Buffers

Libraries to install from CRAN if not installed:

> install.packages("sp")
> install.packages("rgeos")
> install.packages("readxl")
> install.packages("raster")
> install.packages("devtools")
> install.packages("geosphere")

Library to install from GitHub if not installed:

> devtools::install_github("choisy/gadmVN")

Some libraries to load for interactive use:

> library(sp)       # coordinates, CRS, proj4string, spTransform, over
> library(rgeos)    # gIntersection
rgeos version: 0.3-26, (SVN revision 560)
 GEOS runtime version: 3.6.1-CAPI-1.10.1 r0 
 Linking to sp version: 1.2-5 
 Polygon checking: TRUE

The polygon of Vietnam:

> vn <- gadmVN::gadm(level = "country")

The coordinates of the cases:

> cases <- readxl::read_excel("LongLat.xlsx")
> coordinates(cases) <- ~ LONGITUDE + LATITUDE
> proj4string(cases) <- CRS("+proj=longlat")

Making sure that the coordinates of the cases have the same projection as the polygon of Vietnam:

> cases <- spTransform(cases, proj4string(vn))

Defining buffers:

> radius <- 10000 # in meters
> buffers <- raster::buffer(cases, radius, FALSE)

Checking that all the buffers overlay with the polygons of Vietnam:

> any(is.na(sapply(seq_along(cases), function(x) over(vn, buffers[x]))))
[1] FALSE

Calculating the areas of the buffers, defining as the intersection of the buffers with the polygon of Vietnam:

> denom <- 10^6 # to get area in square km
> areas <- sapply(seq_along(buffers),
+                 function(x)
+                   geosphere::areaPolygon(gIntersection(vn, buffers[x])) / denom)

Checking the distribution of these areas:

> hist(areas, col = "grey")