json对象的属性值获取不了？为undefined？

var data1 = {
                "raster_metainfo": [0,0,0,0,0,0,0],
                "navigation_metainfo": [0,0,0,0,0,0,0],
                "thematic_metainfo": [0,0,0,0,0,0,0],
                "text_metainfo":  [0,0,0,0,0,0,0],
                "video_metainfo":  [0,0,0,0,0,0,0],
                "database_metainfo": [0,0,0,0,0,0,0],
                "service_metainfo":  [0,0,0,0,0,0,0]
            }
            jsonData = JSON.stringify(data1);

            var a ='navigation_metainfo';
                        console.log(data1.a);