text: Chapter 6
  Figure 6-10. Horizontal clearance and marker construction.
  MARKING CONSIDERATIONS
  6-104. Place the markings where obstacles will not mask the pilot’s line of sight. The DZ markings must
  be clearly visible to the aircrew on approach as early as possible. If conditions preclude placing the
  markings at the computed point, the DZSTL may have to adjust the location of the intended PI, ensuring
  the new PI location complies with the requirements for the type of airdrop. Advise both the aircrew and
  user of the change in PI location. As a guide, use a mask clearance ratio of 1 to 15 units of horizontal
  clearance. For example, suppose you must position a DZ marker near a terrain mask, such as the edge of a
  forest on the DZ approach. The trees measure 10 meters (33 feet) high. The markings would require 150
  meters (492 feet) of horizontal clearance from the trees (Figure 6-11).