Section 3 - Bit-map section

      Octet
      No.
      Contents
      1-3Length of section (octets)
      4Number of unused bits at end of Section 3
      5-6Table reference:
      If the octets contain zero, a bit-map follows
      If the octets contain a number, it refers to a predetermined bit-map provided by the center
      7-The bit-map - contiguous bits with a bit to data point correspondance, ordered as defined in the grid definition