Paul Regularの優れた作業に基づいて、排他的なポリゴンを保証する（つまり、オーバーラップしない）バージョンを次に示します。
UTMタイプの座標での作業で発見した問題に対処するために、フェアリーダストの新しい引数fdを追加しました。基本的に私が理解しているように、アルゴリズムは等高線から横方向の点をサンプリングして、どちらの側がポリゴンの内側にあるかを判断することによって機能します。線からのサンプルポイントの距離は、たとえば別の輪郭の後ろにある場合、問題を引き起こす可能性があります。したがって、結果のポリゴンが間違って見える場合は、非常に間違っているか、ほぼ正しく見えるまで、値10^±nに設定してみてください。fd
raster2contourPolys <- function(r, levels = NULL, fd = 1) {
## set-up levels
levels <- sort(levels)
plevels <- c(min(values(r)-1, na.rm=TRUE), levels, max(values(r)+1, na.rm=TRUE)) # pad with raster range
llevels <- paste(plevels[-length(plevels)], plevels[-1], sep=" - ")
llevels[1] <- paste("<", min(levels))
llevels[length(llevels)] <- paste(">", max(levels))
## convert raster object to matrix so it can be fed into contourLines
xmin <- extent(r)@xmin
xmax <- extent(r)@xmax
ymin <- extent(r)@ymin
ymax <- extent(r)@ymax
rx <- seq(xmin, xmax, length.out=ncol(r))
ry <- seq(ymin, ymax, length.out=nrow(r))
rz <- t(as.matrix(r))
rz <- rz[,ncol(rz):1] # reshape
## get contour lines and convert to SpatialLinesDataFrame
cat("Converting to contour lines...\n")
cl0 <- contourLines(rx, ry, rz, levels = levels)
cl <- ContourLines2SLDF(cl0)
## extract coordinates to generate overall boundary polygon
xy <- coordinates(r)[which(!is.na(values(r))),]
i <- chull(xy)
b <- xy[c(i,i[1]),]
b <- SpatialPolygons(list(Polygons(list(Polygon(b, hole = FALSE)), "1")))
## add buffer around lines and cut boundary polygon
cat("Converting contour lines to polygons...\n")
bcl <- gBuffer(cl, width = fd*diff(bbox(r)[1,])/3600000) # add small buffer so it cuts bounding poly
cp <- gDifference(b, bcl)
## restructure and make polygon number the ID
polys <- list()
for(j in seq_along(cp@polygons[[1]]@Polygons)) {
polys[[j]] <- Polygons(list(cp@polygons[[1]]@Polygons[[j]]),j)
}
cp <- SpatialPolygons(polys)
cp <- SpatialPolygonsDataFrame(cp, data.frame(id=seq_along(cp)))
# group by elev (replicate ids)
# ids = sapply(slot(cl, "lines"), slot, "ID")
# lens = sapply(1:length(cl), function(i) length(cl[i,]@lines[[1]]@Lines))
## cut the raster by levels
rc <- cut(r, breaks=plevels)
## loop through each polygon, create internal buffer, select points and define overlap with raster
cat("Adding attributes to polygons...\n")
l <- character(length(cp))
for(j in seq_along(cp)) {
p <- cp[cp$id==j,]
bp <- gBuffer(p, width = -max(res(r))) # use a negative buffer to obtain internal points
if(!is.null(bp)) {
xy <- SpatialPoints(coordinates(bp@polygons[[1]]@Polygons[[1]]))[1]
l[j] <- llevels[raster::extract(rc,xy)]
}
else {
xy <- coordinates(gCentroid(p)) # buffer will not be calculated for smaller polygons, so grab centroid
l[j] <- llevels[raster::extract(rc,xy)]
}
}
## assign level to each polygon
cp$level <- factor(l, levels=llevels)
cp$min <- plevels[-length(plevels)][cp$level]
cp$max <- plevels[-1][cp$level]
cp <- cp[!is.na(cp$level),] # discard small polygons that did not capture a raster point
df <- unique(cp@data[,c("level","min","max")]) # to be used after holes are defined
df <- df[order(df$min),]
row.names(df) <- df$level
llevels <- df$level
## define depressions in higher levels (ie holes)
cat("Defining holes...\n")
spolys <- list()
p <- cp[cp$level==llevels[1],] # add deepest layer
p <- gUnaryUnion(p)
spolys[[1]] <- Polygons(p@polygons[[1]]@Polygons, ID=llevels[1])
for(i in seq(length(llevels)-1)) {
p1 <- cp[cp$level==llevels[i+1],] # upper layer
p2 <- cp[cp$level==llevels[i],] # lower layer
x <- numeric(length(p2)) # grab one point from each of the deeper polygons
y <- numeric(length(p2))
id <- numeric(length(p2))
for(j in seq_along(p2)) {
xy <- coordinates(p2@polygons[[j]]@Polygons[[1]])[1,]
x[j] <- xy[1]; y[j] <- xy[2]
id[j] <- as.numeric(p2@polygons[[j]]@ID)
}
xy <- SpatialPointsDataFrame(cbind(x,y), data.frame(id=id))
holes <- over(xy, p1)$id
holes <- xy$id[which(!is.na(holes))]
if(length(holes)>0) {
p2 <- p2[p2$id %in% holes,] # keep the polygons over the shallower polygon
p1 <- gUnaryUnion(p1) # simplify each group of polygons
p2 <- gUnaryUnion(p2)
p <- gDifference(p1, p2) # cut holes in p1
} else { p <- gUnaryUnion(p1) }
spolys[[i+1]] <- Polygons(p@polygons[[1]]@Polygons, ID=llevels[i+1]) # add level
}
cp <- SpatialPolygons(spolys, pO=seq_along(llevels), proj4string=CRS(proj4string(r))) # compile into final object
## make polygons exclusive (i.e. no overlapping)
cpx = gDifference(cp[1,], cp[2,], id=cp[1,]@polygons[[1]]@ID)
for(i in 2:(length(cp)-1)) cpx = spRbind(cpx, gDifference(cp[i,], cp[i+1,], id=cp[i,]@polygons[[1]]@ID))
cp = spRbind(cpx, cp[length(cp),])
## it's a wrap
cp <- SpatialPolygonsDataFrame(cp, df)
cat("Done!")
cp
}
