The Ordnance Survey 25 inch to the mile County Series Map is immensely valuable for local history. It is the most detailed Ordnance Survey mapping for most parts of England and Wales. Only urban areas with a population of over 4,000 people were mapped in more detail.
The maps allow practically every feature in the landscape to be shown. They provide good detail of all buildings, streets, railways, industrial premises, parkland, farms, woodland, and rivers. Their bold style and informative symbols and abbreviations allow easy interpretation for a wide range of uses.
The 25 inch scale is four times larger than 6 inch, and shows more features in a clearer way:
Greater detail for all buildings, including divisions between contiguous houses, and even smaller features such as projecting bay windows and steps
Pavements, garden paths and positions of free standing trees
Railway tracks and stations in plan form, with many smaller features, such as signal boxes
Industrial premises, quarries, lime kilns
Docks, harbours, and quaysides
All public boundaries, including civil parishes, municipal wards, as well as burgh and county boundaries.
Over time, changes in Ordnance Survey policy and the need to make economies also changed the detailed content on the maps. For example:
field names were omitted after 1888
the recording of bay windows, garden paths, gates (except across roads), and hedgerow timber was discontinued after 1892
public buildings were distinguished from 1897
The 25 inch maps record acreages of all land parcels on the map, below each parcel number. Land parcels were numbered consecutively within each parish, and acreages of fields were recorded in separate Parish Area Books or Books of Reference. On the map 1.0018 square inches on the map equals one acre on the ground.
Those counties surveyed before 1879 include land use information.
Until 1922, for fields that crossed sheet lines, the usual practice was to give two or more parcel numbers for each sheet. From 1922, for fields that crossed sheet lines, the usual practice was to give the same parcel number on each sheet, and calculate the area to the sheet edge only. Parcel numbers were not changed between editions if at all possible.
From 1889 until ca. 1907, built up areas for which were too detailed for individual land parcels were edged by a yellow band, and the acreage shown for this wider area. From 1897 to 1909, stipple bands were used. Smaller land parcels were sometimes grouped together through the 'S' (or brace symbol) crossing over parcel boundaries, with the acreage given for the multiple land parcels.
Ordnance Survey Bench marks (BMs) are survey marks made by Ordnance Survey to record height above Ordnance Datum. If the exact height of one BM is known, the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling.
Most commonly, the BMs are found on buildings or other semi-permanent features. Although the main network is no longer being updated, the record is still in existence and the markers will remain until they are eventually destroyed by redevelopment or erosion.
Bench marks are the visible manifestation of Ordnance Datum Newlyn (ODN), which is the national height system for mainland Great Britain and forms the reference frame for heights above mean sea level. ODN is realised on the ground by a network of approximately 190 fundamental bench marks (FBMs). From these FBMs tens of thousands of lower-order BMs were established. The network has had little maintenance for 30 years.
There are approximately 500 000 'lower order' BMs still remaining. This number is reducing due to property development, road widening and so on.