Add geographic_center class method.

lib/geocoder.rb

@@ -214,6 +214,34 @@ module Geocoder
     c * units[options[:units]]
   end
   
+  ##
+  # Compute the geographic center (aka geographic midpoint, center of
+  # gravity) for an array of [lat,lon] points.
+  #
+  def self.geographic_center(points)
+    
+    # convert to Cartesian coordinates
+    x = []; y = [], z = []
+    points.each do |p|
+      x << cos(p[0]) * cos(p[1])
+      y << cos(p[0]) * sin(p[1])
+      z << sin(p[0])
+    end
+    
+    # compute average coordinates
+    avg = [x,y,z].map do |c|
+      c.inject(0){ |tot,i| tot += i }
+    end
+    
+    # convert back to latitude/longitude
+    xa, ya, za = avg
+    lon = atan2(ya, xa)
+    hyp = sqrt(xa * xa + ya * ya)
+    lat = atan2(za, hyp)
+    
+    [lat,lon]
+  end
+
   ##
   # Convert degrees to radians.
   #

test/geocoder_test.rb

@@ -7,4 +7,14 @@ class GeocoderTest < Test::Unit::TestCase
     assert_equal [40.7495760, -73.9916733], v.fetch_coordinates
     assert_equal [40.7495760, -73.9916733], [v.latitude, v.longitude]
   end
+  
+  # sanity check
+  def test_distance_between
+    assert_equal 69, Geocoder.distance_between(0,0, 0,1).round
+  end
+  
+  # sanity check
+  def test_geographic_center
+    assert_equal 69, Geocoder.geographic_center([[0,0], [0,1]]).round
+  end
 end