Ping_M

Ping_M is the number of the ping in the middle of the bounding rectangle of the analysis domain calculated as follows:

pm= (ps + pe) / 2          if (ps + pe) is even
pm= (ps + pe - 1) / 2     if (ps + pe) is odd

where

pe	is the ping number of the last (end) ping of the analysis domain.
ps	is the ping number of the first (start) ping of the analysis domain.

See also

School Detection module algorithms
Other region variables for schools analysis