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What is Satellite Derived Bathymetry?

  • This week our Blog is from Ross Smith, Ross works as a Geospatial Data Specialist in our Denver office.

    Accurate and consistent bathymetric mapping of coastal marine environments has long challenged hydrographers. These euphotic regions are inherently challenging due to shallow depths, shifting seabed features, and often-dynamic ocean currents. Traditional hydrographic survey techniques such as single and multi-beam echosounders are often not suitable, practical or safe under these unique circumstances. Additionally, these methods typically require large investments of money, time and energy, while risking lives and equipment.

    Satellite Derived Bathymetry

    Accurate depictions of coastal bathymetric features are critical to the majority of end-users, luckily modern remote-sensing technology offers a powerful alternative solution. Satellite derived bathymetry is currently at the forefront of the oceanographic industry,  circumventing the above issues whilst simultaneously cutting costs, reducing risk, and increasing the ability for iterative surveying.

    So, how does it work? What are its capabilities? Does it have limitations? Let’s find out.

    How it works

    The principle concepts driving satellite derived bathymetry first came about in the 1970s, when multispectral satellites sensors such as the those aboard the Landsat satellite began orbiting Earth. Multispectral sensors collect specific wavelengths of energy along the electromagnetic spectrum, referred to as “bands”. Typically, sensors collect reflected bands of visible light like red, green, and blue, and may include other non-visible reflected and emitted bands such as near and far infrared. At the core of satellite derived bathymetry is the connection between this reflected energy and water depth. Essentially, for each pixel of a given satellite image where the sea floor is visible, there is a statistical relationship between the amount, and type of energy that the sensor detects, and the depth of the water at that pixel location. This relationship can be exploited with computational algorithms.

    Currently, most established satellite derived bathymetry algorithms require some registration with known depth points. These registration points help calibrate the algorithm to the imagery by giving the computation a frame of reference. This tells the computer that when the seafloor is at a certain depth, specific types and intensities of energy (light) are collected by the sensor. Algorithms can also utilize indexes of known sea-bottom types and their specific reflectance signatures to further refine accuracy. This is important, because while it may stand to reason that deeper water is darker, some seafloor types like seagrass or coral can reflect light that throws sensors off, creating erroneous depth values. Understanding this, and other nuances to the SDB process is just another piece to the puzzle that is bathymetry.

    Multispectral sensors collect specific wavelengths of energy along the electromagnetic spectrum, referred to as bands

    Satellite derived bathymetry sounds great, right? It is! Nevertheless, understanding its capabilities is essential to proper allocation of resources. Limiting factors of satellite derived bathymetry are all about how clearly and accurately you can see the ocean floor from space. This usually means that the process will only resolve depths up to the point of light extinction. Under optimal conditions, light extinction depth is around 20-30 meters. Additionally, coastal areas with lots of algal bloom, suspended sediments, or intense wave action, can all limit satellite derived bathymetry viability. However, with the right conditions, Satellite derived bathymetry directly addresses the issues of bathymetric mapping of coastal regions.

    The resolution of the final product is only limited by the sensor resolution, the process needs no “boots on the ground”, and requires less limited time and energy to produce. The 2m Bathymetry product, which has been created in close collaboration with DHI, and is derived from DigitalGlobe WorldView-2 and WorldView-3 imagery, allows  for finite analysis of bathymetric features at a much lower cost, compared to high-resolution multibeam surveys. Our 10m & 30m resolution satellite derived bathymetry products utilize ESA Sentinel-2 imagery and US Landsat imagery, further lowering costs, and allowing for repetitious surveying; an imperative capability in the dynamic coastal regions.

    Contact TCarta today to find out how we can use this excellent technology to expand your bathymetric horizons!