It is recommended that grid references are obtained from the Ordnance Survey (OS) Explorer Maps at a scale of 1:25,000 or that a web site such as Grid Reference Finder is used. Whilst the 1:50,000 scale Landranger Maps are adequate for identifying the grid references of large features they do not provide the detail needed to identify individual buildings.
Historically the Industrial Archaeology community has always used 6 figure grid references (ie SD 456 789) which identifies the bottom left hand corner of an area 100m x 100m square. Whilst this is adequate for humans to identify major structures, it is not sufficiently accurate to locate individual buildings in an urban area, or to display the site accurately on Google Maps. In these cases it is strongly recommended that 8 figure grid references (ie SE 1234 5678) which identifies the bottom left hand corner of an area 10m x 10m square are used. Further in Geographic Information Systems (GIS) it is normal to use 10 figure grid references (ie SE 12345 67891) which defines a square only 1m by 1m.
It is strongly recommended that wherever possible 8 or preferably 10 figure grid references are always used when inputting data to IHO.
For those who are not familiar with maps the OS grid can be equated to a large grid with an origin located to the south west of Cornwall. In this the horizontal “X” axis equates to the easting and the vertical “Y” axis corresponds to the northing. By convention the easting is always given first followed by the northing. Instructions for obtaining a grid reference are given on each map at the bottom of the key.
For ease of use the grid is sub-divided into a series of 100 km by 100 km grid squares each of which is allocated a two letter code. Grid references have historically been given relative to the individual grid squares and as such they comprises of two letters which identifies the grid square, followed by two blocks of numbers, the easting and northing. (see Fig 1 over)
In GIS applications it is now more normal to use the grid co-ordinates which defines the point to the OS grid origin. As converting between grid references and grid–co-ordinates can cause some difficulties for users the IHO software calculates the required grid co-ordinates and the corresponding latitude and longitude from the supplied grid reference and there is no need for users to record or input these.
Consider the case where you wish to give the grid reference of a point 36.415 km east and 59.687 km north of the NY square origin. Grid square NY has an easting commencing at 300 km east of the origin and a northing commencing at 500 km north of the origin. A grid reference relative to the NY grid square, and accurate to 100 m, therefore has an easting of 364 and a northing of 596. To give the same grid reference relative to the imaginary origin it is necessary to omit the grid letters and to add in the distance from the origin and thus the gird co-ordinate becomes 3364 5596.
| Grid reference | Grid co-ordinate | |
| For a grid reference accurate to 1000 m | NY 36 59 | 336 559 |
| For a grid reference accurate to 100 m | NY 364 596 | 3364 5596 |
| For a grid reference accurate to 10 m | NY 3641 5968 | 33641 55968 |
| For a grid reference accurate to 1 m | NY 36415 59687 | 336415 559687 |
| 12 | HP 4,12 | ||||||
| 11 | HT 3,11 | HU 4,11 | |||||
| 10 | HW 1,10 | HX 2,10 | HY 3,10 | HZ 4,10 | |||
| 9 | NA 0,9 | NB 1,9 | NC 2,9 | ND 3,9 | |||
| 8 | NF 0,8 | NG 1,8 | NH 2,8 | NJ 3,8 | NK 4,8 | ||
| 7 | NL 0,7 | NM 1,7 | NN 2,7 | NO 3,7 | |||
| 6 | NR 1,6 | NS 2,6 | NT 3,6 | NU 4,6 | |||
| 5 | NW 1,5 | NX 2,5 | NY 3,5 | NZ 4,5 | |||
| 4 | SC 2,4 | SD 3,4 | SE 4,4 | TA 5,4 | |||
| 3 | SH 2,3 | SJ 3,3 | SK 4,3 | TF 5,3 | TG 6,3 | ||
| 2 | SM 1,2 | SN 2,2 | SO 3,2 | SP 4,2 | TL 5,2 | TM 6,2 | |
| 1 | SR 1,1 | SS 2,1 | ST 3,1 | SU 4,1 | TQ 5,1 | TR 6,1 | |
| 0 | SV 0,0 | SW 1,0 | SX 2,0 | SY 3,0 | SZ 4,0 | TV 5,0 | |
| 0 | 1 | 2 | 3 | 4 | 5 | 6 |
Distance from the origin in 100 Km’s
Figure 1 OS Grid Reference Code Letters
Whilst 8 digit grid references can be obtained with reasonable accuracy from 1:25,000 OS Maps for 10 digit grid references accurate to 1m it is necessary to either use larger scale maps or on line mapping sites.
A number of internet sites can be used to obtain grid references one such site being Grid Reference Finder (www.gridreferencefinder.com) other sites are however available. When using the Grid Reference Finder site, on the first screen, a box on the left hand side allows you to enter either, a post code, location or approximate grid reference for a site. After entering which ever detail is most convenient press the “GO” button and the site will zoom in to the given location which is shown as a satellite image.
Alternatively you can zoom to the point on the map on the right hand side by placing the cursor over the point of interest and double clicking the left button on the map.
By pressing on the left hand mouse button it is possible to drag the map or satellite image so that the cursor is centred on the building or site of interest. Right clicking on the mouse brings up a “Pin Symbol” and opens a separate screen which gives the grid reference and other information in the chosen point. The site gives both the 10 figure grid reference, and the 12 figure grid co-ordinates for the point both of which are accurate to 1m.
