Geograph Knowledgebase / FAQ


I don't understand Grid References - are there any sites to help me?
Here's a quick link to the Ordnance Survey's beginner's guide to grid references: link

Grid references locate places on the map. They consist of one (Ireland) or two (Great Britain) letters followed by an even number of digits. The letters define a (100x100)km^2 square (a 'myriad' in Geograph-speak) and are best looked up on an overview map. The numbers locate the position within that square. To find a location, split the block of numbers in two. The first block is the easting, i.e. the distance from the western edge of the myriad. The second is the northing - the distance from its southern edge.

Grid references always specify square areas, not points. The more digits there are, the smaller is the square referred to, and the higher is the precision of the grid reference. For each pair of digits, the precision increases by a factor of ten: SN58 defines a square of (10x10)km^2, 50km to the east and 80km to the north of the origin of myriad SN. SN5881 is a (1x1)km^2 square 58km east and 81km north of that origin. SN 58272 81324 is a (1x1)m^2 area. When stating a grid reference, the precision given should be in line with the accuracy of the position (how well do we actually know where we are?) and with the size of the object (a building doesn't fit on a 1m^2 footprint).

The system Geograph still uses in Ireland works in exactly the same way, although the Ordnance Surveys of Ireland and of Northern Ireland have recently introduced a different system for their maps. This article link explains the differences.
Are there any Geograph specific terminology?
Yes, as an established project, with very specific aims, its inevitiable we would develop our own terminology, for more see this article: link

(explains things like hectads, myriads, and the hamsters amoung others)

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