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

Bathymetric Position Index Structures were derived from the gridded multibeam bathymetry (sarigan_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 Sarigan Island. Two fine scale factors (20 and 50) and two broad scale factors (250 and 500) were tested in different combinations. After measuring many of the bathymetric features that indicate possible reef structures when overlaid on the hillshade, initially the values of 500 and 50 were used (also after Lundblad et al. 2006; see Supplemental_Information). However, after running the Benthic Terrain Modeler (BTM), it was apparent that most of the fine scale structures were hard to identify in the classification. Therefore, the other scales of BPI grids were created and used in the Terrain Classification Builder (part of the BTM) to create multiple structure and zone grids. The combination of 20 (or 30) and 250 brought out those fine scale structures, including the broad crests, which are very important in coral reef environments. However, 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 offs on the East side of the island.