calculate SPI
Examples
data(skink_grid)
data(grid)
grid$zone[1:2] <- 2
grid
#> # A tibble: 16 × 8
#> grid X Y left right bottom top zone
#> <dbl> <int> <int> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1 1 1 1 33 98.5 94 171. 2
#> 2 2 2 1 98.5 164 94 171. 2
#> 3 3 3 1 164 230. 94 171. 1
#> 4 4 4 1 230. 295 94 171. 1
#> 5 5 1 2 33 98.5 171. 248. 1
#> 6 6 2 2 98.5 164 171. 248. 1
#> 7 7 3 2 164 230. 171. 248. 1
#> 8 8 4 2 230. 295 171. 248. 1
#> 9 9 1 3 33 98.5 248. 324. 1
#> 10 10 2 3 98.5 164 248. 324. 1
#> 11 11 3 3 164 230. 248. 324. 1
#> 12 12 4 3 230. 295 248. 324. 1
#> 13 13 1 4 33 98.5 324. 401 1
#> 14 14 2 4 98.5 164 324. 401 1
#> 15 15 3 4 164 230. 324. 401 1
#> 16 16 4 4 230. 295 324. 401 1
get_zone_object(grid, skink_grid) |> calc_spi()
#> [1] 0.02157296