code: function SphereMercator(params) {
      /*  =========== parameters  =  ===================== */
    params = params || {};
    SpatialReference.call(this, params);
      this.a_ = (params.semi_major || 6378137.0) / (params.unit || 1);
      this.lamda0_ = (params.central_meridian || 0.0) * RAD_DEG;
  }
docs: Creates a Spatial Reference based on Sphere Mercator Projection.
  The <code>params</code> passed in constructor should have the following properties:
  <code><br/>-wkid: wkid
  <br/>-semi_major:  ellipsoidal semi-major axis
  <br/>-unit: meters per unit
  <br/>-central_meridian: lamda0, longitude of the false origin  (with respect to the prime meridian)
  </code>
  <br/>e.g. The "Web Mercator" used in ArcGIS Server:<br/>
  <code> var web_mercator  = new SphereMercator({wkid: 102113,  semi_major:6378137.0,  central_meridian:0, unit: 1 });
  </code>
  @name SphereMercator
  @class This class (<code>SphereMercator</code>) is the Projection Default Google Maps uses. It is a special form of Mercator.
  @constructor
  @param {Object} params
  @extends SpatialReference