Definition: Bathymetry is the study of under water (ocean, lake) topography where as bathymetric LiDAR is an airborne acquisition technology used to capture a continues topographic data of both land and seafloor at the same time. This is the most effective and cost efficient data capturing method applied in many important issues like:
- Fundamental understanding of risk area of high vulnerability
- To provide detailed 3D elevation models along cost lines
- To capture on both sides of the cost line, over areas stretching more than 100km along the cost, has made it the ‘gold standard for coastal’ vulnerability and nearshore bathymetric habitat modeling.
Principle of Bathymetric LiDAR
It works based on principle on differential timing of laser pulses reflected directly from the water surface and reflected under the water bottom (sea floor).
Bathymetric LiDAR is also slightly different from its correspondent airborne topographic LiDAR in that the airborne topographic LiDAR using a light with wavelength of 1,064nm where as bathymetric LiDAR uses a light with 532nm wavelength.
The simplified components of Bathymetric LiDAR
As its corresponding topographic LiDAR system do the bathymetric LiDAR system can be simplified to four components.
- Gps receiver to define the aircrafts position.
- The IMU (inertial measurement unit) to define the roll, yaw and pitch
- The leaser scanner to emit the signal in a particular pattern.
- The sensor which reads the Return.
The position and orientation from the above components of bathymetric LiDAR enables the system to record accurate measurements. Some of the sensors can measure 100,000 points per second even more, implies 10 points/m2 in shallow water, and with a depth of up to 75m.
It’s Adverse Impacts: By comparison bathymetric surveys are more vulnerable to the adverse impacts of environmental effects, than their counterparts topographic surveys which can result in data gaps, reduced data coverage and measurement quality.
Factors which Should be focused: Understanding and managing of some special conditions in bathymetric LiDAR can mean the difference between success and failure in its whole process. For a successful bathymetric LiDAR the following factors need to be considered such as weather for flying, air traffic controls, turbidity, tides, sea state, vegetation condition and ground control accessibility.
The major hindrance of shallow water from bathymetric LiDAR sensors is water clarity, or lack thereof. Where as high turbidity, sea grasses and low-reflectance seafloors pose risks to the success of a survey with a LiDAR System.
Charactorstics of Bathymetric LiDAR Sensors In shallow and in deep water.
LiDar system for depth water (>10m) survey generally have more laser power/pulse but a lower measurement frequency (low resolution), a larger laser footprint and receiver field-of-view than LiDAR systems for shallow (<10m) water systems. These deep-water bathymetric LiDAR systems vary in depth penetration capability from between 2.0 to 3.0 times the Secchi depth (a disc to measure water transparency) measurement. Bathymetric survey operators nowadays utilizes shallow-water and deep-water sensors simultaneously in twin optical port survey aircraft to maximize detail and coverage.
The scan patterns for sensors are composed of the shape, tilt and method.
The scan shapes vary between:
- Circular arc,
- Elliptical arc
- Non-Oscillating Elliptical Arc
NB: The circular and elliptical scanners are able to look forwards and backwards, increasing the number of times an area is sampled, although this can result in oversampling along the edges of the scan where as the remaining shapes are usually tilted forward or backwards with respect to the aircraft.
The scan methods vary between:
- Oscillating mirrors;
- Rotating prisms;
- Palmer scanners;
- Rotating multi-facet mirrors and
- Oscillating raster scanners
All of which result in slight differences in the scan pattern and can be seen in the subsequent point cloud.
Decision to Buy a Bathymetric LiDAR System: The best Bathimetric LiDAR survey sensors can be sensors which are manufactured by the considerations of all the great qualities that were tried to be incorporated above in our article. Experts recommend that the individual characteristics of the sensor and their environmental impact should be examined as well. Obviously, the practical experience and knowledge and expertise of the operator actuates the final decision at a certain level (i.e. Operators with best experiences in this survey should be involved in the decision). From a LiDAR perspective, points per meter, avoiding shadows and swath width seem to be the most desired features. The components that should be reconsidered during buying such a system can much depend on the often intended purpose of the expected outputs of the survey, its area, environment.
For example: usually, what type of environment is expected to be surveyed?, the water clarity situations that we have seen above. These and all use and usability analysis by reverse engineering should ofcourse be considered to reach the best decision in buying and planning to buy process.