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.

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")