Each rectangle on the grid represents an area of land which is 1 x 1.5 miles (1.6 x 2.4 km) on the ground. The vast majority of all the available maps are drawn at a scale of 1:2500. One of these maps covers the entire rectangular area. They were sufficient in detail for the majority of rural areas: countryside and villages.

In some small towns there was need for greater detail. Here you may find there are four further maps drawn at a scale of 1:1250 which cover the same rectangular grid area as the square above. Each of these four maps therefore represents an area of land which is 0.5 x 0.75 miles (0.8 x 1.2 km) on the ground.

The density of the larger towns and cities meant even greater detail was required. Here there may be 25 further maps drawn at a scale of 1:500 which, put together, cover the same original rectangular grid area. Each of these 25 maps therefore represents an area of land which is just 0.04 x 0.06 miles (64 x 96 metres) on the ground.