Calculate the area of a polygon drawn on google maps in an Android application

I would like to calculate the are of a polygon drawn in a map fragment for a college project.

This is how I draw my polygon.

@Override
    public void onMapClick(LatLng point) {
        //tvLocInfo.setText("New marker added@" + point.toString());
        map.addMarker(new MarkerOptions().position(point).draggable(true).title(point.toString()));

        markerClicked = false;
    }

    @Override
    public void onMapLongClick(LatLng point) {
        //tvLocInfo.setText("New marker added@" + point.toString());
        map.clear();
    }

    @Override
    public boolean onMarkerClick(Marker marker) {

        if(markerClicked){

            if(polygon != null){
                polygon.remove();
                polygon = null;
            }

            polygonOptions.add(marker.getPosition());
            polygonOptions.strokeColor(Color.RED);
            polygonOptions.fillColor(Color.BLUE);
            polygon = map.addPolygon(polygonOptions);
            //Area = google.maps.geometry.spherical.computeArea(polygon.getPath().getArray());
        }else{
            if(polygon != null){
                polygon.remove();
                polygon = null;
            }

            polygonOptions = new PolygonOptions().add(marker.getPosition());
            markerClicked = true;
        }

I have seen this code on how to calculate the area but I am unsure how to implement it in my application and calculate the area of my polygon.

I use this code to calculate an area of a GPS with Android:

  private static final double EARTH_RADIUS = 6371000;// meters

  public static double calculateAreaOfGPSPolygonOnEarthInSquareMeters(final List<Location> locations) {
    return calculateAreaOfGPSPolygonOnSphereInSquareMeters(locations, EARTH_RADIUS);
  }

  private static double calculateAreaOfGPSPolygonOnSphereInSquareMeters(final List<Location> locations, final double radius) {
    if (locations.size() < 3) {
      return 0;
    }

    final double diameter = radius * 2;
    final double circumference = diameter * Math.PI;
    final List<Double> listY = new ArrayList<Double>();
    final List<Double> listX = new ArrayList<Double>();
    final List<Double> listArea = new ArrayList<Double>();
    // calculate segment x and y in degrees for each point
    final double latitudeRef = locations.get(0).getLatitude();
    final double longitudeRef = locations.get(0).getLongitude();
    for (int i = 1; i < locations.size(); i++) {
      final double latitude = locations.get(i).getLatitude();
      final double longitude = locations.get(i).getLongitude();
      listY.add(calculateYSegment(latitudeRef, latitude, circumference));
      Log.d(LOG_TAG, String.format("Y %s: %s", listY.size() - 1, listY.get(listY.size() - 1)));
      listX.add(calculateXSegment(longitudeRef, longitude, latitude, circumference));
      Log.d(LOG_TAG, String.format("X %s: %s", listX.size() - 1, listX.get(listX.size() - 1)));
    }

    // calculate areas for each triangle segment
    for (int i = 1; i < listX.size(); i++) {
      final double x1 = listX.get(i - 1);
      final double y1 = listY.get(i - 1);
      final double x2 = listX.get(i);
      final double y2 = listY.get(i);
      listArea.add(calculateAreaInSquareMeters(x1, x2, y1, y2));
      Log.d(LOG_TAG, String.format("area %s: %s", listArea.size() - 1, listArea.get(listArea.size() - 1)));
    }

    // sum areas of all triangle segments
    double areasSum = 0;
    for (final Double area : listArea) {
      areasSum = areasSum + area;
    }

    // get abolute value of area, it can't be negative
    return Math.abs(areasSum);// Math.sqrt(areasSum * areasSum);
  }

  private static Double calculateAreaInSquareMeters(final double x1, final double x2, final double y1, final double y2) {
    return (y1 * x2 - x1 * y2) / 2;
  }

  private static double calculateYSegment(final double latitudeRef, final double latitude, final double circumference) {
    return (latitude - latitudeRef) * circumference / 360.0;
  }

  private static double calculateXSegment(final double longitudeRef, final double longitude, final double latitude,
      final double circumference) {
    return (longitude - longitudeRef) * circumference * Math.cos(Math.toRadians(latitude)) / 360.0;
  }

I could also use the following polygon which is static if calculating the area of the drawn polygon is not possible.

