The generalisation of detail on the maps used on this website means that roads, for example, often appear wider than they actually are. The apparent location of a particular grid reference with respect to the features on the map tends to vary slightly as the scale of the map is altered. The size of the discrepancy varies with local circumstances, but is often greatest in towns and cities, particularly in the vicinity of dual carriageways, where streets are crowded closely together, and at road junctions.
Most of the OS grid references have been derived from readings taken in situ with a Garmin eTrex GPS Personal Navigator set to the OSGB36 coordinate system that have been subsequently adjusted to match the street level (1:10,000) maps.
Although twelve figure grid references are in theory accurate to plus or minus 0.5 metres in each direction, the estimated accuracy of the published figures is generally at least one order of magnitude less. They have been retained to facilitate the placing of map icons.
The quoted latitudes and longitudes are all WGS84. For places in the UK, they have been derived from the adjusted 12 figure grid references. For locations outside the UK, they have been mainly derived directly from Google Maps. The WGS84 data has been inputted without further adjustment to place the map icons in the aerial views.
The published distances from Greenwich are calculated from the Airy Transit Circle on the WGS84 ellipsoid, using an algorithm developed by Thaddeus Vincenty.
To be listed in the database, there must be clear evidence of a deliberate marking having been made. The word Meridian is extensively used in the names of buildings, businesses and roads. Whilst some are on or close to the line, others most certainly are not – Meridian Close in Ilkeston for example being more than 1° to the west. The use of the word is more widespread in towns such as Peacehaven through which the Meridian actually passes. As a rough rule of thumb, a building, business or road whose name contains the word Meridian has only been included if at least part of its curtilage falls within 120 m of the Meridian as defined by the Airy Transit Circle. This broad sweep takes in the WGS84 meridian, along with the various earlier Greenwich meridians. A wider margin has been allowed for works of art and for locations outside the UK.
It had long been believed that Ordnance Survey maps were based on the Airy Meridian. That they were still based on the earlier Bradley Meridian only re-emerged in the 1950s during the retriangulation of Britain. As a result, a small offset needs to be applied when using an OS map or GPS set to OSGB36 to locate the Airy Meridian. This fact remains largely unknown. As a result, most of the UK markings are (nominally) on the Bradley rather than the Airy Meridian as intended.