High-resolution habitat and bathymetry maps for 65,000 sq. km of Earth’s remotest coral reefs

Abstract

With compelling evidence that half the world's coral reefs have been lost over the last four decades, there is urgent motivation to understand where reefs are located and their health. Without such basic baseline information, it is challenging to mount a response to the reef crisis on the global scale at which it is occurring. To combat this lack of baseline data, the Khaled bin Sultan Living Oceans Foundation embarked on a 10-yr survey of a broad selection of Earth's remotest reef sites-the Global Reef Expedition. This paper focuses on one output of this expedition, which is meter-resolution seafloor habitat and bathymetry maps developed from DigitalGlobe satellite imagery and calibrated by field observations. Distributed on an equatorial transect across 11 countries, these maps cover 65,000 sq. km of shallow-water reef-dominated habitat. The study represents an order of magnitude greater area than has been mapped previously at high resolution. We present a workflow demonstrating that DigitalGlobe imagery can be processed to useful products for reef conservation at regional to global scale. We further emphasize that the performance of our mapping workflow does not deteriorate with increasing size of the site mapped. Whereas our workflow can produce regional-scale benthic habitat maps for the morphologically diverse reefs of the Pacific and Indian oceans, as well as the more depauperate reefs of the Atlantic, accuracies are substantially higher for the former than the latter. It is our hope that the map products delivered to the community by the Living Oceans Foundation will be utilized for conservation and act to catalyze new initiatives to chart the status of coral reefs globally.

Reef Cover: a coral reef classification to guide global habitat mapping from remote sensing

“…Through the 1990s and early 2000s field survey techniques described above enabled more effective linkage of ecological surveys to remote sensing data 55,56 . Accessibility to higher spatial resolution images over larger areas in combination with detailed field data, physical attributes and object-based analysis resulted in large reef area mapping 34,39,40,57 . In the last five years, the increase in daily to weekly global coverage of this type of imagery, in combination with cloud-based processing capability has expanded to a global capability for reef mapping 14 .…”

“…To explore relevancy, a cross-walk was performed between Reef Cover and a selection of major regional to global coral reef classification 42,64,77,78 , mapping 20,34,39 and monitoring efforts 41 , to make sure important classes from established classifications had not been missed 47 (Table 5).…”

Confidence Levels, Sensitivity, and the Role of Bathymetry in Coral Reef Remote Sensing

“…A common challenge when scaling remote sensing analysis from local to regional scales, and beyond, is maintaining model integrity across varying environmental conditions and application domains [21,62,63]. With airborne and satellite derived bathymetry this includes the potential to encounter water conditions and depths that exceed the capabilities of a given modeling approach.…”

“…This study was performed with the intent of developing a framework for processing hyperspectral satellite data to derive bathymetry and benthic reflectance of coral reefs for widespread (ideally global) application [21][22][23]. The results are consistent across multiple sites in the Hawaiian Islands, suggesting applicability across geographic space and time.…”

“…With increasing availability of freely available multispectral data, such as Landsat and Sentinel-2 [18], as well as access to faster computers and algorithms, high spatial resolution commercial satellites [19], and unmanned aerial vehicles (UAVs) [20], new bathymetric maps and classification methods are being developed that cover much broader geographic scales. Two of the most extensive recent examples are the World Reef Map [21,22] funded through the Khaled bin Sultan Living Oceans Foundation, which has mapped 65,000 km 2 of coral reefs in 11 countries using high resolution DigitalGlobe WorldView-2 imagery, and the Allen Coral Atlas [23], which is an ongoing effort using high resolution Planet Dove imagery to map and monitor the world's coral reefs. Previously, the Millennium Coral Reef Mapping Project [24] used more than 1500 Landsat images to develop a hierarchical classification map of coral reefs worldwide.…”