AFD Products: Names & Numbers Manual

AFD Manual: Grid References

APPENDIX D: GRID REFERENCES IN AFD PRODUCTS

Grid References

DataTalk GeoRef is a Postcode level grid reference data supplied by AFD for distance calculations, nearest calculations and data / location analysis. It is made up of a six digit Easting and a six digit Northing. This reference relates the location of the Postcode to the National Grid (or Irish Grid for Northern Ireland Postcodes (start with BT).

DataTalk GeoRef is a good alternative to Postzon. However, Postzon, Code Point and Address Point data options are available at additional cost.

Background to Data

England, Wales and Scotland

The largest units of the National Grid are 500km squares, each designated by a prefix letter - the first letter to be quoted in the National Grid reference.

The 500km squares are then broken down into twenty five - 100km squares; these are also designated by a prefix letter - the second letter of the grid reference.

Within the 100km square, each smaller grid square (at 10km, 1 km, or 100m side) is designated by the distance of its South-West corner from the West (Eastings) and South (Northings) margins, respectively, of the 100km square in which it lies.

To enable these distances, or co-ordinates, to be easily found, their values are printed in the map margins against the grid lines concerned. These provide the numbers - two, four or six digits - which identify each particular grid square.

EXAMPLE
e = easting n = northing

S	K	1	2
		e	n

shows a point 10,000m East and 20,000m North of the South West corner of the SK square.

S	K	1	3	5	2	4	6
		e	e	e	n	n	n

Shows a point 13,500m East and 24,600m North of the South West corner of the SK square.

A point or feature, within one of the smallest grid squares, can be indicated still more closely by estimating the tenths of the appropriate grid square.

When giving a National Grid reference, for any point, the distance Eastwards (Eastings) is always given before the distance Northwards (Northings).

Northern Ireland

Northern Ireland is covered by the Irish Grid System, which contains a single alpha character to represent each 100km square. There are 5 such 100km squares covering Northern Ireland using the letters: C D G H J.

This, however, makes the distance difficult, so AFD products automatically convert the Irish grid system to British system to allow distances / nearest to be correctly calculated. You can opt to convert all grid references to use the British or Irish Grid systems to provide consistency across the whole UK and these options are available in the front-end and developers can gain access to these conversions through the Common API.

Isle of Man, Channel Islands and other postcodes where Grid References are not available (e.g. PO Boxes)

No Grid References are available for these postcodes but to enable them to be used with the Map and Nearest facilities of AFD Products, grid references for the centre of the locality/town have been hard coded. PO Boxes are non-geographic (being located in the Post Office) hence having no grid reference. You can also opt to use these locality/town hard coded values generally in the Front-end and they are accessible through the Common API for developers. In large cities these approximations can be quite far out as the values given are for the centre of that city and not the postcode you selected. However this may be more useful than no grid reference at all to assist in distance calculations or locating somewhere on the map.

Latitude & Longitude

To enable these Grid References to be used with GPS systems and other tools requiring latitude and longitude values AFD Products provide a conversion to latitude and longitude values for a grid reference which is displayed in the front end and can be inserted into your application. Developer's can gain access to these conversions through the Common API. The latitude and longitude values given are based on the WGS84 standard - the one in most common usage with GPS systems. The conversions used are accurate to within 5 metres (the worst case scenario at locations with most distortion). But remember that a grid references at postcode level may cover a wide area especially in rural areas.

Address, Postcode and Bank solutions