Survey of reefs based on Landsat 8 operational land imager (OLI) images in the Nansha Islands, South China Sea

A detailed survey of the reefs in the Nansha Islands’ sea that are of strategic importance is conducive to their future development. In this study, a total of 50 Landsat 8 operational land imager (OLI) images were used to analyze the geological features and spectral characteristics of the Nansha reefs. The exposed islands/sandbanks, reef flats and sub-tidal reefs were identified with the near-infrared band, the red and green bands, and the blue band, respectively. Based on the relationships among various characteristics and bands illustrated in the OLI images, the maximum between-cluster variance threshold algorithm (OTSU method) and the mathematical morphology were employed to extract characteristic parameters of the spatial geometry of the reefs from top to bottom, which were subsequently operated by a series of post-processing methods such as vectorization, simplification and topological analysis. Among the 132 standalone reefs and 16 atolls that are identified in the study, four reefs have yet to be named in the Standard Names of Various Islands in the South China Sea and the website Nansha Islands Online (http://www.nansha.org/). Another 24 ones are exposed to the atmosphere even during high tides. Taiping Island with an area of 0.57 km2 represents the largest exposed geological feature. This study demonstrated the applicability of medium-resolution satellite images to derive the coral reef information effectively and thus provided information for the related departments that are responsible to manage the coast. An improved resolution of multispectral bands with the panchromatic band in higher spatial resolution (15 m for OLI image) is expected to provide an optimum satellite based approach to map marine habitats.