user3910073 user3910073 - 3 months ago 21
R Question

Extract shapefile value to point with R

I want to extract a shapefile value at a particular location. The shapefile I am using is can be found here, downloaded by clicking

Download IHO Sea Areas
. The shape file contains all the possible seas.

I can read it and plot it using:

require("maptools")
require(rgdal)
require(sp)

ogrListLayers("World_Seas.shp")
shape <- readOGR("World_Seas.shp", layer="World_Seas")


However, I would like to extract the sea value for a particular location, say

p <- c(-20, 40)

Answer

there is a probably an easier way but here is my take

require("maptools")
require(rgdal)
require(sp)
library(plyr)
library(dplyr)

setwd("/Users/drisk/Downloads/seas")
ogrListLayers("World_Seas.shp")
shape=readOGR("World_Seas.shp", layer="World_Seas") 

datapol <- data.frame(shape)
pointtoplot <- data.frame(x=-20, y=40)
coordinates(pointtoplot) <- ~ x + y 
proj4string(pointtoplot) <- CRS("+proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0")
#
#function over from package sp
test <- data.frame(xx=over(shape, pointtoplot))
combine <- cbind(test, datapol)
combine <- na.omit(combine) #only one point left

output for your point x=-20, y=40

   xx                 NAME ID Gazetteer_ id
35  1 North Atlantic Ocean 23       1912 35
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