A Benthic Terrain Classification Scheme for American Samoa

would represent patterns of depressions. The scalefactor of the resulting grid is 4, where
scalefactor is the resolution multiplied by the outer radius.
Figure 5 Example of the variables used to derive bathymetric position index (BPI) from
bathymetry. The grid cells represent a derived BPI grid. Negative values are lower than
their neighbors. Positive values are higher than their neighbors. Values of zero are flat
areas or areas with constant slope.
The results of BPI are scale dependent; different scales identify fine or broad
benthic features. To achieve the best BPI zone and structure classifications several large
and small-scale grids were created for each study site. The fine scale grids were created
with scalefactors of 10, 20, and 30, and the broad scale grids were created with
scalefactors of 50, 70, 125, and 250. BPI and BPI were used to classify
Fagatele Bay and Taema Bank. These scalefactors were chosen because, at these sites,
the small seascape features (distance between relatively small ridges) are, on average,
about 20 m across; the large seascape features (e.g. the distance across the deep channel
on the west end of Taema Bank and the length of the peninsula in Fagatele Bay) are
about 250 m across. This is based on close examination of the bathymetry prior to the
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