![]() If, for some reason, a reference is required that is unique in the world, one must look in the margin of the map for the Grid Zone Designation. For most purposes this is sufficiently unambiguous. This reference is still not unique, but the same reference does not occur again for about 2,900 km (1,800 miles). Here the church would be in square NT99 and more precisely at point NT9293. ![]() Figure 3 shows a reference on a 1:250 000 map (where grid lines are usually identified by a single number). ![]() The identifying letters (two of them) are always given before the numbers. This is particularly important in the case of medium- and small-scale maps (1:250 000 and smaller), as unlettered references are ambiguous on a single map. Therefore a method has been devised to identify the 100,000-metre squares by letters which are printed in blue on the face of all NTS maps. The Military Grid Reference System is convenient, but unfortunately reference numbers repeat themselves every 100,000 metres (100 km or about 62 miles). Figure 3 is an illustration of reference numbers in blue which makes the unique referencing.
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