gistfile1.java

/**
 * <p>Represents a point on the surface of a sphere. (The Earth is almost
 * spherical.)</p>
 *
 * <p>To create an instance, call one of the static methods fromDegrees() or
 * fromRadians().</p>
 *
 * <p>This code was originally published at
 * <a href="http://JanMatuschek.de/LatitudeLongitudeBoundingCoordinates#Java">
 * http://JanMatuschek.de/LatitudeLongitudeBoundingCoordinates#Java</a>.</p>
 *
 * @author Jan Philip Matuschek
 * @version 22 September 2010
 */
public class GeoLocation {

	private double radLat;  // latitude in radians
	private double radLon;  // longitude in radians

	private double degLat;  // latitude in degrees
	private double degLon;  // longitude in degrees

	private static final double MIN_LAT = Math.toRadians(-90d);  // -PI/2
	private static final double MAX_LAT = Math.toRadians(90d);   //  PI/2
	private static final double MIN_LON = Math.toRadians(-180d); // -PI
	private static final double MAX_LON = Math.toRadians(180d);  //  PI

	private GeoLocation () {
	}

	/**
	 * @param latitude the latitude, in degrees.
	 * @param longitude the longitude, in degrees.
	 */
	public static GeoLocation fromDegrees(double latitude, double longitude) {
		GeoLocation result = new GeoLocation();
		result.radLat = Math.toRadians(latitude);
		result.radLon = Math.toRadians(longitude);
		result.degLat = latitude;
		result.degLon = longitude;
		result.checkBounds();
		return result;
	}

	/**
	 * @param latitude the latitude, in radians.
	 * @param longitude the longitude, in radians.
	 */
	public static GeoLocation fromRadians(double latitude, double longitude) {
		GeoLocation result = new GeoLocation();
		result.radLat = latitude;
		result.radLon = longitude;
		result.degLat = Math.toDegrees(latitude);
		result.degLon = Math.toDegrees(longitude);
		result.checkBounds();
		return result;
	}

	private void checkBounds() {
		if (radLat < MIN_LAT || radLat > MAX_LAT ||
				radLon < MIN_LON || radLon > MAX_LON)
			throw new IllegalArgumentException();
	}

	/**
	 * @return the latitude, in degrees.
	 */
	public double getLatitudeInDegrees() {
		return degLat;
	}

	/**
	 * @return the longitude, in degrees.
	 */
	public double getLongitudeInDegrees() {
		return degLon;
	}

	/**
	 * @return the latitude, in radians.
	 */
	public double getLatitudeInRadians() {
		return radLat;
	}

	/**
	 * @return the longitude, in radians.
	 */
	public double getLongitudeInRadians() {
		return radLon;
	}

	@Override
	public String toString() {
		return "(" + degLat + "\u00B0, " + degLon + "\u00B0) = (" +
				 radLat + " rad, " + radLon + " rad)";
	}

	/**
	 * Computes the great circle distance between this GeoLocation instance
	 * and the location argument.
	 * @param radius the radius of the sphere, e.g. the average radius for a
	 * spherical approximation of the figure of the Earth is approximately
	 * 6371.01 kilometers.
	 * @return the distance, measured in the same unit as the radius
	 * argument.
	 */
	public double distanceTo(GeoLocation location, double radius) {
		return Math.acos(Math.sin(radLat) * Math.sin(location.radLat) +
				Math.cos(radLat) * Math.cos(location.radLat) *
				Math.cos(radLon - location.radLon)) * radius;
	}

	/**
	 * <p>Computes the bounding coordinates of all points on the surface
	 * of a sphere that have a great circle distance to the point represented
	 * by this GeoLocation instance that is less or equal to the distance
	 * argument.</p>
	 * <p>For more information about the formulae used in this method visit
	 * <a href="http://JanMatuschek.de/LatitudeLongitudeBoundingCoordinates">
	 * http://JanMatuschek.de/LatitudeLongitudeBoundingCoordinates</a>.</p>
	 * @param distance the distance from the point represented by this
	 * GeoLocation instance. Must me measured in the same unit as the radius
	 * argument.
	 * @param radius the radius of the sphere, e.g. the average radius for a
	 * spherical approximation of the figure of the Earth is approximately
	 * 6371.01 kilometers.
	 * @return an array of two GeoLocation objects such that:<ul>
	 * <li>The latitude of any point within the specified distance is greater
	 * or equal to the latitude of the first array element and smaller or
	 * equal to the latitude of the second array element.</li>
	 * <li>If the longitude of the first array element is smaller or equal to
	 * the longitude of the second element, then
	 * the longitude of any point within the specified distance is greater
	 * or equal to the longitude of the first array element and smaller or
	 * equal to the longitude of the second array element.</li>
	 * <li>If the longitude of the first array element is greater than the
	 * longitude of the second element (this is the case if the 180th
	 * meridian is within the distance), then
	 * the longitude of any point within the specified distance is greater
	 * or equal to the longitude of the first array element
	 * <strong>or</strong> smaller or equal to the longitude of the second
	 * array element.</li>
	 * </ul>
	 */
	public GeoLocation[] boundingCoordinates(double distance, double radius) {

		if (radius < 0d || distance < 0d)
			throw new IllegalArgumentException();

		// angular distance in radians on a great circle
		double radDist = distance / radius;

		double minLat = radLat - radDist;
		double maxLat = radLat + radDist;

		double minLon, maxLon;
		if (minLat > MIN_LAT && maxLat < MAX_LAT) {
			double deltaLon = Math.asin(Math.sin(radDist) /
				Math.cos(radLat));
			minLon = radLon - deltaLon;
			if (minLon < MIN_LON) minLon += 2d * Math.PI;
			maxLon = radLon + deltaLon;
			if (maxLon > MAX_LON) maxLon -= 2d * Math.PI;
		} else {
			// a pole is within the distance
			minLat = Math.max(minLat, MIN_LAT);
			maxLat = Math.min(maxLat, MAX_LAT);
			minLon = MIN_LON;
			maxLon = MAX_LON;
		}

		return new GeoLocation[]{fromRadians(minLat, minLon),
				fromRadians(maxLat, maxLon)};
	}

}