Before sites can be displayed on Google Maps it is necessary to convert the grid reference to the corresponding latitude and longitude. Although all these calculations are handled by the software the following is provided by way of background for anyone who may be interested.
Latitudes and longitudes are calculated by the software using the latest implementation of the World Geodetic System map projection (WGS84). This projection is also used by a number of other organisations as there base map projection including:-
It is also compatible with the latitudes and longitudes provided by the Global Positioning System (GPS). Unfortunately Google Maps, uses a slightly different map projection that goes variously under the name Web Mercator, Google Web Mercator, Spherical Mercator, WGS84 Web Mercator or WGS84/Pseudo-Mercator. Whilst it is now the de facto standard for Web mapping applications and is used by Google Maps, Bing Maps, Mapquest, Mapbox, OpenStreetMap, problems arise due to the differences with the standard WGS84 map projection. In effect the projection uses the WGS84 map projection but truncates it at 85.051129° north and south. Thus the underlying geographic coordinates are as defined using WGS84 (European Petroleum Survey Group 3857), but projected as if they were defined on a sphere (as in EPSG 3785). However in the context of the UK the differences are generally small and normally present no major issues when using Google maps to illustrate UK Heritage Sites at their latitudes and longitudes as calculated in accordance with WGS84.
The software uses a two stage process in which the latitude and longitude of the site is initially calculated in accordance with the recommendations given in the Ordnance Survey publication - A guide to coordinate systems in Great Britain, version 2.2, December 2013. As Google Maps uses different map project to the OS it is then necessary to transform the initial latitude and longitude to the new map projection. For a full description of these calculations reference should be made to the OS publication.
The following gives a simplified outline the calculation process used in YIHO. Grid references are initially converted to grid co-ordinates and then to latitudes and longitudes based upon the Modified Airey map projection (OSGB36) as used by the OS. This uses an ellipsoid with a semi major axis (a) of 6377563.396 m and a semi minor axis (b) of 6356256.909 m. A Helmert transformation is then applied to convert these values to the World Geodetic System map projection (WGS84) which uses a semi major axis (a) of 6378137.00 m and a semi minor axis (b) of 6356752.3141 m. In doing this a standard height of 100 m above Ordnance Datum (AOD) is assumed for all points.
The WGS84 map projection is also used by a number of other organisations as their base map projection including:-
International Terrestrial Reference System (ITRS)
Europe-fixed version of WGS84, European Terrestrial Reference System 1989 (ETRS89)
Latitudes and longitudes calculated by this method are also compatible with those available form the Global Positioning System (GPS).
Unfortunately Google Maps uses a slightly modified version of the WGS74 map projection, that goes variously under the name Web Mercator, Google Web Mercator, Spherical Mercator, WGS84 Web Mercator or WGS84/Pseudo-Mercator. Whilst this is now the de-facto standard or web mapping applications and is used by Google Maps, Bing Maps, Mapquest, Mapbox, OpenStreetMap, problems can arise due to the differences with the standard WGS84 map projection. In simple terms the projection uses the WGS84 map projection but truncates it at 85.051129 degrees north and south. Thus the underlying geographic coordinates are as defined using WGS84 but projected as if they were defined on a sphere (as in EPSG 3785, European Petroleum Survey Group 3857).
However in the context of the UK the differences are generally small and normally present no major issues when using Google Maps to illustrate sites at their latitudes and longitudes as calculated in accordance with WGS84.
The major source of error in this process is the conversion from grid reference to grid co-ordinate. As outlined previously the grid reference identifies the bottom left hand corner of the 100m x 100m square(for 6 figure grid references such as SE 123 456) or the 10m x 10m square (for 8 figure grid references such as SE 1234 5678) within which the site lies. Thus latitudes and longitudes calculated from such grid references can have errors of up to -100m or -10m respectively ie the latitude and longitude will always define a point to the south and west of the sites true position.
In contrast by convention both the grid co-ordinate and latitude and longitude define a point location. Thus the grid co-ordinate 438662 445955 identifies a point at the centre of the road bridge in Collingham near Leeds to the nearest metre. The corresponding WGS84 decimal latitude and longitude of the bridge calculated by the software is 53.908316, -1.412979. In minutes and seconds these are 53 degrees 54 minutes and 29.2 seconds north and 1 degree 24 minutes 46.7 seconds west.
It should be noted that the different conversion programs available on the web for calculating latitude and longitudes from grid references use slightly different calculation techniques. Consequently for the same point they typically give values that vary in the 5th or 6th decimal place. In the “real world” and based on the earth's equatorial circumference of 40,075.16 km or the polar circumference of 40,008 km, a change in the 6th decimal place (0.000001 degrees) equates to approximately 1.11 m. Thus whist latitudes and longitudes quoted to 6 decimal places have a nominal accuracy of 1 m the failure to take into account the actual height of the point will in practice result in greater errors. As stated by the OS on page 33 of A guide to coordinate systems in Great Britain, other aspects of the conversion from OSGB36 to WGS84 will in practice result in errors typically in the order of 5m. A limited comparison of the latitudes and longitudes as calculated by the YIHO software against those given by other web sources suggests that errors are typically 1m to 2m in the latitude and up to 5m in the longitude. However for practical purposes such differences are not significant when viewing the location of a typical site on Google Maps.