r - 计算距离栅格避开土地

Question

我正在寻找一种方法来计算距一个点的距离栅格，类似于这样raster::distance做，但要避开陆地。因此，就像我可以制作到一个点的距离栅格一样（没有任何约束），如下所示：

library(sp)
library(raster)


# create cooridnates sp points
col_coord <- data.frame(x      = c(-13.8309),
                        y      = c(28.9942))
coordinates(col_coord) <- ~x+y
col_coord@proj4string <- CRS(projections$WGS84)

#create base raster
baseRaster <- raster(xmn = -80, xmx =55, ymn = -70, ymx = 70, crs = CRS("+proj=longlat +ellps=WGS84"), resolution = 1)

#rasterize spatial point
colraster <- rasterize(col_coord, baseRaster)

#calculate distance
coldist <- raster::distance(colraster, doEdge = T)

#plot
plot(coldist) 

并获得此图像：

我想制作相同的栅格，但要“最短距离避开一个区域”（在这种情况下是土地）。我知道这个包gdistance，我已经尝试遵循这篇文章的建议，但据我所知：（Trans 是一个过渡层，因为计算需要很长时间在这里可用。

library(gdistance)
# load transition layer (change directory if needed). This is in another CRS so we transform everything
load("data/Transition_layer.RData")

col_coord <- spTransform(col_coord, Trans@crs)
A <- accCost(Trans, col_coord)
plot(A) #this shows high cost in contintents which is good

# create a raster for water
mapWGS <- rgeos::gBuffer(rworldmap::getMap(resolution = "high"), width = 0)
water <- rasterize(mapWGS, baseRaster, mask = F, field = 1)
table(is.na(water[]))

water[water == 1] <- 0
water[is.na(water)] <- 1
water[water == 0] <- NA
plot(water)

water <- projectRaster(water, crs = Trans@crs)

# change resolution of A to match that of water
A <- resample(A, water)
A2 <- mask(A, water, inverse = F)

plot(A2) # this doesn't make that much sense. 

因此，任何关于如何从这里获得类似于第一个但包含避开陆地的距离的栅格的想法将不胜感激！

标签： rgisdistancer-raster

Answer 1

我想你可以用raster::gridDistance一个omit论点

library(raster)
xy <- data.frame(x=-13.8309, y=28.9942)
r <- raster(xmn=-120, xmx=55, ymn=-70, ymx=70, res=.25)

library(maptools)
data(wrld_simpl)

r <- rasterize(wrld_simpl, r, field= -1)
rr <- rasterize(xy, r, update=TRUE)

x <- gridDistance(rr, 1, omit=-1)
plot(x / 1000000)
points(xy)

地图显示它适用于太平洋，也适用于红海（因为 wrld_simpl 没有巴拿马运河和苏伊士运河）。

但是因为使用 gridDistance 路径必须通过栅格单元的中心，所以估计的距离比实际距离要长一些。