SceneControl中如何将屏幕坐标转为地理坐标？

 IPoint point = null;

            object ppOwner =null;

            object ppObject = null;

            pSceneGraph.Locate(m_sceneHookHelper.ActiveViewer, X, Y, esriScenePickMode.esriScenePickFastLocation, true, out point, out ppOwner, out ppObject);