Bathymetric Position Index (BPI) Structures 10 m grid derived from gridded bathymetry of Asuncion Island, Commonwealth of the Northern Mariana Islands (CNMI), USA.

Bathymetric Position Index Structures were derived from the gridded multibeam bathymetry (asuncion_10m) using the Benthic Terrain Modeler (BTM). The BTM (written for ArcGIS 8.x or 9.x) may be downloaded from http://dusk2.geo.orst.edu/djl/samoa/tools.html. BPI Structures are derived from two scales of a focal mean analysis on bathymetry and slope. After testing combinations, two scales of bpi (50 & 250) were chosen for Asuncion Island. Two fine scale factors (20 and 50) and two broad scale factors (250 and 500) were tested after measuring many of the bathymetric features that indicate possible reef structures when overlaid on the hillshade (also after Lundblad et al. 2006; see Supplemental_Information). Four combinations of scale factors were created and used in the Terrain Classification Builder (part of the BTM) to create multiple structure and zone grids. Compared to the combinations using a broad scale factor of 500, those using 250 highlight more fine scale structures, including the broad crests, which are very important in coral reef environments. The fine scale factor of 50 defined the fine scale structures as more continuous features and cancelled out a lot of the noise that was falsely defined by using 20 or 30. Additionally, three ranges of slope define flats, slopes, and near vertical: x < 5; 5 < x < 70; x > 70. Depth bounds separated flat structures into shelf (0 to -60m) and deep shelf (-60 to -1000m) environments. The depths were based on the bathymetric drop off on the SW end of the island.