Grid ref lengths

Q

Some grid references are six digits, others eight and some 10. What’s the difference? Olivia Harley, via Facebook

Jeremy says It’s all down to accuracy. The grid lines on UK maps are based around a series of squares. Twenty-five 500km squares (referenced A-Z) cover the entire UK mapped area and these are subdivided by 25 squares 100km in size (again referenced A-Z). This provides the twin alpha part of grid refs, ie NN, SH. The 100km squares are then split down to 1km squares that give the familiar patchwork we see on maps. The size of these 1km squares vary depending on the scale of the map in use: a 1km grid square is 4cm wide on a 1:25,000 map, while on 1:50,000 it is 2cm wide.

All grid references start with two letters (found in the corner of the 100km map) and the most basic grid reference is four figures long – giving you a location within 1km square. For greater accuracy, use the Romer scale on your compass base plate to define a six-figure reference to specify an area of 100m square. In most cases a six-figure reference is sufficient, particularly if there is also a named feature or map icon you can use. Eight or 10-figure grid refs (accurate to 10m and 1m square, respectively) are possible, but can be tricky to calculate on a windy hillside!