$$PARA_{MN} = mean(PARA[patch_{ij}]$$
where \(PARA[patch_{ij}]\) is the perimeter area ratio of each patch.
PARA_MN is a 'Shape metric'. It summarises each class as the mean of
each patch belonging to class i. The perimeter-area ratio describes the patch complexity
in a straightforward way. However, because it is not standarised to a certain shape
(e.g. a square), it is not scale independent, meaning that increasing the patch size
while not changing the patch form will change the ratio.
Because the metric is based on distances or areas please make sure your data
is valid using check_landscape.
Behaviour
Approaches PARA_MN > 0 if PARA for each patch approaches PARA > 0,
i.e. the form approaches a rather small square. Increases, without limit, as PARA increases,
i.e. patches become more complex.