Merge pull request #13 from geocrystal/ring_area

use geocrystal/ring_area for area

shard.yml

@@ -14,6 +14,8 @@ dependencies:
     github: geocrystal/geohash
   geo_bearing:
     github: geocrystal/geo_bearing
+  ring_area:
+    github: geocrystal/ring_area
 
 development_dependencies:
   ameba:

spec/geo/polygon_spec.cr

@@ -85,15 +85,19 @@ describe Geo::Polygon do
 
   describe "#area" do
     coords = [
-      Geo::Coord.new(-3, -2),
-      Geo::Coord.new(-1, 4),
-      Geo::Coord.new(6, 1),
-      Geo::Coord.new(3, 10),
-      Geo::Coord.new(-4, 9),
+      Geo::Coord.new(-15, 125),
+      Geo::Coord.new(-22, 113),
+      Geo::Coord.new(-37, 117),
+      Geo::Coord.new(-33, 130),
+      Geo::Coord.new(-39, 148),
+      Geo::Coord.new(-27, 154),
+      Geo::Coord.new(-15, 144),
+      Geo::Coord.new(-15, 125),
     ]
+
     polygon = Geo::Polygon.new(coords)
 
-    it { polygon.area.should eq(60.0) }
+    it { polygon.area.should eq(7748891609977.457) }
   end
 
   describe "comparisons" do

src/geo.cr

@@ -2,6 +2,7 @@ require "convex_hull"
 require "geohash"
 require "geo_bearing"
 require "haversine"
+require "ring_area"
 require "./geo/utils"
 require "./geo/coord"
 require "./geo/polygon"

src/geo/coord.cr

@@ -188,10 +188,6 @@ module Geo
       true
     end
 
-    def shoelace(other : Geo::Coord)
-      lat * other.lng - lng * other.lat
-    end
-
     private def guard_seconds(pattern : String, result : String) : Array(String)?
       if m = pattern.match(/<(lat|lng)s>/)
         return [result, ""] unless m && result.starts_with?("60")

src/geo/polygon.cr

@@ -25,9 +25,11 @@ module Geo
       @coords
     end
 
-    # Evaluate area of a polygon using shoelace formula
+    # Return the approximate signed geodesic area of the polygon in square meters.
     def area
-      @coords.each_cons_pair.sum { |lat, lng| lat.shoelace(lng) }.abs.fdiv(2)
+      coordinates = @coords.map { |coord| [coord.lng, coord.lat] }
+
+      RingArea.ring_area(coordinates)
     end
 
     # Order coords in lexicographical order.