Ordnance survey: BaseData.GB (NTFD)

BaseData.GB is defined as a geometrically structured "link and node" database. Features are represented as points, that are fixed positionally by one coordinate pair, and lines, a direct connection between two pairs of coordinates. These determine the geometric or positional characteristics of the data. They may, for example, describe the course of a road by position and orientation (line) or the position of a dual carriageway roundabout (point).

BaseData.GB(NTF v2.0 level 3) are supported by the GDB library for import and view.

Structure of features

The structure of each different feature is as follows:

1. Point Feature:
. a Point Record [POINTREC] 15;
. a Geometry Record [GEOMETRY1] 21;
. an associated Attribute Record [ATTREC] 14, which contains
  a measurement for variable sized symbols.
2. Line Feature:
. a Line Record [LINEREC] 23;
. a Geometry Record [GEOMETRY1] 21,and maybe one or more
   continuation Records[CONTREC] 00 to extend the Geometry Record.
. Geometry Continuation Records
. an associated Attribute Record [ATTREC] 14
4. Text Feature:
. a Text Record [TEXTREC] 43;
. a Text Position Record [TEXTPOS] 44; 
. a Text Represenration Record [TEXTREP] 45
. a Geometry Record [GEOMETRY1] 21.
. an associated Attribute Record [ATTREC] 14

Coordinate referencing system

All coordinates within the data are expressed as strings of six numeric characters. Leading zeros are present to complete the six characters.All coordinates are expressed in meters and are measured from the local origin, which is the south-west corner of the 100 km by 100 km square.To convert coordinate data to full National Grid coordinates, add the coordinates of the feature to those of the south-west corner of the 100 km by 100 km square. The south-west corner coordinates are contained within the {X_ORIG} and {Y_ORIG}fields of the Section Header Record ([SECHREC]).

See UK National Transfer Format (NTF) for information on georeferencing.