MercatorMap.java | Utility class to convert between geo-locations and Cartesian screen coordinates, via tillnagel.com

MercatorMap.java

/**
 * Utility class to convert between geo-locations and Cartesian screen coordinates.
 * Can be used with a bounding box defining the map section.
 *
 * (c) 2011 Till Nagel, tillnagel.com
 */
public class MercatorMap {
  
  public static final float DEFAULT_TOP_LATITUDE = 80;
  public static final float DEFAULT_BOTTOM_LATITUDE = -80;
  public static final float DEFAULT_LEFT_LONGITUDE = -180;
  public static final float DEFAULT_RIGHT_LONGITUDE = 180;
  
  /** Horizontal dimension of this map, in pixels. */
  protected float mapScreenWidth;
  /** Vertical dimension of this map, in pixels. */
  protected float mapScreenHeight;

  /** Northern border of this map, in degrees. */
  protected float topLatitude;
  /** Southern border of this map, in degrees. */
  protected float bottomLatitude;
  /** Western border of this map, in degrees. */
  protected float leftLongitude;
  /** Eastern border of this map, in degrees. */
  protected float rightLongitude;

  private float topLatitudeRelative;
  private float bottomLatitudeRelative;
  private float leftLongitudeRadians;
  private float rightLongitudeRadians;

  public MercatorMap(float mapScreenWidth, float mapScreenHeight) {
    this(mapScreenWidth, mapScreenHeight, DEFAULT_TOP_LATITUDE, DEFAULT_BOTTOM_LATITUDE, DEFAULT_LEFT_LONGITUDE, DEFAULT_RIGHT_LONGITUDE);
  }
  
  /**
   * Creates a new MercatorMap with dimensions and bounding box to convert between geo-locations and screen coordinates.
   *
   * @param mapScreenWidth Horizontal dimension of this map, in pixels.
   * @param mapScreenHeight Vertical dimension of this map, in pixels.
   * @param topLatitude Northern border of this map, in degrees.
   * @param bottomLatitude Southern border of this map, in degrees.
   * @param leftLongitude Western border of this map, in degrees.
   * @param rightLongitude Eastern border of this map, in degrees.
   */
  public MercatorMap(float mapScreenWidth, float mapScreenHeight, float topLatitude, float bottomLatitude, float leftLongitude, float rightLongitude) {
    this.mapScreenWidth = mapScreenWidth;
    this.mapScreenHeight = mapScreenHeight;
    this.topLatitude = topLatitude;
    this.bottomLatitude = bottomLatitude;
    this.leftLongitude = leftLongitude;
    this.rightLongitude = rightLongitude;

    this.topLatitudeRelative = getScreenYRelative(topLatitude);
    this.bottomLatitudeRelative = getScreenYRelative(bottomLatitude);
    this.leftLongitudeRadians = getRadians(leftLongitude);
    this.rightLongitudeRadians = getRadians(rightLongitude);
  }

  /**
   * Projects the geo location to Cartesian coordinates, using the Mercator projection.
   *
   * @param geoLocation Geo location with (latitude, longitude) in degrees.
   * @returns The screen coordinates with (x, y).
   */
  public PVector getScreenLocation(PVector geoLocation) {
    float latitudeInDegrees = geoLocation.x;
    float longitudeInDegrees = geoLocation.y;

    return new PVector(getScreenX(longitudeInDegrees), getScreenY(latitudeInDegrees));
  }

  private float getScreenYRelative(float latitudeInDegrees) {
    return log(tan(latitudeInDegrees / 360f * PI + PI / 4));
  }

  protected float getScreenY(float latitudeInDegrees) {
    return mapScreenHeight * (getScreenYRelative(latitudeInDegrees) - topLatitudeRelative) / (bottomLatitudeRelative - topLatitudeRelative);
  }
  
  private float getRadians(float deg) {
    return deg * PI / 180;
  }

  protected float getScreenX(float longitudeInDegrees) {
    float longitudeInRadians = getRadians(longitudeInDegrees);
    return mapScreenWidth * (longitudeInRadians - leftLongitudeRadians) / (rightLongitudeRadians - leftLongitudeRadians);
  }
}