/
WebMercatorProjection.js
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/
WebMercatorProjection.js
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/*global define*/
define([
'./Cartesian3',
'./Cartographic',
'./defaultValue',
'./defined',
'./defineProperties',
'./DeveloperError',
'./Ellipsoid',
'./Math'
], function(
Cartesian3,
Cartographic,
defaultValue,
defined,
defineProperties,
DeveloperError,
Ellipsoid,
CesiumMath) {
"use strict";
/**
* The map projection used by Google Maps, Bing Maps, and most of ArcGIS Online, EPSG:3857. This
* projection use longitude and latitude expressed with the WGS84 and transforms them to Mercator using
* the spherical (rather than ellipsoidal) equations.
*
* @alias WebMercatorProjection
* @constructor
*
* @param {Ellipsoid} [ellipsoid=Ellipsoid.WGS84] The ellipsoid.
*
* @see GeographicProjection
*/
var WebMercatorProjection = function(ellipsoid) {
this._ellipsoid = defaultValue(ellipsoid, Ellipsoid.WGS84);
this._semimajorAxis = this._ellipsoid.maximumRadius;
this._oneOverSemimajorAxis = 1.0 / this._semimajorAxis;
};
defineProperties(WebMercatorProjection.prototype, {
/**
* Gets the {@link Ellipsoid}.
*
* @memberof WebMercatorProjection.prototype
*
* @type {Ellipsoid}
* @readonly
*/
ellipsoid : {
get : function() {
return this._ellipsoid;
}
}
});
/**
* Converts a Mercator angle, in the range -PI to PI, to a geodetic latitude
* in the range -PI/2 to PI/2.
*
* @param {Number} mercatorAngle The angle to convert.
* @returns {Number} The geodetic latitude in radians.
*/
WebMercatorProjection.mercatorAngleToGeodeticLatitude = function(mercatorAngle) {
return CesiumMath.PI_OVER_TWO - (2.0 * Math.atan(Math.exp(-mercatorAngle)));
};
/**
* Converts a geodetic latitude in radians, in the range -PI/2 to PI/2, to a Mercator
* angle in the range -PI to PI.
*
* @param {Number} latitude The geodetic latitude in radians.
* @returns {Number} The Mercator angle.
*/
WebMercatorProjection.geodeticLatitudeToMercatorAngle = function(latitude) {
// Clamp the latitude coordinate to the valid Mercator bounds.
if (latitude > WebMercatorProjection.MaximumLatitude) {
latitude = WebMercatorProjection.MaximumLatitude;
} else if (latitude < -WebMercatorProjection.MaximumLatitude) {
latitude = -WebMercatorProjection.MaximumLatitude;
}
var sinLatitude = Math.sin(latitude);
return 0.5 * Math.log((1.0 + sinLatitude) / (1.0 - sinLatitude));
};
/**
* The maximum latitude (both North and South) supported by a Web Mercator
* (EPSG:3857) projection. Technically, the Mercator projection is defined
* for any latitude up to (but not including) 90 degrees, but it makes sense
* to cut it off sooner because it grows exponentially with increasing latitude.
* The logic behind this particular cutoff value, which is the one used by
* Google Maps, Bing Maps, and Esri, is that it makes the projection
* square. That is, the rectangle is equal in the X and Y directions.
*
* The constant value is computed by calling:
* WebMercatorProjection.mercatorAngleToGeodeticLatitude(Math.PI)
*
* @type {Number}
*/
WebMercatorProjection.MaximumLatitude = WebMercatorProjection.mercatorAngleToGeodeticLatitude(Math.PI);
/**
* Converts geodetic ellipsoid coordinates, in radians, to the equivalent Web Mercator
* X, Y, Z coordinates expressed in meters and returned in a {@link Cartesian3}. The height
* is copied unmodified to the Z coordinate.
*
* @param {Cartographic} cartographic The cartographic coordinates in radians.
* @param {Cartesian3} [result] The instance to which to copy the result, or undefined if a
* new instance should be created.
* @returns {Cartesian3} The equivalent web mercator X, Y, Z coordinates, in meters.
*/
WebMercatorProjection.prototype.project = function(cartographic, result) {
var semimajorAxis = this._semimajorAxis;
var x = cartographic.longitude * semimajorAxis;
var y = WebMercatorProjection.geodeticLatitudeToMercatorAngle(cartographic.latitude) * semimajorAxis;
var z = cartographic.height;
if (!defined(result)) {
return new Cartesian3(x, y, z);
}
result.x = x;
result.y = y;
result.z = z;
return result;
};
/**
* Converts Web Mercator X, Y coordinates, expressed in meters, to a {@link Cartographic}
* containing geodetic ellipsoid coordinates. The Z coordinate is copied unmodified to the
* height.
*
* @param {Cartesian3} cartesian The web mercator Cartesian position to unrproject with height (z) in meters.
* @param {Cartographic} [result] The instance to which to copy the result, or undefined if a
* new instance should be created.
* @returns {Cartographic} The equivalent cartographic coordinates.
*/
WebMercatorProjection.prototype.unproject = function(cartesian, result) {
//>>includeStart('debug', pragmas.debug);
if (!defined(cartesian)) {
throw new DeveloperError('cartesian is required');
}
//>>includeEnd('debug');
var oneOverEarthSemimajorAxis = this._oneOverSemimajorAxis;
var longitude = cartesian.x * oneOverEarthSemimajorAxis;
var latitude = WebMercatorProjection.mercatorAngleToGeodeticLatitude(cartesian.y * oneOverEarthSemimajorAxis);
var height = cartesian.z;
if (!defined(result)) {
return new Cartographic(longitude, latitude, height);
}
result.longitude = longitude;
result.latitude = latitude;
result.height = height;
return result;
};
return WebMercatorProjection;
});