Digimap: Code-Point

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Code-Point^®

Introduction

Code-Point consists of two separate elements, Code-Point itself and Code-Point Polygons:

Code Point provides a National Grid reference for each unit postcode in Great Britain. Multiple postcodes in a single block of flats or offices will share one National Grid reference. With each co-ordinated point there is information about the postal delivery points within the postcode unit and codes for a number of administrative boundaries, which coincide with the postcode unit. It is Code-Point that is used in Digimap when a postcode search is entered to locate a map.

Code-Point Polygons represent postcode unit boundaries in Great Britain. The boundaries are derived from ADDRESS-POINT^®, an Ordnance Survey product which provides a National Grid co-ordinate for each postal delivery address in Great Britain. Each postcode unit boundary is created to surround all addresses with the same postcode. Where appropriate, the boundaries follow major roads, railways, rivers and Royal Mail postcode sector boundaries. Often, tower blocks or similar buildings will be assigned with more than one postcode. Such buildings are called vertical streets and are represented in the polygon dataset as small squares. A separate look-up table links postcodes to vertical streets using a unique vertical street reference code. Some postcode units are omitted from the dataset, due to issues of accuracy or because they are PO boxes (i.e. non-geographic postcodes). These are listed in separate tables, included with the Code-Point polygons.

Contents of Code-Point

Code-Point contains the following information:

Unit postcode (a unique reference);
Positional quality indicator (PQI);
PO box indicator;
Total number of delivery points within the unit postcode;
Number of delivery points with the same PQI as the unit postcode itself;
Number of domestic delivery points;
Number of non-domestic delivery points;
Number of delivery points which are PO boxes;
Number of premises with a matched address;
Number of unmatched delivery points;
National Grid Code-Point location co-ordinate (CPLC);
Country code;
National Health Service regional health authority code;
National Health Service health authority code;
Administrative County code;
Administrative district code (includes Unitary Authorities);
Administrative ward code;
Postcode type.

Contents of Code-Point Polygon Files

Code-Point Polygon Attributes

Code-Point polygon attributes include the following fields:

Postcode (full postcode from Code-Point or vertical street identifier)
UPP (Unique Polygon Identifier)

Vertical Streets Look-up Table

This comma delimited text file provides a look-up table of vertical street reference codes (as present in the Code-Point polygon 'Postcode' field) and the postcodes contained in them.

PO Boxes Look-up Table

This comma delimited text file lists those postcodes that are omitted from the Code-Point polygon dataset because they are PO boxes. The file includes two fields:

Postcode (postcode that has been omitted from the polygon dataset)
Reason (in this case, the reason is always "POBOX")

Discards Look-up Table

This comma delimited text file lists those postcodes that are omitted from the Code-Point polygon dataset because there were no ADDRESS-POINT records of sufficient positional quality to create a useful postcode polygon. The file includes two fields:

Postcode (postcode that has been omitted from the polygon dataset)
Reason (in this case, the reason is always "QUALITY")

Accuracy and Resolution

Code-Point

Each Code-Point location co-ordinate (CPLC) is quoted to a resolution of one metre. The CPLC is allocated to a point that falls within the extent of the unit postcode. This point coincides with the nearest delivery point to the calculated mean position of all the delivery points in the unit. The delivery point mean is calculated using ADDRESS-POINT, which contains a 0.1m resolution Grid Reference for each delivery point.

Code-Point Polygons

Postcode unit boundaries are a type of synthetic boundary. The postcode unit is a collection of delivery points with the same postcode, while the 'boundary' can be drawn anywhere , so long as it contains all delivery points. The postcode unit boundary does not exist in the real world. Where the boundary lines fall makes no difference to the postman, only the delivery points matter. In other words, there is no such thing as a correct postcode boundary.

The Code-Point unit boundaries however, have been created to best represent the postcode unit footprint in a way that allows the dataset to be used for many different applications. ADDRESS-POINT delivery points with sufficient positional quality were used to create the postcode unit polygons, which have been edited to follow major physical features but still enclose each delivery point in the correct postcode unit. Some postcode units do not contain enough delivery points with sufficient positional quality to create an acceptable polygon 'footprint'. These have been left out of the Code Point polygon dataset and are listed separately in a 'Discards' look-up table.