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Satellite-derived near-shore bathymetry (SDB) is becoming an increasingly important method for assessing vulnerability to climate change and natural hazards in lowlying atolls of the northern tropical Pacific Ocean. Satellite imagery has become a cost-effective means for mapping near-shore bathymetry because ships cannot collect soundings safely while operating close to the shore. Also, green laser light detection and ranging (lidar) acquisitions are expensive in remote locations. Previous research has demonstrated that spectral band ratio-based techniques, commonly called the natural logarithm approach, may lead to more precise measurements and modeling of bathymetry because of the phenomenon that different substrates at the same depth have approximately equal ratio values. The goal of this research was to apply the band ratio technique to Landsat 8 at-sensor radiance imagery and WorldView-3 atmospherically corrected imagery in the coastal waters surrounding the Majuro Atoll, Republic of the Marshall Islands, to derive near-shore bathymetry that could be incorporated into a seamless topobathymetric digital elevation model of Majuro. Attenuation of light within the water column was characterized by measuring at-sensor radiance and reflectance at different depths and calculating an attenuation coefficient. Bathymetric lidar data, collected by the U.S. Naval Oceanographic Office in 2006, were used to calibrate the SDB results. The bathymetric lidar yielded a strong linear relation with water depths. The Landsat 8-derived SDB estimates derived from the blue/green band ratio exhibited a water attenuation extinction depth of 6 meters with a coefficient of determination R2=0.9324. Estimates derived from the coastal/red band ratio had an R2=0.9597. At the same extinction depth, SDB estimates derived from WorldView-3 imagery exhibited an R2=0.9574. Because highly dynamic coastal shorelines can be affected by erosion, wetland loss, hurricanes, sea-level rise, urban development, and population growth, consistent bathymetric data are needed to better understand sensitive coastal land/water interfaces in areas subject to coastal disasters. Introduction Satellite-derived near-shore bathymetry (SDB) remote sensing research uses ocean optics to estimate near-shore bathymetry using passive multispectral satellite imagery, such as Landsat 8 (L8) or DigitalGlobe WorldView-3 (WV-3) (Gesch and others, 2016; Pe’eri and others, 2014; 2016a; 2016b; Stumpf and others, 2003). Active sensors, such as light detection and ranging (lidar) or sound navigation and ranging (sonar), have been used to collect bathymetric data; however, mapping islands in the Pacific Ocean with green laser lidar sensors aboard a small, fixed-wing aircraft is expensive. Because surface vessels often cannot collect soundings safely while operating close to the shore, satellite imagery has potential to be a more cost-effective means for mapping near-shore bathymetry to assess vulnerability to natural hazards in lowlying atolls in the northern tropical Pacific. In the scientific literature, the most common method used to derive bathymetry from satellite imagery is an optimization approach using spectral band ratio-based techniques, more commonly called the ratio of natural logarithm approach (Dierssen and others, 2003; International Hydrographic Organization, Intergovernmental Oceanographic Commission, 2016; Pe’eri and others, 2014; 2016a; 2016b; Stumpf and others, 2003). This empirical method may lead to more precise measurements and modeling of bathymetry because different substrates at the same depth have approximately equal ratio values (Legleiter and Overstreet, 2012). A study by Pe’eri and others (2014) indicated that, based on the assumption that the water column is uniformly mixed, the ratio of two bands, such as blue/green, will maintain a near-constant attenuation value that is the difference of the diffuse attenuation coefficient values at two different wavelengths. The concept underlying the ratio transform approach is that bottom radiance of one band will decay faster with depth as compared to the other band. As a result, the ratio between the two bands will vary with depth (Pe’eri and others, 2014). The logarithm of the band-ratio approach accounts for the exponential attenuation of light in the water column; therefore, the image-derived quantity, or the change in ratio between bands, is affected more by depth 2 Near-Shore Bathymetry on the Majuro Atoll, Republic of the Marshall Islands, Using Landsat 8 and WorldView-3 Imagery than by bottom reflectance (Stumpf and others, 2003). Thus, a linear relation between water depth and reflectance, relative radiance, or raw digital numbers in one or more spectral bands is commonly used for SDB (Legleiter and others, 2012; Pe’eri and others, 2014; 2016a; 2016b). According to Stumpf and others (2003), the use of satellite imagery might be the only viable way to synoptically characterize either extensive or remote coral reef environments because of incomplete or spatially limited bathymetric data or difficulty obtaining soundings in remote reef areas. Stumpf and others (2003) indicated that high spatial resolution is required due to the relatively small horizontal spatial features in remote coral atoll locations; therefore, medium resolution (30 meters [m]) and high-resolution (1.6 m) satellite imagery were used to estimate SDB for the Majuro Atoll coral reef environment.

Evaluating the potential for near-shore bathymetry on the Majuro Atoll, Republic of the Marshall Islands, using Landsat 8 and WorldView-3 imagery