Structure of extremal sets and approach to existence of exposed points;

A variational baseline design of the course based on 1:250000 DEM and high accuracy position exposure in which the error is less than 5 meters or 2% of the baseline distance is expounded.

However, the film exposure device of the currently widespread-used X-ray machine has poor quality of image, a low level of automation, poor precision controlled and film exposure and large size.