text: Tracking and Countertracking
  Figure I-3. Types of footprints.
  Box Method
  I-11. To use the 36-inch-box method, mark off a 30- to 36-inch cross-section of the trail, count the
  prints in the box, then divide by two to determine the number of parties who used the trail. The M16 rifle is
  39 inches long. You can use it to measure (Figure I-4).
  Identify a Key Print
  I-12. The figure shows the use of a left boot print as the key print. In this situation, you draw a line
  from the heel across the trail.
  Move Forward
  I-13. Move to the key print for the opposite foot and draw a line through the instep. Your two lines
  form two opposite sides of a box. The edges of the trail form the other two sides. Together, the four lines
  form a box.
  Count
  I-14. Count every whole or partial print that falls inside of the box. Because any person who was
  walking normally would have stepped in the box at least once, the number of footprints in the box will help
  you to determine the total number of people in the party.