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Tuesday, 13 October 2015

Mapping SQL Relay locations using ggplot...

I may have planned to talk about too much in a one hour talk at SQL Relay. In terms of what I planned to show, I wanted to show a little bit of geomapping using ggplot.

Here is the visualisation of the SQL Relay locations drawn with ggplot:

This script illustrates a few interesting things about ggplot:
  • you can plot polygons using the geom_polygon() function - complex shapes like maps
  • you can use different data.frames for different 'layers' in a plot
  • you can sequentially add extra detail to your visualisation - this script uses:


# draw a map of the UK with R. 
# https://www.students.ncl.ac.uk/keith.newman/r/maps-in-r

# install packages if necessary

library(maps)       # Provides functions that let us plot the maps
library(mapdata)    # Contains the hi-resolution maps.
library(ggmap)      # finds longitude and latitude values
library(ggplot2)    # high quality graphing

locs <- c("Nottingham England", "Reading England", "London England", "Bristol England", "Cardiff Wales", "Birmingham England")
locI <- c("N", "R", "L", "Br", "C", "Bi")
locCol <- c("darkgoldenrod1", "yellowgreen", "yellowgreen", "darkgoldenrod1", "blue", "tomato")
lonlat <- geocode(locs)

# draw a simple map of the United Kingdom
    xlim=c(-10,2), ylim=c(50,60))
# plot SQL Relay Locations info using base graphics
       col= locCol,
       cex = 1)
Map made with map() and base graphics

# we can also do this in ggplot better...
# make a data.frame 
data2 <- as.data.frame(locs)

# bind in longitude, latitude, colours and initials
data2 <- cbind(data2, lonlat, locCol, locI)

# use map_data() function to create a data.frame containing the map of the UK
uk<-map_data("worldHires", region = "UK")

# create the object m with the map using geom_polygon() function 
# we can make the map different colours
# see here for more about colours in R
# http://www.stat.columbia.edu/~tzheng/files/Rcolor.pdf
m <- ggplot() +
                  aes(x=long, y=lat, group=group),
                  fill="gray88") +   # more about colours
            scale_x_continuous(limits = c(-10,3)) +
            scale_y_continuous(limits =c(49,60.9))

# add the points on the plot telling us where the longitude and latitude of various places
# N.B. the data is coming from a different data.frame to the map
# first add some black dots
m <- m +  geom_point(data=data2, 
                     aes(x=lon, y=lat), 
                     size = 7) 

# then some smaller coloured dots
m <- m +  geom_point(data=data2, 
                aes(x=lon, y=lat), 
                size = 6)

# add the letters inside the circles  
m <- m + geom_text(data = data2, 
                   aes(x=lon, y=lat, label = locI, size = 4))

# change the theme to make the map nicer
m <- m + theme_bw()

# add a title
m <- m + ggtitle("SQL Relay Locations") +
         theme(legend.position="none") # get rid of the legend

m # show the object

Final map was exported as an A5 PDF which looked nice.


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