Ordnance survey: Meridian (NTFM)

Meridian data is supplied in the British Standard national format common to all Ordnance Survey's digital map data products;namely NTF, and is transferred in Level 3 as variable length records.Meridian (NTF v2.0 level 3) are supported by the GDB library for import and view.

Meridian data within each layer are supplied as vector data, in a link and node structure. Geographical features are represented as data entities either as points or lines. Points are fixed positionally by one coordinate pair, e.g. a railway station. Lines are fixed positionally by a series of connected coordinate points to represent linear map features such as roads, railways, etc. Points and lines within the data model determine the geometric (positional) characteristics of the data.

Structure of features

The structure of each different feature is as follows:

1.Point Feature
. POINT RECORD [POINTREC]
. GEOMETRY RECORD [GEOMETRY1]
. ATTRIBUTE RECORD [ATTREC]
2.Line Feature
. LINE RECORD [LINEREC]
. GEOMETRY RECORD [GEOMETRY1] and GEOMETRY CONTINUATION RECORDS
. ATTRIBUTE RECORDS [ATTREC]
3.Name Detail
. TEXT RECORD [TEXTREC]
. TEXT POSITION RECORD [TEXTPOS]
. TEXT REPRESENTATION RECORD [TEXTREP]
. GEOMETRY RECORD [GEOMETRY1]
. ATTRIBUTE RECORD [ATTREC]

Coordinate referencing system

Coordinate values and the number of coordinate pairs in a feature are transferred in the [GEOMETRY1] NTF record. All coordinates within the data are expressed as strings of five numeric characters. Leading zeros are present to complete the five characters.All coordinates are measured from the local origin, which is the south-west corner of the tile.To convert coordinate data to full National Grid coordinates, add the coordinates of the feature to those of the south-west corner of the tile. 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.