Add calculations for distances btwn lat/lon lines.

lib/geocoder/calculations.rb

@@ -21,6 +21,21 @@ module Geocoder
     #
     KM_IN_MI = 0.621371192
 
+    ##
+    # Calculate the distance between latitude lines in the given units.
+    #
+    def distance_between_latitude_lines(units = :mi)
+      2 * Math::PI * earth_radius(units) / 360
+    end
+
+    ##
+    # Calculate the distance between longitude lines at the given latitude.
+    # This ranges from around 69 miles at the equator to zero at the poles.
+    #
+    def distance_between_longitude_lines(latitude, units = :mi)
+      distance_between_latitude_lines(units) * Math.cos(to_radians(latitude))
+    end
+
     ##
     # Calculate the distance between two points on Earth (Haversine formula).
     # Takes two sets of coordinates and an options hash:

test/geocoder_test.rb

@@ -139,6 +139,18 @@ class GeocoderTest < Test::Unit::TestCase
 
   # --- calcluations ---
 
+  def test_distance_between_longitude_lines_at_equator
+    assert_equal 69, Geocoder::Calculations.distance_between_longitude_lines(0).round
+  end
+
+  def test_distance_between_longitude_lines_at_new_york
+    assert_equal 53, Geocoder::Calculations.distance_between_longitude_lines(40).round
+  end
+
+  def test_distance_between_longitude_lines_at_north_pole
+    assert_equal 0, Geocoder::Calculations.distance_between_longitude_lines(89.98).round
+  end
+
   def test_distance_between
     assert_equal 69, Geocoder::Calculations.distance_between(0,0, 0,1).round
     la_to_ny = Geocoder::Calculations.distance_between(34.05,-118.25, 40.72,-74).round