APPENDIX D: GRID REFERENCES IN POSTCODE PLUS
Grid References
The grid references supplied by AFD Postcode Plus are matched by AFD from the Royal Mail Postzon file and consist of a Northing and an Easting for most postcodes in the UK. These notes are supplied to assist you in understanding and making use of this data, which is of tremendous value in allowing you to plot the location of addresses.
The Grid Reference supplied by Postcode Plus is made up of a 6 character Easting and a six Character Northing - though for most postcodes, only 5 characters are actually used. This reference relates the location of the Postcode to the National Grid, allowing for a 10 metre reference to be included on the file, although it should not be assumed that all grid references are provided to this level of accuracy.
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). Eastings and Northings must always be recorded in the same number of figures even though some of them are zero.
Grid references for Scottish Postcodes have been taken from the Postcode directory maintained in Scotland by GRO(S). The OSGRs were allocated by GRO(S) to the nearest 10 metres of the centre of the building judged by eye to be the centroid of the area covered by the Postcode.
For the most northern grid references, 6 characters are provided to hold the northing numeric value, a conversion of the first two numerics into a single letter takes place to allow the northing to be held in a 5 character field.
The letters used are: P U Z O T Y and are used as follows:
| Northing Letter on PAF | if Easting value begins with | Then Northing Value begins with |
| P | 4 | 12 |
| U | 4 | 11 |
| Z | 4 | 10 |
| O | 3 | 12 |
| T | 3 | 11 |
| Y | 3 | 10 |
Grid Reference's returned by the Postcode Plus API for developers contain these letters at the start of the Grid Northing for these most northern grid references so you will need to expand the letter for distance calculations. The Postcode Plus front-end automatically converts these before displaying or inserting the Grid Northing value.
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.
The Irish Grid references held in Postcode Plus from the Royal Mail Postzon file are to a different base datum than those for the UK mainland. This base datum is used for showing the approximate location of Northern Ireland postcodes with the Map feature.
This, however, makes the distance calculating facilities of Postcode Plus very difficult, so Postcode Plus contains conversions between the British and Irish grid system to allow distances 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 our Utility DLL. The Nearest facility always uses British Grid References across the whole UK to allow distances to be compared between any postcode.
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 Postcode Plus 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 Utility DLL for developers. In large cities these approximations can be quite far out as it's the centre of that city not the postcode you selected, but may be more useful than no grid reference at all to assist in distance calculations or locating somewhere on the map.
To enable these Grid References to be used with GPS systems and other tools requiring latitude and longitude values Postcode Plus provides a conversion to latitude and longitude values for a grid reference which are displayed in the front end and can be inserted into your application. Developer's can gain access to these conversions through the Utility DLL. 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 grid references are only supplied to a 10 metre accuracy, and only available for the centre of the Postcode so this accuracy should be sufficient for the data provided.
Copyright 2004
AFD Computers
01624 811711
