A Benthic Terrain Classification Scheme for American Samoa

BPI was calculated in the ArcGIS raster calculator using the focal mean
calculation described above; the resulting grid values are converted to integers to
minimize the storage size of the grid and to simplify symbolization (Algorithm 1).
Algorithm 1 creates a BPI grid using bathymetry and user defined radii:
scalefactor = outer radius in map units multiplied by bathymetric data resolution
irad = inner radius of annulus in cells
orad = outer radius of annulus in cells
bathy = bathymetric grid
rad = radius (if using circle instead of annulus)
BPI = int((bathy – focalmean(bathy, annulus, irad, orad)) + 0.5)
OR
BPI = int((bathy – focalmean(bathy, circle, rad)) + 0.5)
The cells in the output grid are assigned values within a range of positive and
negative numbers (Figure 4). The 0.5 is added before the integer conversion and is meant
to force floating point values, regardless of the sign of the value, to round up if the value
has a decimal of greater than .5 and to round down if the value has a decimal of less than
.5. The variable is not necessary if a user chooses to allow the floating point values to be
rounded downward consistently for positive and negative values.
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