDAR data that was collected from two scans that were taken from opposite sides of the harbour approximately 1/2 a kilometre apart.

    Bay Bulls Harbour, Avalon Peninsula, Newfoundland. Topographic LIDAR data that was collected from two scans that were taken from opposite sides of the harbour approximately 1/2 a kilometre apart.

  • Bay Bulls Harbour, Avalon Peninsula, Newfoundland. Topographic LIDAR data that was collected from two scans that were taken from opposite sides of the harbour approximately 1/2 a kilometre apart.

    Bay Bulls Harbour, Avalon Peninsula, Newfoundland. Topographic LIDAR data that was collected from two scans that were taken from opposite sides of the harbour approximately 1/2 a kilometre apart.

Topographic lidar can measure elevations on land using the travel time of a red laser pulse. Airborne lidar mapping (ALM) uses a large number of near-vertical laser pulses to develop a digital elevation model (DEM) of the terrain. As topographic lidar also uses backscattered energy to determine distance, multiple travel times are possible for each beam in areas with tree cover. Many lidar applications require “bare-earth” DEM to be developed, so one task of lidar analysis involves identifying and removing the objects that sit on the ground, such as trees and buildings. For CHS, topographic lidar is useful for delimiting the nearshore uplands and the foreshore. It can also be used to estimate the location of the high water line.

Lidar reflectivity

Measuring time of flight of a signal relies on some energy being backscattered from the object of interest. This is true for echosounders requiring an acoustic backscattered signal from the seabed. It is also true for backscattered light energy from any lidar system. And just as backscattered acoustic energy can give us information about the properties of the seabed, so can lidar backscatter provide information about the seabed or topography from the strength of the reflectance.

Scanning laser systems

Lidar systems do not need to be aimed vertically to be useful for mapping. Horizontally aimed scanning laser systems have been used for decades in all types of surveying applications, from monitoring deformations of earthen dams, to monitoring the stability of ship wrecks. These systems can also be used from moving platforms, such as a truck or a launch. In the launch application, a scanning laser pointed at the shoreline allows precise measurements of all shoreline construction and the natural shape of the foreshore, high water line and nearshore topography. Of course, these systems can only map what they can “see” so objects that are obscured by, for example, a moored ship will not be mapped until the ship leaves its berth.