In the paper, perspective transformation is used to project points from the front horizon of the camera to an image plane and thus to measure the distance between the detected object and the camera. With the information about points on the ground, the projective positions of every tip in a rigid object can be figured out through transformation. Features of the object's projection at different distances, such as size and shape, can also be predicted. Besides, the paper has analyzed difference in the result of the measurement and errors caused by the application of different parameters. The information assists engineers of vision-based detection system in determining the parameters of the system and identifying features of an object's projection to accelerate the detection. Also, the camera parameters compensate automatically when being influenced by outer force to promote the effects of detection and make a robust system.