IPoint从自定义的投影坐标系转换到自定义的地理坐标系

 IPoint pointStart = new PointClass();
                     pointStart = xyPolyline.FromPoint;

                     ISpatialReferenceFactory pSpatialRefFtry = new SpatialReferenceEnvironmentClass();
                     ISpatialReference fromSpatialRef = pSpatialRefFtry.CreateESRISpatialReferenceFromPRJFile(Application.StartupPath + "\\CoordinateSystems\\2000_3_Degree_117E.prj");
                     ISpatialReference toSpatialRef = pSpatialRefFtry.CreateESRISpatialReferenceFromPRJFile(Application.StartupPath + "\\CoordinateSystems\\China Geodetic Coordinate System 2000.prj");
                     ISpatialReferenceFactory pSRF = new SpatialReferenceEnvironmentClass();
                     pointStart.SpatialReference = fromSpatialRef;
                     pointStart.Project(toSpatialRef);