Polygon UCCpolygon = map.addPolygon(new PolygonOptions()
    .add(new LatLng(51.893728, -8.491865), 
         new LatLng(51.893550, -8.492479), 
         new LatLng(51.893216, -8.492224), 
         new LatLng(51.893404, -8.491598))
 .strokeColor(Color.RED)
 .fillColor(Color.BLUE));

Thanks for the help!

Sean

There's already a library for that.

import com.google.maps.android.SphericalUtil;

//...

List<LatLng> latLngs = new ArrayList<>();
latLngs.add(new LatLng(51.893728, -8.491865));
latLngs.add(new LatLng(51.893550, -8.492479));
latLngs.add(new LatLng(51.893216, -8.492224));
latLngs.add(new LatLng(51.893404, -8.491598));
Log.i(TAG, "computeArea " + SphericalUtil.computeArea(latLngs));

For me the output is computeArea 1920.8879882782069

If you want to use SphericalUtils code without any library, you can use following code. it's taken from opensource code from SphericalUtils.java and other class. I have taken this code and used it as i was using MapBox and MapBox does not have implemented the calculateArea function in Turf.

import java.util.List;

import pojo.LatLng;

import static java.lang.Math.PI;
import static java.lang.Math.abs;
import static java.lang.Math.atan2;
import static java.lang.Math.cos;
import static java.lang.Math.sin;
import static java.lang.Math.tan;
import static java.lang.Math.toRadians;

public class PolygonUtils {

    /**
     * The earth's radius, in meters.
     * Mean radius as defined by IUGG.
     */
    static final double EARTH_RADIUS = 6371009;

    /**
     * Returns the area of a closed path on Earth.
     * @param path A closed path.
     * @return The path's area in square meters.
     */
    public static double computeArea(List<LatLng> path) {
        return abs(computeSignedArea(path,EARTH_RADIUS));
    }

    /**
     * Returns the signed area of a closed path on a sphere of given radius.
     * The computed area uses the same units as the radius squared.
     * Used by SphericalUtilTest.
     */
    static double computeSignedArea(List<LatLng> path, double radius) {
        int size = path.size();
        if (size < 3) { return 0; }
        double total = 0;
        LatLng prev = path.get(size - 1);
        double prevTanLat = tan((PI / 2 - toRadians(prev.getLatitude())) / 2);
        double prevLng = toRadians(prev.getLongitude());
        // For each edge, accumulate the signed area of the triangle formed by the North Pole
        // and that edge ("polar triangle").
        for (LatLng point : path) {
            double tanLat = tan((PI / 2 - toRadians(point.getLatitude())) / 2);
            double lng = toRadians(point.getLongitude());
            total += polarTriangleArea(tanLat, lng, prevTanLat, prevLng);
            prevTanLat = tanLat;
            prevLng = lng;
        }
        return total * (radius * radius);
    }

    /**
     * Returns the signed area of a triangle which has North Pole as a vertex.
     * Formula derived from "Area of a spherical triangle given two edges and the included angle"
     * as per "Spherical Trigonometry" by Todhunter, page 71, section 103, point 2.
     * See http://books.google.com/books?id=3uBHAAAAIAAJ&pg=PA71
     * The arguments named "tan" are tan((pi/2 - latitude)/2).
     */
    private static double polarTriangleArea(double tan1, double lng1, double tan2, double lng2) {
        double deltaLng = lng1 - lng2;
        double t = tan1 * tan2;
        return 2 * atan2(t * sin(deltaLng), 1 + t * cos(deltaLng));
    }
}

来源：https://stackoverflow.com/questions/28838287/calculate-the-area-of-a-polygon-drawn-on-google-maps-in-an-android-application

标签

android

google-maps

google-maps-android-api-2

polygon