Build a wind connectivity graph

Constructs the directed network that the least-cost functions (least_cost(), least_cost_paths(), and pairwise_least_cost()) find paths through. Those functions build the graph from a wind rose automatically, so you don't normally need to call this function. Building the graph yourself can save time when making many least-cost calls on a large grid: pass the graph in place of the rose.

Usage

wind_graph(x, direction = "downwind", wrap = NULL)

Arguments

x

A wind_rose.

direction

Either "downwind" (the default) or "upwind": whether links follow the wind, or run against it.

wrap

Logical: join the east and west edges of the grid, linking cells across them? The default, NULL, does so if x is a global grid spanning all 360 degrees of longitude, where -180 and 180 are the same meridian, and not otherwise. TRUE or FALSE overrides this; TRUE on a longitude/latitude grid that isn't global gives a warning.

Value

A wind_graph: a gdistance Transition-class object, with its direction recorded.

Details

The graph links each grid cell to its eight neighbors. Each link's conductance is the mean, over its two cells, of the rose's conductance in the link's direction. In a downwind graph, links point the way the wind carries material; an upwind graph is the same network with every link reversed, for measuring travel toward a site.

Examples

rose <- windscape_example("wind_rose")
graph <- wind_graph(rose)
sites <- cbind(c(-110, -100), c(40, 40))
pairwise_least_cost(graph, sites)
#>          [,1]     [,2]
#> [1,]    0.000 299.9684
#> [2,] 1043.173   0.0000