Grid Reference Precision
During the submission process, and on the final photo page, we feature a small extract from the 1:50,000 scale Landranger™ Ordnance Survey® raster map. Due to the limitations of scale of these maps, some features are not accurately located on the map, e.g. roads are much wider than in reality.
When the map markers are dragged to locate a feature on these maps we calculate an 8-figure grid reference (excluding the grid letters). We do this so that the markers on the map on the photo page appear to be located over the appropriate features, and also in helping to calculate a view direction.
However this would normally imply a much greater degree of accuracy for the recorded location than is in reality possible from direct referencing on the map, therefore grid references derived by using the map markers are "corrected" to 6 figures for display on the actual photo page. (This however is optional and can be turned off using the tickbox)
Contributors wishing to use a high degree of precision in recording the positions of the photo's subject and the camera's location should obtain those measurements from GPS or other, larger-scale, mapping sources. GPS users will be aware that the accuracy of measurements obtained by that method will vary according to the number of satellites visible at any one time and should adjust the precision of their recorded grid references accordingly.
Geograph will accept grid references with as little precision as the grid square itself, i.e. 4 figures, or up to 10 figures, which would theoretically locate a feature on the ground with a precision of a 1 metre by 1 metre square. When obtaining a position from a map scaled at 1:50,000 or 1:25,000; it is appropriate to assume an accuracy of 100 metres, i.e. a 6-figure grid reference. A 6-figure grid reference, therefore, identifies a point anywhere in a square 100 metres by 100 metres.
For more information on the National Grid System and on Grid References, see Interactive Guide to the National Grid in Great Britain. The Irish National Grid is very similar, but using a single letter prefix, see Overview Map for the layout of the squares.