bpi: bathytopografic position index
In janhoo/crecs: quantitative species distribution modelling

Returns different tpi's.

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bpi(dem, orad, irad = NULL, slope = NULL, fac = 1, simple = TRUE,
  simpler = TRUE, scale = FALSE, inv = FALSE, verbose = FALSE,
  raw = FALSE, ...)

`dem`	the elevation model as `RasterLayer`, `SpatialGridDataFrame` or `SpatialPointsDataFrame`
`orad`	outer circle radius in map units for which the tpi is calculated
`irad`	inner circle radius. If given, an annulus is used fot tpi caculation. If 0, the inner radius contains exacly one pixel. Default is `NULL`
`slope`	threshold to delineate flat from slope. Unit is 'degrees'. In case of default `NULL`, single standart deviation of slopes is used
`fac`	modifier to slope threshold. Applies only if `simple=TRUE`.
`simple`	output only 3 categories: (1) valley; (2) slope/flat; (3) ridge/top. Overrides `simpler`
`simpler`	simplify the non-simple schema to 6 categories: (1) valley; (2) lower slope; (3) flat; (4) middle slope; (5) upper slope; (6) ridge/top
`scale`	apply preemptive `scale`
`inv`	switch sign (in case of bathemetry with positive depth values)
`verbose`	be verbose and plot