Create convex hull polygon from points and save as shapefile

Question

Needing some help re a conversion problem in R.

I've got calculated the convex hull of a cloud of points. I'd like, from the points forming the convex hull, to build a polygon object and save that as a shapefile that can be read by a GIS software (ArcMap or the like).

My code looks like this:

gps <- read.csv(f)  ##reads the lat-long coordinates  file 
x <- gps$LONGITUDE  ##tells R which columns is which
y <- gps$LATITUDE
z<-chull(x,y)       ##calculates the convex hull --this is just a list of x-y points, N vertex 
dfHull <-cbind(x[z],y[z])  ##the convex hull expressed as a list of selected x-y points
plot(dfHull)     ##this plots the vertex of the polygon, just a check
lines(dfhull)    ##plots the polygon in screen

##generate polygon shapefile, from dfHull, and save it externally as a shapefile ???

The source file only contains lat-long coordinates, e.g:

52.73336     N  0.365974
52.7332  N  0.366051
52.73289     N  0.36636
52.73297     N  0.366258
52.73298     N  0.366243
52.733   N  0.366112
52.73308     N  0.365942
52.73317     N  0.365881
52.73321     N  0.36593
52.73328     N  0.365942
52.73352     N  0.36579
52.73362     N  0.365678
52.73391     N  0.365536
52.7373  N  0.36543
52.73289     N  0.36728

I know there are packages (rgdal,maptools,..) to help with these, but I'm very unfamiliar with spatial stuff. Really all I need is to generate the polygon object and save that as shapefile.

Any help appreciated. Thanks in advance, dev.

Nananne

The shapefiles package provides an easy example of how to write a shapefile out of simple R objects. – Alecalecia

Answer 1

Here is a simple example to create a SpatialPolygonsDataFrame, which can be saved as a shapefile with rgdal::writeOGR():

set.seed(1)
dat <- matrix(stats::rnorm(2000), ncol = 2)
ch <- chull(dat)
coords <- dat[c(ch, ch[1]), ]  # closed polygon

plot(dat, pch=19)
lines(coords, col="red")

library("sp")
library("rgdal")

sp_poly <- SpatialPolygons(list(Polygons(list(Polygon(coords)), ID=1)))
# set coordinate reference system with SpatialPolygons(..., proj4string=CRS(...))
# e.g. CRS("+proj=longlat +datum=WGS84")
sp_poly_df <- SpatialPolygonsDataFrame(sp_poly, data=data.frame(ID=1))
writeOGR(sp_poly_df, "chull", layer="chull", driver="ESRI Shapefile")

Relationship

Still have to test, but it seems to solve my problem perfectly. Many thanks!! – Nananne

hi rcs, A quick questions. I get "Error: could not find function "writeOGR"". Is this because you need ArcGIS installed on your PC? And second: does the shapefile include just the hull points, the polygon shape, everything? – Nananne

writeOGR is in the rgdal package, you have to install and load this package (ArcGIS is not required). The shp contains a polygon defined by the hull points. – Relationship

I try this with points around the south pole and it doesnt work... I think I need to handle projection before doing chull – Perianth