沿SpatialLine的R栅格提取：将提取的值与实际距离相关联

Question

在R中沿SpatialLine提取栅格的值时，如何将这些值与沿此线的实际距离联系起来？

假设我想沿着以下行提取R徽标的值：

library(raster)
r <- raster(system.file("external/rlogo.grd", package="raster"))
x=c(5, 95)
y=c(20, 50)
line = SpatialLines(list(Lines(Line(cbind(x,y)), ID="a")))
plot(r)
plot(line, add=TRUE)

我可以提取值并绘制它们 - 但是如何用实际距离替换x值（下面的1:length(vals)）（从例如线的左侧开始，例如0）？

vals <- extract(r, line)[[1]]
plot(1:length(vals), vals, type='o')

我可以将细胞提取与xyFromCell结合起来，按照建议here获取提取细胞的坐标，但我不清楚如何更进一步。

Answer 1

我不确定你要问的是什么，但是如果你想寻找线段最左边坐标和线穿过的单元中心之间的距离，那么你可以找到像这样的距离：

x <- extract(r, l, cellnumbers=TRUE)[[1]]
xy <- xyFromCell(r, x[,1]) # get cell coordinates where the line passes
start <- xy[which.min(xy[,1]),] # leftmost coordinate of the line
d <- apply(xy, 1, function(x, start) sqrt(sum((x-start)^2)), start=start) # find distances between the line segment start and the cells
plot(1:length(d), d, type='o')

Answer 2

这是一个解决方案（部分基于@jvj的输入），试图计算线上raster::extract提供的单元中心的正交投影，然后计算沿着该线的距离线。

（这是一个R-beginners脚本，可能很容易改进，但似乎有用（当然只适用于有距离投影的栅格））

vals <- extract(r, line, cellnumbers=TRUE)[[1]]
cellsxy <- xyFromCell(r, vals[,1]) # coordinates of intersected cells (likely not ON the line)
linexy = spsample(line, 1000, "regular") # get the line as points
linexy <- matrix(cbind(linexy$x, linexy$y), ncol=2) # easier than Spatial object for later
orthoproj <- c() # to store the orthogonal projections of cells centres on the line
for (i in 1:nrow(cellsxy)) {
  xypt = cellsxy[i,]
  min.index <- which.min(spDistsN1(linexy, xypt)) 
  orthopt <- linexy[min.index, ] # orthogonal projections = smaller distance 
  orthoproj <- c(orthoproj, c(orthopt[1], orthopt[2]))
}
orthoproj <- matrix(orthoproj, ncol=2, byrow=T)
orthoproj <- data.frame(x=orthoproj[,1], y=orthoproj[,2])
orthoproj <- orthoproj[order(orthoproj[,1]),] # reorder with increasing distance
orthoproj <- data.frame(x=orthoproj$x, y=orthoproj$y)
start <- linexy[which.min(linexy[,1]),] # leftmost coordinate of the line
dists <- apply(orthoproj, 1, 
               function(xy, start) sqrt(sum((xy-start)^2)), 
               start=start) # distances between 'start' and  the orthogonal projections

plot(dists, rev(vals[,2]), type='o') # !! beware: order of 'vals' and 'dists' 
                    # depending on the order in which cellnumbers are returned 
                    # in raster::extract and the shape of your line !!