From 6d9708e7e8688727fbb6e1db04cf667854addba7 Mon Sep 17 00:00:00 2001
From: Anton Tananaev <anton.tananaev@gmail.com>
Date: Thu, 19 Nov 2020 22:09:54 -0800
Subject: Fix polygon area calculation

---
 .../java/org/traccar/geofence/GeofencePolygon.java | 40 +++++++---------------
 .../org/traccar/geofence/GeofencePolygonTest.java  | 13 +++++--
 2 files changed, 22 insertions(+), 31 deletions(-)

(limited to 'src')

diff --git a/src/main/java/org/traccar/geofence/GeofencePolygon.java b/src/main/java/org/traccar/geofence/GeofencePolygon.java
index c03a71225..cd2cbf16a 100644
--- a/src/main/java/org/traccar/geofence/GeofencePolygon.java
+++ b/src/main/java/org/traccar/geofence/GeofencePolygon.java
@@ -15,9 +15,16 @@
  */
 package org.traccar.geofence;
 
+import org.locationtech.spatial4j.context.SpatialContext;
+import org.locationtech.spatial4j.context.jts.JtsSpatialContextFactory;
+import org.locationtech.spatial4j.shape.ShapeFactory;
+import org.locationtech.spatial4j.shape.jts.JtsShapeFactory;
+
 import java.text.ParseException;
 import java.util.ArrayList;
 
+import static org.locationtech.spatial4j.distance.DistanceUtils.DEG_TO_KM;
+
 public class GeofencePolygon extends GeofenceGeometry {
 
     public GeofencePolygon() {
@@ -107,37 +114,14 @@ public class GeofencePolygon extends GeofenceGeometry {
         return oddNodes;
     }
 
-    private double toRadians(double input) {
-        return input / 180.0 * Math.PI;
-    }
-
-    private double polarTriangleArea(double tan1, double lng1, double tan2, double lng2) {
-        double deltaLng = lng1 - lng2;
-        double t = tan1 * tan2;
-        return 2 * Math.atan2(t * Math.sin(deltaLng), 1 + t * Math.sin(deltaLng));
-    }
-
     @Override
     public double calculateArea() {
-        if (coordinates.size() < 3) {
-            return 0;
-        }
-
-        double total = 0;
-        Coordinate prev = coordinates.get(coordinates.size() - 1);
-        double prevTanLat = Math.tan((Math.PI / 2 - toRadians(prev.getLat())) / 2);
-        double prevLng = toRadians(prev.getLon());
-
-        for (Coordinate point : coordinates) {
-            double tanLat = Math.tan((Math.PI / 2 - toRadians(point.getLat())) / 2);
-            double lng = toRadians(point.getLon());
-            total += polarTriangleArea(tanLat, lng, prevTanLat, prevLng);
-            prevTanLat = tanLat;
-            prevLng = lng;
+        JtsShapeFactory jtsShapeFactory = new JtsSpatialContextFactory().newSpatialContext().getShapeFactory();
+        ShapeFactory.PolygonBuilder polygonBuilder = jtsShapeFactory.polygon();
+        for (Coordinate coordinate : coordinates) {
+            polygonBuilder.pointXY(coordinate.getLon(), coordinate.getLat());
         }
-
-        double earthRadius = 6371009;
-        return total * (earthRadius * earthRadius);
+        return polygonBuilder.build().getArea(SpatialContext.GEO) * DEG_TO_KM * DEG_TO_KM;
     }
 
     @Override
diff --git a/src/test/java/org/traccar/geofence/GeofencePolygonTest.java b/src/test/java/org/traccar/geofence/GeofencePolygonTest.java
index 94b73af3a..cc9c46c94 100644
--- a/src/test/java/org/traccar/geofence/GeofencePolygonTest.java
+++ b/src/test/java/org/traccar/geofence/GeofencePolygonTest.java
@@ -5,10 +5,17 @@ import java.text.ParseException;
 import org.junit.Test;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 
 public class GeofencePolygonTest {
 
+    @Test
+    public void testCalculateArea() throws ParseException {
+        String test = "POLYGON((-23.559204099194772 148.8653145299711, -23.6000443437826 148.85956016213583, -23.600411843430095 148.89462111436828, -23.5626384786532 148.90278297873897, -23.5574863232753 148.88137329347367, -23.559204099194772 148.8653145299711))";
+        assertEquals(17, new GeofencePolygon(test).calculateArea(), 1);
+    }
+
     @Test
     public void testPolygonWkt() throws ParseException {
         String test = "POLYGON ((55.75474 37.61823, 55.75513 37.61888, 55.7535 37.6222, 55.75315 37.62165))";
@@ -23,7 +30,7 @@ public class GeofencePolygonTest {
         GeofenceGeometry geofenceGeometry = new GeofencePolygon();
         geofenceGeometry.fromWkt(test);
         assertTrue(geofenceGeometry.containsPoint(55.75476, 37.61915));
-        assertTrue(!geofenceGeometry.containsPoint(55.75545, 37.61921));
+        assertFalse(geofenceGeometry.containsPoint(55.75545, 37.61921));
 
     }
     
@@ -34,7 +41,7 @@ public class GeofencePolygonTest {
         geofenceGeometry.fromWkt(test);
         assertTrue(geofenceGeometry.containsPoint(66.9015, -180.0096));
         assertTrue(geofenceGeometry.containsPoint(66.9015, 179.991));
-        assertTrue(!geofenceGeometry.containsPoint(66.8368, -179.8792));
+        assertFalse(geofenceGeometry.containsPoint(66.8368, -179.8792));
 
     }
     
@@ -45,7 +52,7 @@ public class GeofencePolygonTest {
         geofenceGeometry.fromWkt(test);
         assertTrue(geofenceGeometry.containsPoint(51.0466, -0.0165));
         assertTrue(geofenceGeometry.containsPoint(51.0466, 0.018));
-        assertTrue(!geofenceGeometry.containsPoint(50.9477, 0.5836));
+        assertFalse(geofenceGeometry.containsPoint(50.9477, 0.5836));
 
     }
 
-- 
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