Ordnance Survey maps were originally drawn up by military surveyors, in the first half of the 19th century, to cover the whole country with a uniform mapping and there is no intention of attempting to describe the history, or the very wide range of generalist and specialist maps that have been produced over the last two centuries, except to say that the modern mapping is fully digitised, and that virtually all modern road and street maps are derived from the data, and most old maps are covered by the Old Maps site. I will concentrate on the maps which are readily accessible and useful to the family historian.

First Edition 1 inch maps: These have been reprinted and are at a scale of 1 inch to the mile. The original survey was engraved on copper plates and major changes - such as the building of railways - were added later. Rural areas are usually much as the original survey. The scale is too small to show individual fields or smaller buildings. The latest reprints, which have transferred the map to modern grid lines at 1:50,000 for direct comparison with modern O.S. maps, are available from www.cassinimaps.com and are highly recommended.

 

The above shows the kind of detail available on the 1880 maps
(from A Short History of Bernards Heath)

Large scale maps from circa 1880: The 6 inch to the mile maps have been digitised and are available online from the free site British History Online or old-maps - who will sell you a map centred on the area of interest to you. This shows individual woods and fields and even buildings, and for a larger farm it should be possible to identify a number of buildings. It should be noted that for most rural districts there would have been very little change over the preceding centuries, except where open fields had been enclosed.

The above shows the kind of detail available on the 1900 reprints
(from A Short History of Bernards Heath)