from the reference corner. Greater precision can be obtained by using six, eight, or even more digits, but always an even number with the same precision on both coordinates. The complete grid reference includes both the grid square letters and the numerical reference giving the location within the grid square. For instance, wQ 683 825 is a location in South London. Grid references can also be given in an all numeral notation. To do this, square vV (the South-West corner of the grid out in the Atlantic) is taken as square zero and then the grid squares are counted from there to the location being referenced. For instance qY 985 613 (somewhere around Carlisle) can also be expressed as 3985 5613. This is because square qY is three 100 km squares East of square vV and five 100 km squares North of vV. However, it is frequently not necessary to explicitly give the grid square at all, this can be implied from context. Transition to National Grid The Ordnance Survey National Grid was introduced in 1938 and the intention was to replace the Cassini Grid system with this. However, it had not become established when World War II broke out and the Cassini Grid continued in use throughout the war. Work on the National Grid maps stopped for the duration of the war. In fact, progress went backwards due to the Southampton offices of the Ordnance Survey being bombed by the Germans in 1940 destroying some of their data. National Grid maps began to be issued after the war, but it was some time before there was full coverage. In the interim, the public were allowed to purchase War Office Cassini Grid maps. National Grid maps at 1:25,000 scale (often referred to as 2 inches to the mile, or just 2-inch maps) were not introduced until as late as 1950. Cassini Grid maps at this scale had been available, but apparently the public were discouraged from using them by the Ordnance Survey. They were poor quality, rushed into production for wartime needs, and had been produced by merely photographically reducing six-inch maps. The long period of introduction of the National Grid resulted in National Grid and Cassini Grid maps being used side by side for an extended time. Converting between the two systems is problematic. Although the two grids are superficially similar (both use 100 km squares), different projections, baselines