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options("repos"="http://cran.us.r-project.org") | |
library("rgdal") | |
myPolygons = readOGR("/path/to/file",layer="mylayer") | |
alpha=0.75 | |
library(alphahull) | |
pts = spsample(myPolygons, n=1000, type="regular") | |
x.as = ashape(coordinates(pts),alpha=alpha) | |
# need to convert x.as to a SpatialPolygon | |
# code to convert ahull into sp: | |
ah2sp <- function(x, increment=360, rnd=10, proj4string=CRS(as.character(NA))){ | |
require(alphahull) | |
require(maptools) | |
if (class(x) != "ahull"){ | |
stop("x needs to be an ahull class object") | |
} | |
# Extract the edges from the ahull object as a dataframe | |
xdf <- as.data.frame(x$arcs) | |
# Remove all cases where the coordinates are all the same | |
xdf <- subset(xdf,xdf$r > 0) | |
res <- NULL | |
if (nrow(xdf) > 0){ | |
# Convert each arc to a line segment | |
linesj <- list() | |
prevx<-NULL | |
prevy<-NULL | |
j<-1 | |
for(i in 1:nrow(xdf)){ | |
rowi <- xdf[i,] | |
v <- c(rowi$v.x, rowi$v.y) | |
theta <- rowi$theta | |
r <- rowi$r | |
cc <- c(rowi$c1, rowi$c2) | |
# Arcs need to be redefined as strings of points. Work out the number of points to allocate in this arc segment. | |
ipoints <- 2 + round(increment * (rowi$theta / 2),0) | |
# Calculate coordinates from arc() description for ipoints along the arc. | |
angles <- anglesArc(v, theta) | |
seqang <- seq(angles[1], angles[2], length = ipoints) | |
x <- round(cc[1] + r * cos(seqang),rnd) | |
y <- round(cc[2] + r * sin(seqang),rnd) | |
# Check for line segments that should be joined up and combine their coordinates | |
if (is.null(prevx)){ | |
prevx<-x | |
prevy<-y | |
} else if (x[1] == round(prevx[length(prevx)],rnd) && y[1] == round(prevy[length(prevy)],rnd)){ | |
if (i == nrow(xdf)){ | |
#We have got to the end of the dataset | |
prevx<-append(prevx,x[2:ipoints]) | |
prevy<-append(prevy,y[2:ipoints]) | |
prevx[length(prevx)]<-prevx[1] | |
prevy[length(prevy)]<-prevy[1] | |
coordsj<-cbind(prevx,prevy) | |
colnames(coordsj)<-NULL | |
# Build as Line and then Lines class | |
linej <- Line(coordsj) | |
linesj[[j]] <- Lines(linej, ID = as.character(j)) | |
} else { | |
prevx<-append(prevx,x[2:ipoints]) | |
prevy<-append(prevy,y[2:ipoints]) | |
} | |
} else { | |
# We have got to the end of a set of lines, and there are several such sets, so convert the whole of this one to a line segment and reset. | |
prevx[length(prevx)]<-prevx[1] | |
prevy[length(prevy)]<-prevy[1] | |
coordsj<-cbind(prevx,prevy) | |
colnames(coordsj)<-NULL | |
# Build as Line and then Lines class | |
linej <- Line(coordsj) | |
linesj[[j]] <- Lines(linej, ID = as.character(j)) | |
j<-j+1 | |
prevx<-NULL | |
prevy<-NULL | |
} | |
} | |
# Promote to SpatialLines | |
lspl <- SpatialLines(linesj) | |
# Convert lines to polygons | |
# Pull out Lines slot and check which lines have start and end points that are the same | |
lns <- slot(lspl, "lines") | |
polys <- sapply(lns, function(x) { | |
crds <- slot(slot(x, "Lines")[[1]], "coords") | |
identical(crds[1, ], crds[nrow(crds), ]) | |
}) | |
# Select those that do and convert to SpatialPolygons | |
polyssl <- lspl[polys] | |
list_of_Lines <- slot(polyssl, "lines") | |
sppolys <- SpatialPolygons(list(Polygons(lapply(list_of_Lines, function(x) { Polygon(slot(slot(x, "Lines")[[1]], "coords")) }), ID = "1")), proj4string=proj4string) | |
# Create a set of ids in a dataframe, then promote to SpatialPolygonsDataFrame | |
hid <- sapply(slot(sppolys, "polygons"), function(x) slot(x, "ID")) | |
areas <- sapply(slot(sppolys, "polygons"), function(x) slot(x, "area")) | |
df <- data.frame(hid,areas) | |
names(df) <- c("HID","Area") | |
rownames(df) <- df$HID | |
res <- SpatialPolygonsDataFrame(sppolys, data=df) | |
res <- res[which(res@data$Area > 0),] | |
} | |
return(res) | |
} | |
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