求翻译：覆盖件的外形轮廓普遍具有不规则、呈自由曲面等特点，因此外形轮廓检测的主要依据是检具的型面，同时配合采用其它相应的检测手段，如划线比较、齐平比对、厚薄规检查、游标卡尺及专用手持量具测量等。　是什么意思？

Generally have irregular contour of the cover, showing the characteristics of free surface, contour detection is mainly based on the fixture surface, in conjunction with the use of other appropriate means of detection, such as crossed relatively flush than the thickness compliance checks, vernier ca