Coastlines, shoals and reefs are some of the most dynamic and constantly changing regions of the globe.
Monitoring and measuring these changes is critical to marine navigation and an important tool in understanding our environment. Near shore bathymetry is currently calculated using high-resolution multispectral satellite imagery. However, with the introduction of WorldView-2’s higher resolution, increased agility and Coastal Blue band (400-450 nm), bathymetric measurements will substantially improve both in depth and accuracy, which can be cost-effectively used to apply in operation potentially to replace the tradtional marine surveying, in particular, up to 10 - 20 m in depth.
Blue band (396-460 nm) in WorldView-2 & -3 panatrating shallow water (coastal, lake, etc.)
Marine surveyors perform inspections of vessels of all types including oil rigs, ferries, cargo vessels and warships, pleasure craft, passenger vessels, tugboats, barges, dredges, as well as marine cargo, marine engines and facilities such as canals, drydocks, loading docks and more for the purpose of pre-purchase evaluation, insurance eligibility, insurance claim resolution and regulation compliance.
There are two established techniques for calculating bathymetry using multispectral satellite imagery: a radiometric approach and a photogrammetric approach.
The Radiometric Approach
The radiometric approach exploits the fact that different wavelengths of light are attenuated by water to differing degrees, with red light being attenuated much more rapidly than blue light.
Analysts have leveraged existing multispectral satellites’ ability to detect light in the blue (450 – 510 nm), green (510 – 580 nm) and red bands (630 – 690 nm) to achieve good depth estimates, in water up to 15 meters in depth. And, with the addition of sonar based ground truth measurements, they have achieved vertical and horizontal accuracies of less than 1 meter.
In order to improve bathymetric measurements, analysts have turned to airborne, high-resolution multispectral platforms. These sensors are able to detect light between 400 and 450 nm – the spectrum that provides the deepest penetration of clear water.Studies using these data have shown that accurate bathymetric measurements can be achieved up to 20 meters and deeper.
WorldView-2 is the first commercial high-resolution satellite to provide 1.84 m resolution multispectral imagery, plus a Coastal Blue detector focused on the 400 – 450 nm range. WorldView-2’s large single-pass collection capabilities will also make the application of ground truth data more accurate and reliable. Multiple small collections contain differences in sun angle, sea state and other parameters and it is challenging to calibrate one series of measurements and then apply them across a broad area.
Large synoptic collections, enabled by WorldView-2’s agility and rapid retargeting capabilities, allow analysts to compare the differing absorption of the Coastal Blue, Blue and Green bands, calibrate their bathymetric estimations using a few known points, and then reliably extend the model across the entire collection area.
The Photogrammetric Approach
In this method, stereoscopic images are collected over the target area, and a data elevation model (DEM) of the shallow ocean floor is produced from the imagery. Early studies with both satellite imagery, and digital photography appeared promising, and demonstrate that this technique can be used to provide accurate bathymetric models of shallow environments without ground truth. However, the technique has not been widely studied due to limitations in the capabilities of current sensors.
The challenge with collecting stereoscopic imagery of the shallow ocean floor is in how light interacts with the air/water interface. At high angles of incidence, light is completely reflected off the surface of the water, preventing any sub-aquatic features from being observed. Current multispectral satellite sensors are not able to collect enough high-resolution stereoscopic imagery within the narrow angle necessary to penetrate the ocean surface. In addition, none of them are able to measure the shorter wavelength blue light necessary for maximum depth penetration.
WorldView-2 will make this new method for measuring bathymetry possible. The Coastal Blue band will deliver maximum water penetration, and WorldView-2’s enhanced agility will enable the collection of large amounts of high-resolution intrack stereo imagery at the ideal angle for water penetration. The advantage of this approach is that multiple images can be registered using tie points that are visible on land and in the water, and the resulting stereo composite can be used to calculate water depth without relying on ground truth measurements. No other satellite is able to deliver this unique combination of high spatial and spectral resolution, agility and stereo collection capacity.
Please refer to the White Paper from DigitalGlobe in 2010 (attachment), and also the earlier study by Lee (2010).
Worth to mention, Lee 's study didn't showed an obvious correlation between the Coastal depth and Blue bands, because of only using the traditional classification methods in thier study...