Section 3: GRID DEFINITION SECTION
| Octets | Key | Type | Content |
| 1-4 | section3Length | unsigned | Length of section in octets (nn) |
| 5 | numberOfSection | unsigned | Number of section (3) |
| 6 | sourceOfGridDefinition | codetable | Source of grid definition (see Code Table 3.0 and Note 1) |
| 7-10 | numberOfDataPoints | unsigned | Number of data points |
| 11 | numberOfOctectsForNumberOfPoints | unsigned | Number of octets for optional list of numbers (see Note 2) |
| 12 | interpretationOfNumberOfPoints | codetable | Interpretation of list of numbers (see Code Table 3.11) |
| 13-14 | gridDefinitionTemplateNumber | codetable | Grid Definition Template Number (= N) (see Code Table 3.1) |
| 15-xx | Grid Definition Template (see Template 3.N, where N is the Grid Definition Template Number given in octets 13-14) | ||
| [xx+1]-nn | Optional list of numbers defining number of points (see Notes 2, 3 and 4) |
( 1) If octet 6 is not zero, octets 15-xx (15-nn if octet 11 is zero) may not be supplied. This should be documented with all bits set to 1 (missing value) in Grid Definition Template Number.
( 2) An optional list of numbers may be used to document a quasi-regular grid In such a case, octet 11 is non zero, and gives the number of octets used per item in the list. For all other cases, such as regular grids, octets 11 and 12 are zero and no list is appended to the Grid Definition Template.
( 3) If a list of numbers defining number of points is present, it is appended at the end of Grid Definition Template (or directly after Grid Definition Template Number if template is missing), the length of the list is given by the grid definition. When the Grid Definition Template is present, the length is given according to bit 3 of scanning mode flag octet (length is Nj or Ny for flag value 0). List ordering is implied by data scanning.
( 4) Depending on code value given in octet 12, the list of numbers either:
- corresponds to the coordinate lines as given in the grid definition, or
- corresponds to a full circle, or
- does not apply