Finding shortest path without barriers using the sf package in R

Question

Say I have three ships near PNG in the pacific. I want to know the shortest path between all three, while not going aground. This can be done with the ArcGIS function "cost distance" and "shortest path", but I would like to do it in R with the cool new "sf" package. Alternativly using gDistance. Anyone know how to?

Example data:

library(marmap)
library(raster)
library(sf)

#Make three ship positions near PNG
pts <- data.frame(nr=1:3, x=c(145,145,150), y=c(-10,-1,-7)) %>% 
  sf::st_as_sf(coords = c("x","y")) %>% 
  sf::st_set_crs(4326)

#Get bathymetric data from the area
papoue <- getNOAA.bathy(lon1 = 140, lon2 = 155,
                        lat1 = -13, lat2 = 0, resolution = 4)

#Turn into raster and keep only depth lower than 0 meter (water)
r <- marmap::as.raster(papoue)
plot(r)

r@data@values[r@data@values>=0] <- NA
r@data@values[r@data@values<0] <- 1

plot(r)
plot(pts, add=T)

Answer 1

An alternative with the gdistance package: after constructing the raster r as you do, transform your ships' positions pts into a SpatialPoints object:

pts <- sf::as(pts, "Spatial")

Then, you can use the shortestPath() function from the gdistance package. The needed input would be a raster with infinite -- NA -- cost of crossing for all landmass (which you are doing in the r raster). You need to transform the raster into a transition layer matrix with the transition() function (some extra correction might be needed, check reference):

# transforming it:
r <- transition(r, mean, directions = 8)
# Hypothetical distance from the first two ships:
distance <- shortestPath(r, pts[1,], pts[2,], output = "SpatialLines")

Good reference: http://freigeist.devmag.net/economics/683-computing-maritime-routes-in-r.html