Compare two points and conclude direction

Question

Is there a simple method in geotools or JTS to conclude the direction between two geospatial points?

I would like to conclude if point A is (north, south, east, west, NW, NE, SW, SE) when compared with point B?

Or would I need to manually compute the angle between the points (possibly similar to http://docs.geotools.org/stable/userguide/library/referencing/calculator.html) and bin it into the aforementioned directions?

Answer 1

As it happens I have some code lying around that does most of this, so this should do what you need.

import java.util.ArrayList;

import org.geotools.geometry.DirectPosition2D;
import org.geotools.referencing.GeodeticCalculator;
import org.geotools.referencing.crs.DefaultGeographicCRS;
import org.geotools.util.factory.GeoTools;
import org.locationtech.jts.geom.Point;
import org.opengis.referencing.operation.TransformException;

import com.ianturton.cookbook.utilities.GenerateRandomData;

/**
 * given a direction in degrees (from North) return the Compass Point on an 8
 * element compass rose.
 * 
 * @author ian
 */

public class CompassRose8 {
  private static final int SIZE = 8;
  static final float STEP = 360f / SIZE;
  static final float HALF_STEP = STEP / 2.0f;
  static final GeodeticCalculator gc = new GeodeticCalculator();

  static final String points[] = { "N", "NE", "E", "SE", "S", "SW", "W", "NW", "N" };
  static ArrayList<Direction> directions = new ArrayList<>();

  static {
    int k = 0;
    for (float i = 0; i <= 360; i += (360.0 / SIZE), k++) {
      Direction d = new Direction(points[k], i);
      directions.add(d);
    }
  }

  public static String getDirection(Point p1, Point p2) throws TransformException {
    gc.setStartingPosition(new DirectPosition2D(DefaultGeographicCRS.WGS84, p1.getX(), p1.getY()));
    gc.setDestinationPosition(new DirectPosition2D(DefaultGeographicCRS.WGS84, p2.getX(), p2.getY()));
    float heading = (float) (gc.getAzimuth() + 180.0f);
    return getDirection(heading);
  }

  public static String getDirection(float heading) {
    while (heading < 0) {
      heading += 360;
    }
    while (heading > 360) {
      heading -= 360;
    }
    for (Direction d : directions) {
      if (d.contains(heading)) {
        return d.name;
      }
    }
    return "bad heading " + heading;
  }

  public static void main(String[] args) throws TransformException {
    // Make sure we know our axis order (doesn't matter which we pick)
    System.setProperty(GeoTools.FORCE_LONGITUDE_FIRST_AXIS_ORDER, "true");
    Point p1 = GenerateRandomData.createRandomPoint();
    Point p2 = GenerateRandomData.createRandomPoint();
    System.out.println(p1 + " is " + CompassRose8.getDirection(p1, p2) + " of " + p2);
  }

  static public class Direction {
    private float start;
    private float end;
    private float centre;
    final String name;

    Direction(String name, float centre) {
      this.name = name;
      this.centre = centre;
      start = this.centre - HALF_STEP;
      end = this.centre + HALF_STEP;
    }

    public boolean contains(float degree) {
      return (start < degree && end > degree);
    }
  }

}