• Quote
• Contact Us
• FAQ

Direction analysis

In this example we'll learn how to plot a diagram showing what directions couriers travelled in:


This example plots a crude radar chart of the direction couriers travel in. North is up, East is right etc as in a compass. The goal isn't perfect information visualisation but to provide a jumping point for you to perfect it.

You may expect that in a loosely gridded city like London, couriers may travel along those grid lines. The above graphic suggests this may be the case but is by no means conclusive - it's taken from one hour of data. This example assumes basic knowledge of the UNIX environment and ruby. Let's see how it works:

#!/usr/bin/ruby

require 'xml/libxml'
require 'RMagick'

Start the ruby interpreter and load in a XML library (libxml) to parse the data and the ruby image magick library to draw the image.

xml = `curl -s 'http://api.ecourier.co.uk/xmlapi/0.1/API_KEY/PASSWORD/courier/time_range/2006-08-22%2010:00/2006-08-22%2012:00'`

p = XML::Parser.new
p.string = xml
doc = p.parse

Use curl to load some XML data in to a string. Load this an XML object and parse it so that we can iterate over the data.

buckets = [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]

doc.find('courier').each do |courier|
  i = (courier['heading'].to_f % 24).to_i
  buckets[i] = buckets[i] + 1
end

Go through all the data and put the direction of the courier in to a bucket 15 degrees wide (24 buckets on a circle).

biggest = 0

24.times do |i|
  biggest = buckets[i] if buckets[i] > biggest
end

Figure out what the biggest bucket size is so that we can scale all results by this later

canvas = Magick::Image.new(400,400)
gc = Magick::Draw.new
gc.stroke('black')
gc.stroke_width(3)

Create a new image 400x400 pixels across. Make a graphics context to draw on with a black pen 3 pixels wide

24.times do |i|
  buckets[i] = buckets[i].to_f / biggest
  angle = ((i * 15) + 7.5)
  x = buckets[i] * 200 * Math.sin(angle * 3.1415 / 180)
  y = buckets[i] * 200 * Math.cos(angle * 3.1415 / 180)
  gc.line(200,200, 200 + x, 200 + y)
end

For each bucket: Scale the bucket size, find the angle to draw the line, find the end point of the line. Finally draw a line from the center out to that end point.

gc.draw(canvas)
canvas.write('courier_directions.jpg')

Draw the context on to the image and save it with the given filename.