[R-sig-Geo] Concave hull of lat lon points?

[Gareth Davies]

Hi,
One approach is make a buffer around all your points and then merge, so 
that the resulting polygon contains all your points and is connected. 
Then, you can somewhat undo this buffer if needed, by taking a buffer 
with negative width. For example:

library(sp)
library(rgeos)

# Make up some 'banana shaped' points
mypts=cbind(runif(100),runif(100))
keep=4*(mypts[,1]-0.5)**2 + 0.2 > mypts[,2]
mypts=mypts[keep,]

# Coerce to SpatialPointsDataframe
mypts2=SpatialPointsDataFrame(mypts,data=data.frame(NA*mypts[,1]), 
match.ID=F)

# Now take a buffer that covers up all the points
# You will need to decide on an appropriate 'width' argument so that the 
region is connected
buf1=gBuffer(mypts2, width=0.3,byid=T)
buf1_union=gUnionCascaded(buf1) # Take the union -- so this polygon will 
contain all your points
# Now partly 'undo' the buffer -- again, experimentation is needed to 
choose the width
buf_final=gBuffer(buf1_union,width=-0.2)

# This should look okay
plot(buf_final)
points(mypts,col=2)



Cheers,
Gareth.