Section 3: Bit-map section
| Octets | Key | Type | Content |
| 1-3 | section3Length | unsigned | Length of section |
| 4 | numberOfUnusedBitsAtEndOfSection3 | unsigned | Number of unused bits at end of Section 3 |
| 5-6 | tableReference | unsigned | Table 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 centre |
| 7-nn | The bit-map contiguous bits with a bit to data point correspondence, ordered as defined in the grid definition |