Bathymetry and Water Column Correction using 4SM – A HyPhoon user success story

This is part of a series on user success stories that showcase applications and accomplishments using HyPhoon datasets. Send us your story!

The Accomplishment: Dr. Yann Morel derived bathymetry and water column corrected reflectance for Heron Reef using his Self-calibrated Supervised Spectral Shallow-sea Modeler (4SM). Output includes water depth, as shown below, and water column corrected reflectance at the seafloor. Both products are being added to the Heron Reef dataset, and will soon be available for the community to download.

4SM Heron Reef

CASI hyperspectral (left) and 4SM derived bathymetry (right)

Data: The source data used for this success story was the 2002 CASI hyperspectral imagery from the Heron Reef dataset provided courtesy of the Center for Spatial Environmental Research at the University of Queensland. Heron Reef is located at the southern end of the Great Barrier Reef in Australia. The imagery has 1 m pixels and a spectral range from 400-800 nm with 19 spectral bands.

4SM Overview: The 4SM model is based on established physical principles of shallow water optics, operates without need for field data or atmospheric correction, and works with both multispectral and hyperspectral imagery. The model assumes both water and atmospheric conditions are uniform throughout the given scene, requires the presence of both deep water and bare land pixels, and is most applicable to clear shallow water at 0-30 meters depth.

At its core, 4SM utilizes a variant of Lyzenga’s method to calculate the slopes for all bi-dimensional band pairs, which are then used to interpolate diffuse attenuation coefficients for all visible bands. Surface glint is minimized using information from a NIR or SWIR band, bare land pixels are used to derive the slope of the soil line and the water volume reflectance, and deep water pixels are used to approximate deep water radiance. All of this information is then combined to drive an optimization approach for estimating water depth.

Output ultimately includes both water depth and water column corrected reflectance, which can both be used for further habitat and geomorphic analyses.

For more on 4SM:

To access HyPhoon data:

Satellite-Derived Bathymetry – New products available from DigitalGlobe

DigitalGlobe recently hosted a webinar describing the new bathymetry and benthic products that are being derived using imagery from their WorldView-2 sensor. The methodology uses a set of algorithms developed ‘in-house’ at DigitalGlobe, and while current capabilities show excellent results, plans are already underway to deliver additional improvements.

DigitalGlobe bathymetry

As a quick refresher, WorldView-2 is a high-resolution satellite with 8 multispectral bands at 1.85m spatial resolution and one panchromatic band at 0.46m resolution. Included in the multispectral bands is a new coastal band (400-450nm) that provides enhanced water penetration and hence improved capabilities for aquatic remote sensing. Note that Landsat 8 has adopted a similar approach to band selection, now including a similar coastal band (430-450nm), albeit at the coarser spatial resolution of 30m. WorldView-2 was launched on October 8, 2009 from Vandenberg Air Force Base, and interested users have the option of either ordering archived imagery or scheduling new acquisitions for their particular study area.

The WorldView-2 benthic products include options for water depth, water quality and bottom type classification. Since all three products are inherently interrelated in terms of what the satellite sensor measures, rather than independently derive the individual products, DigitalGlobe utilizes a methodology that resolves all three products simultaneously.

For those interested in specifics, the methodology uses HydroLight to generate a table of surface reflectance spectra based on discrete combinations of water depth, water optical properties and bottom reflectance, and then employs a spectral matching technique to find the simulated spectra that best match the spectra for each measured pixel. The output for each pixel is then assumed equivalent to the parameters used for generating the matching spectra in HydroLight.

The recent webinar focused specifically on output for the water depth product, with example validation results shown for the Bahamas (Lee Stocking Island, Princess Cays and West End), Puerto Rico and St. Croix. In all cases the WorldView-2 output shows strong agreement with acoustic and lidar bathymetry measurements. However, it is noted that accuracy decreases with increasing turbidity, which is to be expected given the inherent physical limitations of this imaging problem (i.e., light needs to be able to penetrate through the water column, reflect off the bottom, and propagate back to the sensor).

DigitalGlobe estimates the following specifications for these products. Water depth is accurate to 10-15% actual depth in clear water over sand to a maximum depth of 20-30m, and accurate to 15-20% actual depth in clear water over dark substrates (sea grass, algae, coral) to a maximum depth of 15m. The other products are still being evaluated, but current indications are that water quality output will be valid to 20-30m in clear water and 5m in turbid water, and bottom classification will be valid to 20m over sand and 15m over dark substrates.

DigitalGlobe bathymetry

Whereas the current technology for these products relies on the spectral domain, future improvements will focus on incorporating the angular and temporal domains. Specifically, this includes taking advantage of WorldView-2 capabilities to obtain multiple sequential views from different angles. This allows calculations of stereoscopic relationships as well as wave kinematics (i.e., wave motion), which can both be used to derive information on bathymetry and thereby improve the product.

If you’re interested in seeing the full webinar:

Images supplied courtesy DigitalGlobe.