converting shape coordinate system in R

Question

Hello stackoverflow community!

Could someone kindly please assist me with an R GIS issue I am facing. I am trying to assign an identifiable variable to a list of addresses that I have geocoded. The geocodes were obtained from Google Maps API, and I therefore have latitude and longitude information (ie, -155.6019 18.99883). I would like to use this information with a particular shape file. My dilema is that the shape file I have is not using the same latitude and longitude system. I have attached the code so that you can see what coordinate system is used in the shape file (ie, 843662.6 2132942).

What I would like to please know is how to match the coordinates between my address list and this shape file so that I may use the "overlay" function to match the two together.

Thank you for your time!

#function to download shapefile from web
dlshapefile <- function(shploc,shpfile) {
temp <- tempfile()
download.file(shploc, temp)
unzip(temp)
}

temp <- tempfile()

require(maptools)
dlshapefile(shploc="http://files.hawaii.gov/dbedt/op/gis/data/highdist_n83.shp.zip", temp)
P4S.latlon <- CRS("+proj=longlat +datum=WGS84")
shapeFile <- readShapePoly("highdist_n83.shp", verbose=TRUE, proj4string=P4S.latlon)

Answer 1

I would use readOGR from the rgdal package as that preserves the projection information.

require(rgdal)

in.dir <- "your_directory_name"

sh <- readOGR(in.dir, layer = "highdist_n83")
sh@proj4string # note projection details
sh2 <- spTransform(sh, CRS("+proj=longlat +datum=WGS84"))
sh2@proj4string # and after transforming
plot(sh2)

This gives me:

> sh@proj4string # note projection details
CRS arguments:
 +proj=utm +zone=4 +datum=NAD83 +units=m +no_defs +ellps=GRS80
+towgs84=0,0,0 

and:

> sh2@proj4string # and after transforming
CRS arguments:
 +proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0 

See if that works for you with the address data you wish to add. EDIT If you want to go down the ggplot2 route you could do something like this:

# Plotting with ggplot - first transform shapefile to data frame
sh2.df <- fortify(sh2)

# Fake address data taken from Google maps
add <- data.frame(name = c("University of Hawaii", "Mokuleia Beach Park"),
                      lat = c(21.298971, 21.580508),
                      long = c(-157.817722, -158.191017))

# And plot
ggplot(data = sh2.df, aes(x = long, y = lat, group = group)) +
     geom_polygon(colour = "grey20", fill = "white", size = 0.8) +
     geom_point(data = add, size = 3, aes(x = long, y = lat, group = NULL), pch = 21, fill = "red") +
     coord_equal() +
     theme()

Which gives this: