The Ordnance Survey was established in 1791 and the survey commenced in 1794 the first sheets to a scale of one inch to a mile were published in 1801. We have facsimile copies of Ordnance Surveyor’s Drawings for the London Area 1799-1808 which are to the scale of two inches to a mile and form the basis of the one-inch series.
The larger scale maps are, of course, of more use to local historians than the one-inch series. Fortunately, it was soon recognised that there was a need for a larger scale map for property transactions and tax valuation and in 1853 it was decided to map at the 1:2500 scale. Although a metric scale, its attraction is that, at this scale, one acre is almost exactly a square of one-inch sides. (1.0018 inches). Since going metric it is also easier to use without a special scale, as 1 centimetre equals 25 metres.
The whole country was mapped at the scale of 6 inches to a mile. The six-inch sheets for each county covered an area of 6 miles by 4 miles and were numbered sequentially in Roman numerals. Each six-inch sheet was divided into 16 parts (4 x 4) each 1:2500 map covering one part with an area of 1.5 miles by 1 mile. The six-inch sheet covering most of Hornsey is Middlesex XII (12) Middlesex XII NW etc. The 1: 2500 sheets are numbered Middlesex XII.1 to 16.
A National Grid
In 1938 the OS had decided to relate the mapping to a National Grid reference system but before any maps were published on this system the war intervened. In the early part of the war one-inch maps were provided with a grid superimposed for military purposes. This military grid was superseded in the second half of the war and became the National Grid in use today.
After 1946 for most urban areas maps were produced based on the National Grid. The 1: 2500 maps covered an area of 1 square kilometre and were referenced by the co-ordinates e.g. TQ2889 and the 1:1250 sheets covered a 500 metre square being one quarter of each of the larger sheets and numbered by quadrant e.g. TQ2889NE. After metrication the scales of the one inch and six inch to a mile maps were changed to 1:50,000 and 1:10,000 respectively.