A categorical raster object: SpatRaster; Raster* Layer, Stack, Brick; stars or a list of SpatRasters.
directions
The number of directions in which patches should be
connected: 4 (rook's case) or 8 (queen's case).
Details
$$FRAC = \frac{2 * \ln * (0.25 * p_{ij})} {\ln a_{ij}}$$
where \(p_{ij}\) is the perimeter in meters and \(a_{ij}\) is the
area in square meters
FRAC is a 'Shape metric'. The index is based on the patch perimeter and
the patch area and describes the patch complexity. Because it is standardized,
it is scale independent, meaning that increasing the patch size while not changing the
patch form will not change the ratio.
Units
None
Range
1 <= FRAC <= 2
Behaviour
Approaches FRAC = 1 for a squared patch shape form and FRAC = 2
for a irregular patch shape.
References
McGarigal K., SA Cushman, and E Ene. 2023. FRAGSTATS v4: Spatial Pattern Analysis
Program for Categorical Maps. Computer software program produced by the authors;
available at the following web site: https://www.fragstats.org
Mandelbrot, B. B. 1977. Fractals: Form, Chance, and Dimension.
San Francisco. W. H. Freeman and Company.