openlayers/lib/OpenLayers/Layer/SphericalMercator.js

/**
 * @requires OpenLayers/Layer.js
 * @requires OpenLayers/Projection.js
 */

/**
 * Class: OpenLayers.Layer.SphericalMercator
 * A mixin for layers that wraps up the pieces neccesary to have a coordinate
 *     conversion for working with commercial APIs which use a spherical
 *     mercator projection.  Using this layer as a base layer, additional
 *     layers can be used as overlays if they are in the same projection.
 *
 * A layer is given properties of this object by setting the sphericalMercator
 *     property to true.
 *
 * More projection information:
 *  - http://spatialreference.org/ref/user/google-projection/
 *
 * Proj4 Text:
 *     +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0
 *     +k=1.0 +units=m +nadgrids=@null +no_defs
 *
 * WKT:
 *     900913=PROJCS["WGS84 / Simple Mercator", GEOGCS["WGS 84",
 *     DATUM["WGS_1984", SPHEROID["WGS_1984", 6378137.0, 298.257223563]], 
 *     PRIMEM["Greenwich", 0.0], UNIT["degree", 0.017453292519943295], 
 *     AXIS["Longitude", EAST], AXIS["Latitude", NORTH]],
 *     PROJECTION["Mercator_1SP_Google"], 
 *     PARAMETER["latitude_of_origin", 0.0], PARAMETER["central_meridian", 0.0], 
 *     PARAMETER["scale_factor", 1.0], PARAMETER["false_easting", 0.0], 
 *     PARAMETER["false_northing", 0.0], UNIT["m", 1.0], AXIS["x", EAST],
 *     AXIS["y", NORTH], AUTHORITY["EPSG","900913"]]
 */
OpenLayers.Layer.SphericalMercator = {

    /**
     * Method: getExtent
     * Get the map's extent.
     *
     * Returns:
     * {<OpenLayers.Bounds>} The map extent.
     */
    getExtent: function() {
        var extent = null;
        if (this.sphericalMercator) {
            extent = this.map.calculateBounds();
        } else {
            extent = OpenLayers.Layer.FixedZoomLevels.prototype.getExtent.apply(this);
        }
        return extent;
    },

    /** 
     * Method: initMercatorParameters 
     * Set up the mercator parameters on the layer: resolutions,
     *     projection, units.
     */
    initMercatorParameters: function() {
        // set up properties for Mercator - assume EPSG:900913
        this.RESOLUTIONS = [];
        var maxResolution = 156543.0339;
        for(var zoom=0; zoom<=this.MAX_ZOOM_LEVEL; ++zoom) {
            this.RESOLUTIONS[zoom] = maxResolution / Math.pow(2, zoom);
        }
        this.units = "m";
        this.projection = "EPSG:900913";
    },

    /**
     * APIMethod: forwardMercator
     * Given a lon,lat in EPSG:4326, return a point in Spherical Mercator.
     *
     * Parameters:
     * lon - {float} 
     * lat - {float}
     * 
     * Returns:
     * {<OpenLayers.LonLat>} The coordinates transformed to Mercator.
     */
    forwardMercator: function(lon, lat) {
        var x = lon * 20037508.34 / 180;
        var y = Math.log(Math.tan((90 + lat) * Math.PI / 360)) / (Math.PI / 180);

        y = y * 20037508.34 / 180;
        
        return new OpenLayers.LonLat(x, y);
    },

    /**
     * APIMethod: inverseMercator
     * Given a x,y in Spherical Mercator, return a point in EPSG:4326.
     *
     * Parameters:
     * x - {float} A map x in Spherical Mercator.
     * y - {float} A map y in Spherical Mercator.
     * 
     * Returns:
     * {<OpenLayers.LonLat>} The coordinates transformed to EPSG:4326.
     */
    inverseMercator: function(x, y) {

        var lon = (x / 20037508.34) * 180;
        var lat = (y / 20037508.34) * 180;

        lat = 180/Math.PI * (2 * Math.atan(Math.exp(lat * Math.PI / 180)) - Math.PI / 2);
        
        return new OpenLayers.LonLat(lon, lat);
    },

    /**
     * Method: projectForward 
     * Given an object with x and y properties in EPSG:4326, modify the x,y
     * properties on the object to be the Spherical Mercator projected
     * coordinates.
     *
     * Parameters:
     * point - {Object} An object with x and y properties. 
     * 
     * Returns:
     * {Object} The point, with the x and y properties transformed to spherical
     * mercator.
     */
    projectForward: function(point) {
        var lonlat = OpenLayers.Layer.SphericalMercator.forwardMercator(point.x, point.y);
        point.x = lonlat.lon;
        point.y = lonlat.lat;
        return point;
    },
    
    /**
     * Method: projectInverse
     * Given an object with x and y properties in Spherical Mercator, modify
     * the x,y properties on the object to be the unprojected coordinates.
     *
     * Parameters:
     * point - {Object} An object with x and y properties. 
     * 
     * Returns:
     * {Object} The point, with the x and y properties transformed from
     * spherical mercator to unprojected coordinates..
     */
    projectInverse: function(point) {
        var lonlat = OpenLayers.Layer.SphericalMercator.inverseMercator(point.x, point.y);
        point.x = lonlat.lon;
        point.y = lonlat.lat;
        return point;
    }

};

/**
 * Note: Two transforms declared
 * Transforms from EPSG:4326 to EPSG:900913 and from EPSG:900913 to EPSG:4326
 *     are set by this class.
 */
OpenLayers.Projection.addTransform("EPSG:4326", "EPSG:900913",
    OpenLayers.Layer.SphericalMercator.projectForward);
OpenLayers.Projection.addTransform("EPSG:900913", "EPSG:4326",
    OpenLayers.Layer.SphericalMercator.projectInverse);